DI-Friendly Library

Monday, 19 May 2014 08:20:00 UTC

How to create a Dependency Injection-friendly software library.

In my book, I go to great lengths to explain how to develop loosely coupled applications using various Dependency Injection (DI) patterns, including the Composition Root pattern. With the great emphasis on applications, I didn't particularly go into details about making DI-friendly libraries. Partly this was because I didn't think it was necessary, but since one of my highest voted Stack Overflow answers deal with this question, it may be worth expanding on.

In this article, I will cover libraries, and in a later article I will deal with frameworks. The distinction I usually make is:

  • A Library is a reusable set of types or functions you can use from a wide variety of applications. The application code initiates communication with the library and invokes it.
  • A Framework consists of one or more libraries, but the difference is that Inversion of Control applies. The application registers with the framework (often by implementing one or more interfaces), and the framework calls into the application, which may call back into the framework. A framework often exists to address a particular general-purpose Domain (such as web applications, mobile apps, workflows, etc.).

Most well-designed libraries are already DI-friendly - particularly if they follow the SOLID principles, because the Dependency Inversion Principle (the D in SOLID) is the guiding principle behind DI.

Still, it may be valuable to distil a few recommendations.

Program to an interface, not an implementation #

If your library consists of several collaborating classes, define proper interfaces between these collaborators. This enables clients to redefine part of your library's behaviour, or to slide cross-cutting concerns in between two collaborators, using a Decorator.

Be sure to define these interfaces as Role Interfaces.

An example of a small library that follows this principle is Hyprlinkr, which defines two interfaces used by the main RouteLinker class:

public interface IRouteValuesQuery
    IDictionary<stringobject> GetRouteValues(
        MethodCallExpression methodCallExpression);


public interface IRouteDispatcher
    Rouple Dispatch(
        MethodCallExpression method,
        IDictionary<stringobject> routeValues);

This not only makes it easier to develop and maintain the library itself, but also makes it more flexible for users.

Use Constructor Injection #

Favour the Constructor Injection pattern over other injection patterns, because of its simplicity and degree of encapsulation.

As an example, Hyprlinkr's main class, RouteLinker, has this primary constructor:

private readonly HttpRequestMessage request;
private readonly IRouteValuesQuery valuesQuery;
private readonly IRouteDispatcher dispatcher;
public RouteLinker(
    HttpRequestMessage request,
    IRouteValuesQuery routeValