Why DRY?

Thursday, 07 August 2014 20:11:00 UTC

Code duplication is often harmful - except when it isn't. Learn how to think about the trade-offs involved.

Good programmers know that code duplication should be avoided. There's a cost associated with duplicated code, so we have catchphrases like Don't Repeat Yourself (DRY) in order to remind ourselves that code duplication is evil.

It seems to me that some programmers see themselves as Terminators: out to eliminate all code duplication with extreme prejudice; sometimes, perhaps, without even considering the trade-offs involved. Every time you remove duplicated code, you add a level of indirection, and as you've probably heard before, all problems in computer science can be solved by another level of indirection, except for the problem of too many levels of indirection.

Removing code duplication is important, but it tends to add a cognitive overhead. Therefore, it's important to understand why code duplication is harmful - or rather: when it's harmful.

Rates of change #

Imagine that you copy a piece of code and paste it into ten other code bases, and then never touch that piece of code again. Is that harmful?

Probably not.

Here's one of my favourite examples. When protecting the invariants of objects, I always add Guard Clauses against nulls:

if (subject == null)
    throw new ArgumentNullException("subject");

In fact, I have a Visual Studio code snippet for this; I've been using this code snippet for years, which means that I have code like this Guard Clause duplicated all over my code bases. Most likely, there are thousands of examples of such Guard Clauses on my hard drive, with the only variation being the name of the parameter. I don't mind, because, in my experience, these two lines of code never change.

Yet many programmers see that as a violation of DRY, so instead, they introduce something like this:

Guard.AgainstNull(subject, "subject");

The end result of this is that you've slightly increased the cognitive overhead, but what have you gained? As far as I can tell: nothing. The code still has the same number of Guard Clauses. Instead of idiomatic if statements, they are now method calls, but it's hardly DRY when you have to repeat those calls to Guard.AgainstNull all over the place. You'd still be repeating yourself.

The point here is that DRY is a catchphrase, but shouldn't be an excuse for avoiding thinking explicitly about any given problem.

If the duplicated code is likely to change a lot, the cost of duplication is likely to be high, because you'll have to spend time making the same change in lots of different places - and if you forget one, you'll be introducing bugs in the system. If the duplicated code is unlikely to change, perhaps the cost is low. As with all other risk management, you conceptually multiply the risk of the adverse event happening with the cost of the damage associated with that event. If the product is low, don't bother addressing the risk.

The Rule of Three #

It's not a new observation that unconditional elimination of duplicated code can be harmful. The Rule of Three exists for this reason:

  1. Write a piece of code.
  2. Write the same piece of code again. Resist the urge to generalise.
  3. Write the same piece of code again. Now you are allowed to consider generalising it.
There are a couple of reasons why this is a valuable rule of thumb. One is that the fewer examples you have of the duplication, the less evidence you have that the duplication is real, instead of simply a coincidence.

Another reason is that even if the duplication is 'real' (and not coincidental), you may not have enough examples to enable you to make the correct refactoring. Often, even duplicated code comes with small variations:

  • The logic is the same, but a string value differs.
  • The logic is almost the same, but one duplicate performs an extra small step.
  • The logic looks similar, but operates on two different types of object.
  • etc.
How should you refactor? Should you introduce a helper method? Should the helper method take a method argument? Should you extract a class? Should you add an interface? Should you apply the Template Method pattern? Or the Strategy pattern?

If you refactor too prematurely, you may perform the wrong refactoring. Often, people introduce helper methods, and then when they realize that the axis of variability was not what they expected, they add more and more parameters to the helper method, and more and more complexity to its implementation. This leads to ripple effects. Ripple effects lead to thrashing. Thrashing leads to poor maintainability. Poor maintainability leads to low productivity.

This is, in my experience, the most important reason to follow the Rule of Three: wait, until you have more facts available to you. You don't have to take the rule literally either. You can wait until you have four, five, or six examples of the duplication, if the rate of change is low.

The parallel to statistics #

If you've ever taken a course in statistics, you would have learned that the less data you have, the less confidence you can have in any sort of analysis. Conversely, the more samples you have, the more confidence can you have if you are trying to find or verify some sort of correlation.

The same holds true for code duplication, I believe. The more samples you have of duplicated code, the better you understand what is truly duplicated, and what varies. The better you understand the axes of variability, the better a refactoring you can perform in order to get rid of the duplication.

Summary #

Code duplication is costly - but only if the code changes. The cost of code duplication, thus, is C*p, where C is the cost incurred, when you need to change the code, and p is the probability that you'll need to change the code. In my experience, for example, the Null Guard Clause in this article has a cost of duplication of 0, because the probability that I'll need to change it is 0.

There's a cost associated with removing duplication - particularly if you make the wrong refactoring. Thus, depending on the values of C and p, you may be better off allowing a bit of duplication, instead of trying to eradicate it as soon as you see it.

You may not be able to quantify C and p (I'm not), but you should be able to estimate whether these values are small or large. This should help you decide if you need to eliminate the duplication right away, or if you'd be better off waiting to see what happens.


Markus Bullmann #

I would add one more aspect to this article: Experience.

The more experience you gain the more likely you will perceive parts of your code which could possibly benefit from a generalization; but not yet because it would be premature.

I try to keep these places in mind and try to simplify the potential later refactoring. When your code meets the rule of three, you’re able to adopt your preparations to efficiently refactor your code. This leads to better code even if the code isn't repeated for the third time.

Needless to say that experience is always crucial but I like to show people, who are new to programing, which decisions are based on experience and which are based on easy to follow guidelines like the rule of three.

2014-08-20 22:03 UTC

Markus, thank you for writing. Yes, I agree that experience is always helpful.

2014-08-21 8:48 UTC
Sergey Telshevsky #

Wonderful article but I'd like to say that I prefer sticking to the Zero one infinity rule instead of the Rule Of Three

That way I do overcome my laziness of searching for duplicates and extracting them to a procedure. Also it does help to keep code DRY without insanity to use it only on 2 or more lines of code. If it's a non-ternary oneliner I'm ok with that. If it's 2 lines of code I may consider making a procedure out of it and 3 lines and more are extracted every time I need to repeat them.

2014-12-23 14:00 UTC

I generally agree with the points raised here, but I'd like to point out another component to the example used of argument validation. I realize this post is fairly old now, so I'm not entirely sure if you'd agree or not considering new development since then.

While it is fair to say that argument validation is simple enough not to warrant some sort of encapsulation like the Guard class, this can quickly change once you add other factors into the mix, like number of arguments, and project-specific style rules. Take this sample code as an example:

if (subject1 == null)
    throw new ArgumentNullException("subject1");

if (subject2 == null)
    throw new ArgumentNullException("subject2");

if (subject3 == null)
    throw new ArgumentNullException("subject3");

if (subject4 == null)
    throw new ArgumentNullException("subject4");

if (subject5 == null)
    throw new ArgumentNullException("subject5");

This uses "standard" StyleCop rules with 5 parameters (probably the very limit of what is acceptable before having to refactor the code). Every block has the explicit scope added, and a line break after the scope terminator, as per style guidelines. This can get incredibly verbose and distract from the main logic of the method, which in cases like this, can become smaller than the argument validation code itself! Now, compare that to a more modern approach using discards:

_ = subject1 ?? throw new ArgumentNullException("subject1");
_ = subject2 ?? throw new ArgumentNullException("subject2");
_ = subject3 ?? throw new ArgumentNullException("subject3");
_ = subject4 ?? throw new ArgumentNullException("subject4");
_ = subject5 ?? throw new ArgumentNullException("subject5");

Or even better, using C#6 expression name deduction and the new, native guard method:


Now, even that can be improved upon, and C# is moving in the direction of making contract checks like this even easier.

I only wanted to make the point that, in this particular example, there are other benefits to the refactor besides just DRY, and those can be quite important as well since we read code much more than we write it: any gains in clarity and conciseness can net substantial productivity improvements.

2022-01-13 21:54 UTC

Juliano, thank you for writing. I agree that the first example is too verbose, but when addressing it, I'd start from the position that it's just a symptom. The underlying problem, here, is that the style guide is counterproductive. For the same reason, I don't write null guards that way.

I'm not sure that the following was your intent, so please excuse me as I take this topic in the direction of one of my pet peeves. In general, I think that 'we' have a tendency towards action bias. Instead of taking a step back and consider whether a problem even ought to be a problem, we often launch directly into producing a solution.

In the spirit of five whys, we may first find that the underlying explanation of the problem is the style guide.

Asking one more time, we may begin to consider the need for null guards altogether. Why do we need them? Because objects may be null. Is there a way to avoid that? In modern C#, you can turn on the nullable reference types feature. Other languages like Haskell famously have no nulls.

I realise that this line of inquiry is unproductive when you have existing C# code bases.

Your point, however, remains valid. It's perfectly fine to extract a helper method when the motivation is to make the code more readable. And while my book Code That Fits in Your Head is an attempt to instrumentalise readability to some extend, a subjective component will always linger.

2022-01-15 13:29 UTC

I don't agree that the style guide is counter productive per se. There are fair justifications for prefering the explicit braces and the blank lines in the StyleCop documentation. They might not be the easiest in the eyes in this particular scenario, but we have to take into consideration the ease of enforcement in the entire project of a particular style. If you deviate from the style only in certain scenarios, it becomes much harder to manage on a big team. Applying the single rule everywhere is much easier. I do understand however that this is fairly subjective, so I won't pursue the argument here.

Now, regarding nullable reference types, I'd just like to point out that one cannot only rely on that feature for argument validation. As you are aware, that's an optional feature, that might not be enabled on consumer code. If you are writing a reusable library for example, and you rely solely on Nullable Reference Types as your argument validation strategy, this will still cause runtime exceptions if a consumer that does not have NRTs enabled calls you passing null, and this would be a very bad developer experience for them as the stack trace will be potentially very cryptic. For that reason, it is a best practice to keep the runtime checks in place even when using nullable reference types.

2022-01-17 15:28 UTC

Juliano, thank you for writing. I'm sure that there are reasons for that style guide. I admit that I haven't read them (you didn't link to them), but I can think of some arguments myself. Unless you're interested in that particular discussion, I'll leave it at that. Ultimately, this blog post is about DRY in general, and not particularly about Guard Clauses.

I'm aware of the limitations of nullable reference types. This once again highlights the difference between developing and maintaining a reusable library versus a complete application.

Understanding the forces that apply in a particular context is, I believe, a key to making good decisions.

2022-01-18 10:20 UTC

I love the call out of risk * probability. I use threat matrixes to guide decisions all the time.

I'd also like to elaborate that duplication is determined not just by similar code, but by similar forces that change the code. If code looks similar, but changes for different reasons, then centralizing will cause tension between the different cases.

Using the example above, guard clauses are unlikely to change together. Most likely, the programmmer changes a parameter and just the single guard clause is effected. Since the clauses don't change for the same reason, the effect of duplication is very low.

Different forces might effect all guard clauses and justify centralizing. For example, if a system wanted to change its global null handling strategy at debug-time versus production.

However, I often see semantically separate code struggling to share an implementation. I think header interfaces and conforming container-like library wrappers are companion smells of this scenario. For example, I frequently see multiple flows trying to wrap email notifications behind some generic email notification abstraction. The parent flow is still coupled to the idea of email, the two notifications still change separately, and there's an extra poor abstraction between us and sending emails.

2022-02-19 21:00 UTC

Encapsulation and SOLID Pluralsight course

Wednesday, 06 August 2014 12:19:00 UTC

My latest Pluralsight course is now available. This time it's about fundamental programming techniques.

Most programmers I meet know about encapsulation and Object-Oriented Design (OOD) - at least, until I start drilling them on specifics. Apparently, the way most people have been taught OOD is at odds with my view on the subject. For a long time, I've wanted to teach OOD the way I think it should be taught. My perspective is chiefly based on Bertrand Meyer's Object-Oriented Software Construction and Robert C. Martin's SOLID principles (presented, among other sources, in Agile Principles, Patterns, and Practices in C#). My focus is on Command Query Separation, protection of invariants, and favouring Composition over Inheritance.

Course screenshot

In my new Pluralsight course, I'm happy to be able to teach OOD the way I think it should be taught. The course is aimed at professional developers with a couple of years of experience, but it's based on decades of experience (mine and others'), so I hope that even seasoned programmers can learn a thing or two from watching it.

What you will learn #

How do you make your code easily usable for other people? By following the actionable, measurable rules laid forth by Command/Query Separation as well as Postel’s law.

Learn how to write maintainable software that can easily respond to changing requirements, using object-oriented design principles. First, you'll learn about the fundamental object-oriented design principle of Encapsulation, and then you'll learn about the five SOLID principles - also known as the principles of object-oriented design. There are plenty of code examples along the way; they are in C#, but written in such a way that they should be easily understandable to readers of Java, or other curly-brace-based languages.

Is OOD still relevant? #

Functional Programming is becoming increasingly popular, and regular readers of this blog will have noticed that I, myself, write a fair amount of F# code. In light of this, is OOD still relevant?

There's a lot of Object-Oriented code out there, or at least, code written in a nominally Object-Oriented language, and it's not going to go away any time soon. Getting better at maintaining and evolving Object-Oriented code is, in my opinion, still important.

Some of the principles I cover in this course are also relevant in Functional Programming.


I'm really enjoying your presentation! It is easy to follow and understand your explanations. I have a bit of a tangent question about your TryRead method in the Encapsulation module. You mention that your final implementation has a race condition in it. Can you explain the race condition?
2014-08-06 06:55 UTC

Dan, thank you for writing. You are talking about this implementation, I assume:

public bool TryRead(int id, out string message)
    message = null;
    var path = this.GetFileName(id);
    if (!File.Exists(path))
        return false;
    message = File.ReadAllText(path);
    return true;

This implementation isn't thread-safe, because File.Exists(path) may return true, and then, before File.ReadAllText(path) is invoked, another thread or process could delete the file, causing an exception to be thrown when File.ReadAllText(path) is invoked.

It's possible to make the TryRead method thread-safe (I know of at least two alternatives), but I usually leave that as an exercise for the reader :)

2014-08-10 10:51 UTC
Bart van Nierop #

The only safe implementation that I am aware of would be something along the lines of:

public bool TryRead(int id, out string message)
    var path = this.GetFileName(id);
        message = File.ReadAllText(path);
        return true;
        message = null;
        return false;

Am I missing something?

2014-08-13 16:43 UTC

Bart, no, you aren't missing anything; that looks about right :) The other alternative is just a variation on your solution.

2014-08-13 17:56 UTC


Wednesday, 23 July 2014 14:25:00 UTC

A Drain is a filter abstraction over an Iterator, with the purpose of making out-of-process queries more efficient.

For some years now, the Reused Abstractions Principle has pushed me towards software architectures based upon fewer and fewer abstractions, where each abstraction, on the other hand, is reused over and over again.

In my Pluralsight course about A Functional Architecture with F#, I describe how to build an entire mainstream application based on only two abstractions:

More specifically, I use Reactive Extensions for Commands, and the Seq module for Queries (but if you're on C#, you can use LINQ instead).

The problem #

It turns out that these two abstractions are enough to build an entire, mainstream system, but in practice, there's a performance problem. If you have only Iterators, you'll have to read all your data into memory, and then filter in memory. If you have lots of data on storage, this is obviously going to be prohibitively slow, so you'll need a way to select only a subset out of your persistent store.

This problem should be familiar to every student of software development. Pure abstractions tend not to survive contact with reality (there are examples in both Object-Oriented, Functional, and Logical or Relational programming), but we should still strive towards keeping abstractions as pure as possible.

One proposed solution to the Query Problem is to use something like IQueryable, but unfortunately, IQueryable is an extremely poor abstraction (and so are F# query expressions, too).

In my experience, the most important feature of IQueryable is the ability to filter before loading data; normally, you can perform projections in memory, but when you read from persistent storage, you need to select your desired subset before loading it into memory.

Inspired by talks by Bart De Smet, in my Pluralsight course, I define custom filter interfaces like:

type IReservations =
    inherit seq<Envelope<Reservation>>
    abstract Between : DateTime -> DateTime -> seq<Envelope<Reservation>>


type INotifications =
    inherit seq<Envelope<Notification>>
    abstract About : Guid -> seq<Envelope<Notification>>

Both of these interfaces derive from IEnumerable<T> and add a single extra method that defines a custom filter. Storage-aware implementations can implement this method by returning a new sequence of only those items on storage that match the filter. Such a method may

  • make a SQL query against a database
  • make a query against a document database
  • read only some files from the file system
  • etc.
For more details, examples, and full source code, see my Pluralsight course.

Generalized interface #

The custom interfaces shown above follow a common template: the interface derives from IEnumerable<T> and adds a single 'filter' method, which filters the sequence based on the input argument(s). In the above examples, IReservations define a Between method with two arguments, while INotifications defines an About method with a single argument.

In order to generalize, it's necessary to find a common name for the interface and its single method, as well as deal with variations in method arguments.

All the really obvious names like Filter, Query, etc. are already 'taken', so I hit a thesaurus and settled on the name Drain. A Drain can potentially drain a sequence of elements to a smaller sequence.

When it comes to variations in input arguments, the solution is to use generics. The Between method that takes two arguments could also be modelled as a method taking a single tuple argument. Eventually, I've arrived at this general definition:

module Drain =
    type IDrainable<'a, 'b> =
        inherit seq<'a>
        abstract On : 'b -> seq<'a>
    let on x (d : IDrainable<'a, 'b>) = d.On x

As you can see, I decided to name the extra method On, as well as add an on function, which enables clients to use a Drain like this:

match tasks |> Drain.on id |> Seq.toList with

In the above example, tasks is defined as IDrainable<TaskRendition, string>, and id is a string, so the result of draining on the ID is a sequence of TaskRendition records.

Here's another example:

match statuses |> Drain.on(id, conversationId) |> Seq.toList with

Here, statuses is defined as IDrainable<string * Guid, string * string> - not the most well-designed instance, I admit: I should really introduce some well-named records instead of those tuples, but the point is that you can also drain on multiple values by using a tuple (or a record type) as the value on which to drain.

In-memory implementation #

One of the great features of Drains is that an in-memory implementation is easy, so you can add this function to the Drain module:

let ofSeq areEqual s =
    { new IDrainable<'a, 'b> with
        member this.On x = s |> Seq.filter (fun y -> areEqual y x)
        member this.GetEnumerator() = s.GetEnumerator()
        member this.GetEnumerator() = 
            (this :> 'a seq).GetEnumerator() :> System.Collections.IEnumerator }

This enables you to take any IEnumerable<T> (seq<'a>) and turn it into an in-memory Drain by supplying an equality function. Here's an example:

let private toDrainableTasks (tasks : TaskRendition seq) =
    |> Drain.ofSeq (fun x y -> x.Id = y)

This little helper function takes a sequence of TaskRendition records and defines the equality function as a comparison on each TaskRendition record's Id property. The result is a drain that you can use to select one or more TaskRendition records based on their IDs.

I use this a lot for unit testing.

Empty Drains #

It's also easy to define an empty drain, by adding this value to the Drain module:

let empty<'a, 'b> = Seq.empty<'a> |> ofSeq (fun x (y : 'b) -> false)

Here's a usage example:

let mappedUsers = Drain.empty<UserMappedstring>

Again, this can be handy when unit testing.

Other implementations #

While the in-memory implementation is useful when unit testing, the entire purpose of the Drain abstraction is to enable various implementations to implement the On method to perform a custom selection against a well-known data source. As an example, you could imagine an implementation that translates the input arguments of the On method into a SQL query.

If you want to see examples of this, my Pluralsight course demonstrates how to implement IReservations and INotifications with various data stores - I trust you can extrapolate from those examples.

Summary #

You can base an entire mainstream application on the two abstractions of Iterator and Observer. However, the problem when it comes to Iterators is that conceptually, you'll need to iterate over all potentially relevant elements in your system - and that may be millions of records!

However impure it is to introduce a third interface into the mix, I still prefer to introduce a single generic interface, instead of multiple custom interfaces, because once you and your co-workers understand the Drain abstraction, the cognitive load is still quite low. A Drain is an Iterator with a twist, so in the end, you'll have a system built on 2½ abstractions.

P.S. 2018-06-20. While this article is a decent attempt to generalise the query side of a fundamentally object-oriented approach to software architecture, I've later realised that dependency injection, even when disguised as partial application, isn't functional. The problem is that querying a database, reading files, and so on, is essentially non-deterministic, even when no side effects are incurred. The functional approach is to altogether reject the notion of dependencies.


A diff-output from the A Functional Architecture with F# master branch, after applying the Drain abstraction, is available here. Notice how Drain cuts the maintenance of multiple homogenous abstractions, and makes the code cleaner and easier to reason about.
2014-07-28 09:00 UTC

Hire me

Tuesday, 22 July 2014 08:30:00 UTC

July-October 2014 I have some time available, if you'd like to hire me.

Since I became self-employed in 2011, I've been as busy as always, but it looks like I have some time available in the next months. If you'd like to hire me for small or big tasks, please contact me. See here for details.

Passive Attributes

Friday, 13 June 2014 09:59:00 UTC

Passive Attributes are Dependency Injection-friendly.

In my article about Dependency Injection-friendly frameworks, towards the end I touched on the importance of defining attributes without behaviour, but I didn't provide a constructive example of how to do this. In this article, I'll outline how to write a Dependency Injection-friendly attribute for use with ASP.NET Web API, but as far as I recall, you can do something similar with ASP.NET MVC.

Problem statement #

In ASP.NET Web API, you can adorn your Controllers and their methods with various Filter attributes, which is a way to implement cross-cutting concerns, such as authorization or error handling. The problem with this approach is that attribute instances are created by the run-time, so you can't use proper Dependency Injection (DI) patterns such as Constructor Injection. If an attribute defines behaviour (which many of the Web API attributes do), the most common attempt at writing loosely coupled code is to resort to a static Service Locator (an anti-pattern).

This again seems to lead framework designers towards attempting to make their frameworks 'DI-friendly' by introducing a Conforming Container (another anti-pattern).

The solution is simple: define attributes without behaviour.

Metering example #

Common examples of cross-cutting concerns are authentication, authorization, error handling, logging, and caching. In these days of multi-tenant on-line services, another example would be metering, so that you can bill each user based on consumption.

Imagine that you're writing an HTTP API where some actions must be metered, whereas others shouldn't. It might be nice to adorn the metered actions with an attribute to indicate this:

public IHttpActionResult Get(int id)

Metering is a good example of a cross-cutting concern with behaviour, because, in order to be useful, you'd need to store the metering records somewhere, so that you can bill your users based on these records.

A passive Meter attribute would simply look like this:

[AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
public class MeterAttribute : Attribute

In order to keep the example simple, this attribute defines no data, and can only be used on methods, but nothing prevents you from adding (primitive) values to it, or extend its usage to classes as well as methods.

As you can tell from the example, the MeterAttribute has no behaviour.

In order to implement a metering cross-cutting concern, you'll need to define an IActionFilter implementation, but that's a 'normal' class that can take dependencies:

public class MeteringFilter : IActionFilter
    private readonly IObserver<MeterRecord> observer;
    public MeteringFilter(IObserver<MeterRecord> observer)
        if (observer == null)
            throw new ArgumentNullException("observer");
        this.observer = observer;
    public Task<HttpResponseMessage> ExecuteActionFilterAsync(
        HttpActionContext actionContext,
        CancellationToken cancellationToken,
        Func<Task<HttpResponseMessage>> continuation)
        var meterAttribute = actionContext
        if (meterAttribute == null)
            return continuation();
        var operation = actionContext.ActionDescriptor.ActionName;
        var user = actionContext.RequestContext.Principal.Identity.Name;
        var started = DateTimeOffset.Now;
        return continuation().ContinueWith(t =>
                var completed = DateTimeOffset.Now;
                var duration = completed - started;
                var record = new MeterRecord
                    Operation = operation,
                    User = user,
                    Started = started,
                    Duration = duration
                return t.Result;
    public bool AllowMultiple
        get { return true; }

This MeteringFilter class implements IActionFilter. It looks for the [Meter] attribute. If it doesn't find the attribute on the method, it immediately returns; otherwise, it starts collecting data about the invoked action:

  1. From actionContext.ActionDescriptor it retrieves the name of the operation. If you try this out for yourself, you may find that ActionName alone doesn't provide enough information to uniquely identify the method - it basically just contains the value "Get". However, the actionContext contains enough information about the action that you can easily build up a better string; I just chose to skip doing that in order to keep the example simple.
  2. From actionContext.RequestContext.Principal you can get information about the current user. In order to be useful, the user must be authenticated, but if you need to meter the usage of your service, you'll probably not allow anonymous access.
  3. Before invoking the continuation, the MeteringFilter records the current time.
  4. After the continuation has completed, the MeteringFilter again records the current time and calculates the duration.
  5. Finally, it publishes a MeterRecord to an injected dependency.
Notice that MeteringFilter uses normal Constructor Injection, which means that it can protect its invariants. In this example, I'm using IObserver<T> as a dependency, but obviously, you could use any dependency you'd like.

Configuring the service #

MeteringFilter is a normal class with behaviour, which you can register as a cross-cutting concern in your Web API service as easily as this:

var filter = new MeteringFilter(observer);

where observer is your preferred implementation of IObserver<MeterRecord>. This example illustrates the Pure DI approach, but if you rather prefer to resolve MeteringFilter with your DI Container of choice, you can obviously do this as well.

The above code typically goes into your Global.asax file, or at least a class directly or indirectly invoked from Application_Start. This constitutes (part of) the Composition Root of your service.

Summary #

Both ASP.NET Web API and ASP.NET MVC supports cross-cutting concerns in the shape of filters that you can add to the service. Such filters can look for passive attributes in order to decide whether or not to trigger. The advantage of this approach is that you can use normal Constructor Injection with these filters, which completely eliminates the need for a Service Locator or Conforming Container.

The programming model remains the same as with active attributes: if you want a particular cross-cutting concern to apply to a particular method or class, you adorn it with the appropriate attribute. Passive attributes have all the advantages of active attributes, but none of the disadvantages.


Jonathan Ayoub #
Thanks for the article. I have a simple filter that I need to add, and I was just going to use service locator to get my dependency, but realized that would force me to do things I don't want to when writing a test for the filter.

What if the dependency I'm injecting needs to be a transient dependency? Injecting a transient service into a singleton (the filter), would cause issues. My initial idea is to create an abstract factory as a dependency, then when the filter action executes, create the transient dependency, use it, and dispose. Do you have any better ideas?
2016-01-21 18:34 UTC

Jonathan, thank you for writing. Does this article on the Decoraptor pattern (and this Stack Overflow answer) answer your question?

2016-01-21 19:51 UTC
Jonathan Ayoub #

Yes, that's a good solution for this situation. Thanks!

2016-01-26 14:32 UTC

Reading through Asp.Net Core Type and Service Filters, do you think it's sufficient to go that way (I know that TypeFilter is a bit clumsy), but let's assume that I need simple injection in my filter - ServiceFilterAttribute looks promising. Or you still would recomment to implement logic via traditional filter pipeline: `services.AddMvc(o => o.Filters.Add(...));`?

2016-08-21 17:20 UTC

Valdis, thank you for writing. I haven't looked into the details of ASP.NET Core recently, but even so: on a more basic level, I don't understand the impulse to put behaviour into attributes. An attribute is an annotation. It's a declaration that a method, or type, is somehow special. Why not keep the annotation decoupled from the behaviour it 'triggers'? This would enable you to reuse the behaviour in other scenarios than by putting an attribute on a method.

2016-08-22 16:21 UTC

I got actually very similar feelings, just wanted to get your opinion. And by the way - there is a catch, you can mismatch type and will notice that only during runtime. For instance: `[ServiceFilter(typeof(HomeController))]` will generate exception, because given type is not derived from `IFilterMetadata`

2016-08-22 20:46 UTC

Indeed, one of the most problematic aspects of container-based DI (as opposed to Pure DI) is the loss of compile-time safety - to little advantage, I might add.

2016-08-23 05:49 UTC

I'm concerned with using attributes for AOP at all in these cases. Obviously using attributes that define behaviors that rely on external depedencies is a bad idea as you and others have already previously covered. But is using attributes that define metadata for custom behaviors all that much better? For example, if one provides a framework with libraries containing common controllers, and another pulls those controllers into their composition root in order to host them, there is no indication at compile time that these custom attributes may be present. Would it not be better to require that behavior be injected into the constructor and simply consume the behavior at the relevant points within the controller? Or if attributes must be used, would it not be better for the component that implements the behavior to somehow be injected into the controller and given the opportunity to intercept requests to the controller earlier in the execution pipeline so that it can check for the attributes? Due to the nature of the Web API and .Net MVC engines, attributes defined with behavior can enforce their behaviors to be executed by default. And while attributes without behavior do indicate the need for a behavior to be executed for the class they are decorating, it does not appear that they can enforce said behavior to be executed by default. They are too easy to miss or ignore. There has got to be a better way. I have encountered this problem while refactoring some code that I'm working on right now (retro fitting said code with more modern, DI based code). I'm hoping to come up with a solution that informs the consuming developer in the composition root that this behavior is required, and still be able enforce the behavior with something closer to a decoration rather than a function call.

2016-11-16 23:10 UTC

Tyree, thank you for writing. That's a valid concern, but I don't think it's isolated to passive attributes. The problem you outline is also present if you attempt to address cross-cutting concerns with the Decorator design pattern, or with dynamic interception as I describe in chapter 9 of my book. You also have this problem with the Composite pattern, because you can't have any compile-time guarantee that you've remembered to compose all required elements into the Composite, or that they're defined in the appropriate order (if that matters).

In fact, you can extend this argument to any use of polymorphism: how can you guarantee, at compile-time, that a particular polymorphic object contains the behaviour that you desire, instead of, say, being a Null Object? You can't. That's the entire point of polymorphism.

Even with attributes, how can you guarantee that the attributes stay there? What if another developer comes by and removes an attribute? The code is still going to compile.

Ultimately, code exists in order to implement some desired behaviour. There are guarantees you can get from the type system, but static typing can't provide all guarantees. If it could, you be in the situation where, 'if it compiles, it works'. No programming language I've heard of provides that guarantee, although there's a spectrum of languages with stronger or weaker type systems. Instead, we'll have to get feedback from multiple sources. Attributes often define cross-cutting concerns, and I find that these are often best verified with a set of integration tests.

As always in software development, you have to find the balance that's right for a particular scenario. In some cases, it's catastrophic if something is incorrectly configured; in other cases, it's merely unfortunate. More rigorous verification is required in the first case.

2016-11-19 9:39 UTC

Thanks for the post, I tried to do the same for a class attribute (AttributeTargets.Class) and I am getting a null object every time I get the custom attributes. Does this only work for Methods? Or how can I make it work with classes? Thanks.

2017-04-05 13:03 UTC

Cristian, thank you for writing. The example code shown in this article only looks in the action context's ActionDescriptor, which is an object that describes the action method. If you want to look for the attribute on the class, you should look in the action context's ControllerDescriptor instead, like this:

var meterAttribute = actionContext

Obviously, if you want to support putting the attribute both on the class and the method, you'd need to look in both places, and decide which one to use if you find more than one.

2017-04-25 6:12 UTC
Ian Thomas #

I'm really struggling to get everything hooked up so it's initialised in the correct order, with one DbContext created per web request or scheduled task. The action filter is my biggest sticking point.

I had previously split an IAuthorizationFilter off from a WebAPI AuthorizationFilterAttribute following the advice of this article. I create my filter in WebApiConfig.Register, which is called from MvcApplication.Application_Start(). My problem is that I can't inject my UserService here, because this code is shared across requests, and I want to use one DbContext per request (UserService depends on DbContext). It works if I inject the DependencyResolver, but I realise that's an anti-pattern.

What am I missing? Should I new up a DbContext and UserService just for my AuthorizationFilter, that then gets shared acorss requests? Is there somewhere else I can add my ActionFilter, that gets called once per request? I can inject my service into the created AuthorizationFilterAttribute using the steps described at https://michael-mckenna.com/dependency-injection-for-asp-net-web-api-action-filters-in-3-easy-steps/ but would that just make it a captive dependency of an AuthorizationFilterAttribute that gets shared across multiple requests?

2019-07-26 17:28 UTC

Ian, thank you for writing. It sounds like you need a Decoraptor.

2019-07-28 12:58 UTC

Web API Raygun error handler

Thursday, 12 June 2014 06:38:00 UTC

Adding a Raygun error handler to ASP.NET Web API is easy.

In my Pluralsight course on a Functional Architecture with F#, I show you how to add a global error handler to an ASP.NET Web API site. The error handler you see in the course just saves the error as a text file in Azure BLOB storage, which is fine for a demo. For production software, though, you may want something a bit more sophisticated, like Raygun.

Here's how to add a Raygun exception filter to an ASP.NET Web API site:

let raygunHandler = { new System.Web.Http.Filters.IExceptionFilter with 
    member this.AllowMultiple = true
    member this.ExecuteExceptionFilterAsync(actionExecutedContext, cancellationToken) =
        let raygunKey = CloudConfigurationManager.GetSetting "raygunKey"
        let raygunClient = Mindscape.Raygun4Net.RaygunClient raygunKey
            fun () -> raygunClient.Send actionExecutedContext.Exception) }

This creates an Adapter from the ASP.NET Web API IExceptionFilter interface to a RaygunClient instance. As you can see, I use CloudConfigurationManager.GetSetting to get the Raygun API key from the configuration store.

The only remaining step is to add the error handler to an HttpConfiguration instance:

config.Filters.Add raygunHandler

That's it. Now you can use the Raygun service to manage your errors.

Pure DI

Tuesday, 10 June 2014 06:10:00 UTC

Pure DI is Dependency Injection without a DI Container.

TL;DR: the term Pure DI replaces the term Poor Man's DI.

This post essentially proposes a change of terminology. In my book about Dependency Injection (DI), I was careful to explain the principles and patterns of DI in the pure form, without involving DI Containers. Only in Part 4 do you get extensive coverage of various DI Containers, and even here, what you learn is how the DI principles and patterns map to the various containers.

DI is a set of principles and patterns; DI Containers are optional helper libraries.

However, when I wrote the book, I made a mistake (I probably made many, but here, I'll address a single, specific mistake): in the book, DI without DI Containers is called Poor Man's DI. There are reasons for that, but eventually, I've learned that Poor Man's DI is poor terminology (pun intended). The problem is that it sounds slightly derogatory, or at least unattractive; it also doesn't communicate the message that DI without a DI Container is, in many cases, better than DI with a DI Container - on the contrary, it sounds like it's not as good.

Apart from my book, I've attempted to describe the trade-off involved in going from Poor Man's DI to using a DI Container in various articles:

Based on the reactions I've received, it seems like my readers really like their DI Containers. Perhaps they're just afraid of the alternative, because it's called Poor Man's DI.

For these reasons, from now on, I'll retire the term Poor Man's DI, and instead start using the term Pure DI. Pure DI is when you use the DI principles and patterns, but not a DI Container; it's what I've been doing for the last 1½ years, as well as many years before I wrote my book.

P.S. 2018-04-15. Naming is hard. When I came up with the alternative name of Pure DI, I was already in the process of changing my focus to functional programming, but at that time, I'd still not realised how big a role referential transparency plays in strictly functional languages like Haskell, with its emphasis on pure functions.

The purity implied by Pure DI, unfortunately, has nothing to do with purity in the functional sense of the word. In fact, DI makes everything impure.

When I decided on the term Pure DI, I did it because it sounds vaguely like Poor Man's DI, so that it'd be easy to remember. Additionally, I picked the word pure because it can mean essence, and I thought that, in a sense, Pure DI can be viewed as the Platonic ideal of DI. Finally, pure sounds like something desirable, so because of those reasons, I believed that it'd be a good term. People seem to have picked it up, so in that sense I think that I chose the right word, but it can be confusing if viewed through the lens of functional programming terminology.

Pure DI has nothing to do with pure functions.

Compile-Time Lifetime Matching

Tuesday, 03 June 2014 10:06:00 UTC

When using hand-coded object composition, the compiler can help you match service lifetimes.

In my previous post, you learned how easy it is to accidentally misconfigure a DI Container to produce Captive Dependencies, which are dependencies that are being kept around after they should have been released. This can lead to subtle or catastrophic bugs.

This problem is associated with DI Containers, because Container registration APIs let you register services out of order, and with any particular lifestyle you'd like:

var builder = new ContainerBuilder();
var container = builder.Build();

In this Autofac example, CommerceContext is registered before SqlProductRepository, even though SqlProductRepository is a 'higher-level' service, but ProductService is registered first, and it's even 'higher-level' than SqlProductRepository. A DI Container doesn't care; it'll figure it out.

The compiler doesn't care if the various lifetime configurations make sense. As you learned in my previous article, this particular configuration combination doesn't make sense, but the compiler can't help you.

Compiler assistance #

The overall message in my Poka-yoke Design article series is that you can often design your types in such a way that they are less forgiving of programming mistakes; this enables the compiler to give you feedback faster than you could otherwise have gotten feedback.

If, instead of using a DI Container, you'd simply hand-code the required object composition (also called Poor Man's DI in my book, but now called Pure DI), the compiler will make it much harder for you to mismatch object lifetimes. Not impossible, but more difficult.

As an example, consider a web-based Composition Root. Here, the particular IHttpControllerActivator interface belongs to ASP.NET Web API, but it could be any Composition Root:

public class SomeCompositionRoot : IHttpControllerActivator
    // Singleton-scoped services are declared here...
    private readonly SomeThreadSafeService singleton;
    public SomeCompositionRoot()
        // ... and (Singleton-scoped services) are initialised here.
        this.singleton = new SomeThreadSafeService();
    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
        // Per-Request-scoped services are declared and initialized here
        var perRequestService = new SomeThreadUnsafeService();
        if(controllerType == typeof(FooController))
            // Transient services are created and directly injected into
            // FooController here:
            return new FooController(
                new SomeServiceThatMustBeTransient(),
                new SomeServiceThatMustBeTransient());
        if(controllerType == typeof(BarController))
            // Transient service is created and directly injected into
            // BarController here, but Per-Request-scoped services or
            // Singleton-scoped services can be used too.
            return new BarController(
                new SomeServiceThatMustBeTransient());
        throw new ArgumentException("Unexpected type!""controllerType");

Notice the following:

  • There's only going to be a single instance of the SomeCompositionRoot class around, so any object you assign to a readonly field is effectively going to be a Singleton.
  • The Create method is invoked for each request, so if you create objects at the beginning of the Create method, you can reuse them as much as you'd like, but only within that single request. This means that even if you have a service that isn't thread-safe, it's safe to create it at this time. In the example, the BarController depends on two arguments where the Per-Request Service fits, and the instance can be reused. This may seem contrived, but isn't at all if SomeThreadUnsafeService implements more that one (Role) interface.
  • If you need to make a service truly Transient (i.e. it must not be reused at all), you can create it within the constructor of its client. You see an example of this when returning the FooController instance: this example is contrived, but it makes the point: for some unfathomable reason, FooController needs two instances of the same type, but the SomeServiceThatMustBeTransient class must never be shared. It's actually quite rare to have this requirement, but it's easy enough to meet it, if you encounter it.
It's easy to give each service the correct lifetime. Singleton services share the lifetime of the Composition Root, Per-Request services are created each time the Create method is called, and Transient services are created Just-In-Time. All services go out of scope at the correct time, too.

Commerce example #

In the previous article, you saw how easy it is to misconfigure a ProductService, because you'd like it to be a Singleton. When you hand-code the composition, it becomes much easier to spot the mistake. You may start like this:

public class CommerceCompositionRoot : IHttpControllerActivator
    private readonly ProductService productService;
    public CommerceCompositionRoot()
        this.productService = new ProductService();
    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
        // Implementation follows here...

Fortunately, that doesn't even compile, because ProductService doesn't have a parameterless constructor. With a DI Container, you could define ProductService as a Singleton without a compilation error:

var container = new StandardKernel();

If you attempt to do the same with hand-coded composition, it doesn't compile. This is an excellent example of Poka-Yoke Design: design your system in such a way that the compiler can give you as much feedback as possible.

Intellisense will tell you that ProductService has dependencies, so your next step may be this:

public CommerceCompositionRoot()
    this.productService = 
        new ProductService(
            new SqlProductRepository(
                new CommerceContext())); // Alarm bell!

This will compile, but at this point, an alarm bell should go off. You know that you mustn't share CommerceContext across threads, but you're currently creating a single instance. Now it's much clearer that you're on your way to doing something wrong. In the end, you realise, simply by trial and error, that you can't make any part of the ProductService sub-graph a class field, because the leaf node (CommerceContext) isn't thread-safe.

Armed with that knowledge, the next step is to create the entire object graph in the Create method, because that's the only safe implementation left:

public IHttpController Create(
    HttpRequestMessage request,
    HttpControllerDescriptor controllerDescriptor,
    Type controllerType)
    if(controllerType == typeof(HomeController))
        return new HomeController(
            new ProductService(
                new SqlProductRepository(
                    new CommerceContext())));
    // Handle other controller types here...
    throw new ArgumentException("Unexpected type!""controllerType");

In this example, you create the object graph in a single statement, theoretically giving all services the Transient lifestyle. In practice, there's no difference between the Per Request and the Transient lifestyle as long as there's only a single instance of each service for each object graph.

Concluding remarks #

Some time ago, I wrote an article on when to use a DI Container. In that article, I attempted to explain how going from Pure DI (hand-coded composition) to a DI Container meant loss of compile-time safety, but I may have made an insufficient job of providing enough examples of this effect. The Captive Dependency configuration error, and this article together, describe one such effect: with Pure DI, lifetime matching is compiler-assisted, but if you refactor to use a DI Container, you lose the compiler's help.

Since I wrote the article on when to use a DI Container, I've only strengthened my preference for Pure DI. Unless I'm writing a very complex code base that could benefit from Convention over Configuration, I don't use a DI Container, but since I explicitly architect my systems to be non-complex these days, I haven't used a DI Container in production code for more than 1½ years.


I don't think it's a problem with the container, but a problem with the registrations. I use a Autofac as my DI Container registration, and I always have a root application lifetime scope, and a separate scope for each request. If the product service is registered in the root scope as single instance, it will throw a DependencyResolutionException

In this case, I would have the ProductService registered in the root scope as single instance, and the other types in the request scope.

If ProductService is resolved, a DependencyResolutionException is thrown, and the app is unusable - "fail fast" is followed. To fix the issue, the registration needs to be moved to to the request scope.

Here's an example of a safe MVC Controller Factory using Autofac.

public class AutofacControllerFactory : DefaultControllerFactory
    private readonly IContainer container;
    private Dictionary<IController, ILifetimeScope> scopes = new Dictionary<IController, ILifetimeScope>();

    public AutofacControllerFactory()
        var builder = new ContainerBuilder();

        this.container = builder.Build();

    private void RegisterRootTypes(ContainerBuilder builder)

            .Where(t => t.Name.Contains("Controller"))
    protected internal override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
        var requestScope = container.BeginLifetimeScope(RegisterRequestTypes);
        var controller = (IController)requestScope.Resolve(controllerType);
        scopes[controller] = requestScope;
        return controller;

    private void RegisterRequestTypes(ContainerBuilder builder)

    public override void ReleaseController(IController controller)

Sorry for the lack of code formatting - I'm not sure what you use to format code

2014-06-04 13:20 UTC

Steve, thank you for writing. Indeed, you can make a DI Container detect the Captive Dependency error at run-time. I pointed that out in the defining article about the Captive Dependency problem, and as qujck points out in the comments, Simple Injector has this feature, too.

The point with the present article is that, instead of waiting until run-time, you get a chance to learn about potential lifetime mismatches already at design-time. In C#, F#, and other compiled languages, you can't perform a run-time test until you've compiled. While I'm all for fail fast, I don't think it'd be failing fast enough, if you can catch the problem at compile time, but then deliberately wait until run-time.

Another concern is to go for the simplest thing that could possibly work. Why use a complex piece of code like your AutofacControllerFactory above, instead of just writing the code directly? It's no secret that I'm not a big fan of the Lifetime Scope idiom, and your code provides an excellent example of how complicated it is. You may have omitted this for the sake of the example, but that code isn't thread-safe; in order to make it thread-safe, you'd need to make it even more complicated.

You probably know how to make it thread-safe, as do I, so this isn't an attempt at pointing fingers. The point I'm attempting to make is that, by using a DI Container, you

  • Add complexity
  • Get slower feedback
There are costs associated with using a DI Container; what are the benefits?

2014-06-04 16:45 UTC

Thanks for the speedy response, Mark - I can't keep up. I think an issue I have with poor-man's DI is that I'm yet to see it in anything more than a trivial example. Indeed, the only time I have seen it used in a professional context is a 300 line file, 280 lines of which have the 'new' keyword in it, with plenty of repetition.

Do you know of any medium sized code bases around that use it to good effect? I'm thinking an application with at least 100 types, I'd like to see how the complexity of the graph is managed.

To answer your question, here's the advantages I see of using a container and lifetime scopes.

  • Clearer lifetimes: Your statement that the compiler is detecting the captive dependency isn't quite correct - it's still the developer doing that at design time. They have to see that new CommerceContext() is not a smart thing to do at application start, and move it accordingly. The compiler has nothing to do with that - either way, the check is happening at coding time. Whether that's while typing new CommerceContext() or when typing builder.Register<CommerceContext>(), it's the same thing.

    I'd argue that the code that registers CommerceContext in an application scope is a much clearer alarm bell. After fixing the issue, you'll end up with the registration appearing in a RegisterRequestScopedStuff() method, which is a much better way to tell future developers to be careful about this guy in the future.

  • Simplicity: I would argue that the Autofac controller factory is simpler than the poor mans one. Using poor man style, you have a switch (or bunch of if statements) on the controller type, and need keep track of correct lifetimes in a deeply-nested object graph. I think a (thread safe) dictionary and disposal is significantly simpler that those things - at the very least, has fewer branches - and provides great access points to define expected lifetimes of objects. It probably seems more complicated because there's only a few types, mine has no syntax highlighting (very important for readability!) and I've documented which bits are app-wide and which are request-wide lifetimes, via method names and registration types.

  • Speed of development: I find the overall development speed is faster using a container, as you don't have to micromanage the dependencies. While you do get slower feedback times on dependency failures, you have far fewer failures overall. It's been several months since I've seen a DependencyResolutionException. On the flip side, the javascript development I've done (which doesn't use a container) often has a missing a dependency or 2 - which would be equivalent to a compile error in a strongly typed language.

    What's more, I can write my classes and tests without having to worry about composition until it's time to run the application. To be fair, this is also achieved with good domain/application separation - since the app failing to compile does not prevent the tests from running - but I still like to write tests for my application project.

  • Disposables: As you mentioned, my simple example was not thread safe, due to having to store the lifetime scope for disposal when the controller is released. The only reason I need to store that is so Autofac can clean up any IDisposable dependencies I may have, and trivially at that - how do you do this with poor man's DI, while keeping it simple?

If I can wire up Autofac in my application in 10 minutes, have the computer do all the heavy lifting, while making it clearer to myself and future people what I want the lifetimes of things to be, why would I want to manage a dependency graph myself?

2014-06-05 13:30 UTC

Before we continue this discussion, I think it's important to establish how you use a DI Container. If you refer to my article on the benefits of using a DI Container, which approach are you using?

2014-06-05 14:22 UTC

I'd say I sit somewhere between convention and explicit register, but I guess I disagree about the "pointless" value for it, and place less importance on the value of strong/weak typing. As I said, I very rarely have the dependency exceptions be thrown anyway. In practice, I have a class of services that are wired up by convention (type name ends in "Factory" or "Controller", for example), and explicitly register others. No hard and fast rules about it.

2014-06-06 14:00 UTC

That makes the discussion a little less clear-cut, because you are getting some of the benefits out of Convention over Configuration, but perhaps not as much as you could... Depending on how you put the balance between these two, I would agree with you that using a DI Container is beneficial.

My point isn't that there are no benefits from using a DI Container, but that there are also serious disadvantages. The benefits should outweigh the disadvantages, and that is, in my experience, far from given that they do. YMMV.

Do I know of any medium-sized code bases that use Pure DI to good effect? Perhaps... I don't know what a 'medium-sized' code base is to you. In any case, while I may know of such code bases, I know of none where the source code is public.

300-odd lines of code for composition sounds like a lot, but as I have previously demonstrated, using Explicit Register will only increase the line count.

Another criticism of manual composition is that every time you change something, you'll need to edit the composition code. That's true, but this is equally as true for Explicit Register. The difference is that with manual composition, you learn about this at compile-time, while with Explicit Register, you don't learn about changes until run-time. This, in isolation, is a clear win for manual composition.

Now, if you move to Convention over Configuration, this particular criticism of using a DI Container disappears again, but I never claimed anything else.

2014-06-07 7:15 UTC

Captive Dependency

Monday, 02 June 2014 13:01:00 UTC

A Captive Dependency is a dependency with an incorrectly configured lifetime. It's a typical and dangerous DI Container configuration error.

This post is the sixth in a series about Poka-yoke Design.

When you use a Dependency Injection (DI) Container, you should configure it according to the Register Resolve Release pattern. One aspect of configuration is to manage the lifetime of various services. If you're not careful, though, you may misconfigure lifetimes in such a way that a longer-lived service holds a shorter-lived service captive - often with subtle, but disastrous results. You could call this misconfiguration a Captive Dependency.

A major step in applying DI is to compose object graphs, and service lifetimes in object graphs are hierarchical:

Hierarchical lifetime nature of object graphs

This figure illustrates the configured and effective lifetimes of an object graph. Node A1 should have a Transient lifetime, which is certainly possible. A new instance of C1 should be created Per Request (if the object graph is part of a web application), which is also possible, because A1 has a shorter lifetime than Per Request. Similarly, only a single instance of B3 should ever be created, which is also possible, because the various instances of C1 can reuse the same B3 instance.

The A2 node also has a Singleton lifetime, which means that only a single instance should exist of this object. Because A2 holds references to B1 and A3, these two object are also effectively Singletons. It doesn't matter how you'd like the lifetimes of B1 and A3 to be: the fact is that the single instance of A2 holds on to its injected instances of B1 and A3 means that these instances are going to stick around as long as A2. This effect is transitive, so A2 also causes B2 to have an effective Singleton lifetime.

This can be problematic if, for example, B1, A3, or B2 aren't thread-safe.

Commerce example #

This may make more sense if you see this in a more concrete setting than just an object graph with A1, A2, B1, etc. nodes, so consider the introductory example from my book. It has a ProductService, which depends on an IProductRepository interface (actually, in the book, the Repository is an Abstract Base Class):

public class ProductService
    private readonly IProductRepository repository;
    public ProductService(IProductRepository repository)
        this.repository = repository;
    // Other members go here...

One implementation of IProductRepository is SqlProductRepository, which itself depends on an Entity Framework context:

public class SqlProductRepository : IProductRepository
    private readonly CommerceContext context;
    public SqlProductRepository(CommerceContext context)
        this.context = context;
    // IProductRepository members go here...

The CommerceContext class derives from the Entity Framework DbContext class, which, last time I looked, isn't thread-safe. Thus, when used in a web application, it's very important to create a new instance of the CommerceContext class for every request, because otherwise you may experience errors. What's worse is that these errors will be threading errors, so you'll not discover them when you test your web application on your development machine, but when in production, you'll have multiple concurrent requests, and then the application will crash (or perhaps 'just' lose data, which is even worse).

(As a side note I should point out that I've used neither Entity Framework nor the Repository pattern for years now, but the example explains the problem well, in a context familiar to most people.)

The ProductService class is a stateless service, and therefore thread-safe, so it's an excellent candidate for the Singleton lifestyle. However, as it turns out, that's not going to work.

NInject example #

If you want to configure ProductService and its dependencies using Ninject, you might accidentally do something like this:

var container = new StandardKernel();

With Ninject you don't need to register concrete types, so there's no reason to register the CommerceContext class; it wouldn't be necessary to register the ProductService either, if it wasn't for the fact that you'd like it to have the Singleton lifestyle. Ninject's default lifestyle is Transient, so that's the lifestyle of both SqlProductRepository and CommerceContext.

As you've probably already predicted, the Singleton lifestyle of ProductService captures both the direct dependency IProductRepository, and the indirect dependency CommerceContext:

var actual1 = container.Get<ProductService>();
var actual2 = container.Get<ProductService>();
// You'd want this assertion to pass, but it fails
Assert.NotEqual(actual1.Repository, actual2.Repository);

The repositories are the same because actual1 and actual2 are the same instance, so naturally, their constituent components are also the same.

This is problematic because CommerceContext (deriving from DbContext) isn't thread-safe, so if you resolve ProductService from multiple concurrent requests (which you could easily do in a web application), you'll have a problem.

The immediate fix is to make this entire sub-graph Transient:

var container = new StandardKernel();

Actually, since Transient is the default, stating the lifetime is redundant, and can be omitted:

var container = new StandardKernel();

Finally, since you don't have to register concrete types with Ninject, you can completely omit the ProductService registration:

var container = new StandardKernel();

This works:

var actual1 = container.Get<ProductService>();
var actual2 = container.Get<ProductService>();
Assert.NotEqual(actual1.Repository, actual2.Repository);

While the Captive Dependency error is intrinsically tied to using a DI Container, it's by no means particular to Ninject.

Autofac example #

It would be unfair to leave you with the impression that this problem is a problem with Ninject; it's not. All DI Containers I know of have this problem. Autofac is just another example.

Again, you'd like ProductService to have the Singleton lifestyle, because it's thread-safe, and it would be more efficient that way:

var builder = new ContainerBuilder();
var container = builder.Build();

Like Ninject, the default lifestyle for Autofac is Transient, so you don't have to explicitly configure the lifetimes of SqlProductRepository or CommerceContext. On the other hand, Autofac requires you to register all services in use, even when they're concrete classes; this is the reason you see a registration statement for CommerceContext as well.

The problem is exactly the same as with Ninject:

var actual1 = container.Resolve<ProductService>();
var actual2 = container.Resolve<ProductService>();
// You'd want this assertion to pass, but it fails
Assert.NotEqual(actual1.Repository, actual2.Repository);

The reason is the same as before, as is the solution:

var builder = new ContainerBuilder();
var container = builder.Build();
var actual1 = container.Resolve<ProductService>();
var actual2 = container.Resolve<ProductService>();
Assert.NotEqual(actual1.Repository, actual2.Repository);

Notice that, because the default lifetime is Transient, you don't have to state it while registering any of the services.

Concluding remarks #

You can re-create this problem with any major DI Container. The problem isn't associated with any particular DI Container, but simply the fact that there are trade-offs associated with using a DI Container, and one of the trade-offs is a reduction in compile-time feedback. The way typical DI Container registration APIs work, they can't easily detect this lifetime configuration mismatch.

It's been a while since I last did a full survey of the .NET DI Container landscape, and back then (when I wrote my book), no containers could detect this problem. Since then, I believe Castle Windsor has got some Captive Dependency detection built in, but I admit that I'm not up to speed; other containers may have this feature as well.

When I wrote my book some years ago, I considered including a description of the Captive Dependency configuration error, but for various reasons, it never made it into the book:

  • As far as I recall, it was Krzysztof Koźmic who originally made me aware of this problem. In emails, we debated various ideas for a name, but we couldn't really settle on something catchy. Since I don't like to describe something I can't name, it never really made it into the book.
  • One of the major goals of the book was to explain DI as a set of principles and patterns decoupled from DI Containers. The Captive Dependency problem is specifically associated with DI Containers, so it didn't really fit into the book.
Since then, I've thought of the name Captive Dependency, which may not be super-catchy, but at least accurately describes the problem. A longer-lived object (e.g. a Singleton) holds a shorter-lived object captive, past its due release time. Although the shorter-lived object should be released, it's not, because of a bureaucratic error.

In a follow-up post to this, I'll demonstrate why you don't have the same problem when you hand-code your object graphs.


Simple Injector has built in support for a number of container verifications including lifestyle mismatches (Captive Dependency is a lifestyle mismatch) through its Diagnostic Services.

The configuration for Simple Injector looks like this:

var container = new Container();

The crucial difference with Simple Injector is that once you have finished configuring the container you make a call to the Verify() method to catch misconfigurations such as Captive Dependency.

Here's an example test to demonstrate that the container correctly identifies the lifestyle mismatch:

var results = Analyzer.Analyze(container);
Assert.That(results[0].Description, Is.StringContaining("CaptiveDependency"));
2014-06-02 20:07 UTC
bitbonk #

And for completeness we should also mention how to solve the captive dependency problem. From the really awsome SimpleInjector documentation:

  • Change the lifestyle of the component to a lifestyle that is as short or shorter than that of the dependency.
  • Change the lifestyle of the dependency to a lifestyle as long or longer than that of the component.
  • Instead of injecting the dependency, inject a factory for the creation of that dependency and call that factory every time an instance is required.

For the above example you would probably want to introduce a factory for the DbContexts.

2017-02-28 08:30 UTC

Feedback on ASP.NET vNext Dependency Injection

Monday, 26 May 2014 20:26:00 UTC

ASP.NET vNext includes a Dependency Injection API. This post offers feedback on the currently available code.

As part of Microsoft's new openness, the ASP.NET team have made the next version of ASP.NET available on GitHub. Obviously, it's not yet done, but I suppose that the reasons for this move is to get early feedback, as well as perhaps take contributions. This is an extremely positive move for the ASP.NET team, and I'm very grateful that they have done this, because it enables me to provide early feedback, and offer my help.

It looks like one of the proposed new features of the next version of ASP.NET is a library or API simply titled Dependency Injection. In this post, I will provide feedback to the team on that particular sub-project, in the form of an open blog post. The contents of this blog post is also cross-posted to the official ASP.NET vNext forum.

Dependency Injection support #

The details on the motivation for the Dependency Injection library are sparse, but I assume that the purpose is provide 'Dependency Injection support' to ASP.NET. If so, that motivation is laudable, because Dependency Injection (DI) is the proper way to write loosely coupled code when using Object-Oriented Design.

Some parts of the ASP.NET family already have DI support; personally, I'm most familiar with ASP.NET MVC and ASP.NET Web API. Other parts have proven rather hostile towards DI - most notably ASP.NET Web Forms. The problem with Web Forms is that Constructor Injection is impossible, because the Web Forms framework doesn't provide a hook for creating new Page objects.

My interpretation #

As far as I can tell, the current ASP.NET Dependency Injection code defines an interface for creating objects:

public interface ITypeActivator
    object CreateInstance(
        IServiceProvider services,
        Type instanceType,
        params object[] parameters);

In addition to this central interface, there are other interfaces that enable you to configure a 'service collection', and then there are Adapters for

  • Autofac
  • Ninject
  • StructureMap
  • Unity
  • Caste Windsor
As far as I can tell, there's no web code in the ASP.NET Dependency Injection code base. In other words, this is a poster example of a Conforming Container.

My recommendations #

It's an excellent idea to add 'Dependency Injection support' to ASP.NET, for the few places where it's not already present. However, as I've previously explained, a Conforming Container isn't the right solution. The right solution is to put the necessary extensibility points into the framework:

  • ASP.NET MVC already has a good extensibility point in the IControllerFactory interface. I recommend keeping this interface, and other interfaces in MVC that play a similar role.
  • ASP.NET Web API already has a good extensibility point in the IHttpControllerActivator interface. I recommend keeping this interface, and other interfaces in Web API that play a similar role.
  • ASP.NET Web Forms have no extensibility point that enables you to create custom Page objects. I recommend adding an IPageFactory interface, as described in my article about DI-Friendly frameworks. Other object types related to Web Forms, such as Object Data Sources, suffer from the same shortcoming, and should have similar factory interfaces.
  • There may be other parts of ASP.NET with which I'm not particularly familiar (SignalR?), but they should all follow the same pattern of defining Abstract Factories for user classes, in the cases where these don't already exist.
In addition to adding these required extensibility points, I recommend completely abandoning the project of defining a Conforming Container. The extensibility points should be added where they're used - the MVC Factories as part of MVC, the Web Form Factories as part of Web Forms, etc. This will have the added benefit of making the ASP.NET Dependency Injection project redundant. Less code is better than more code.
"perfection is attained not when there is nothing more to add, but when there is nothing more to remove." - Antoine de Saint Exupéry
These are my general recommendations to the team, but if desired, I'd also like to offer my assistance with any particular issues the team might encounter.

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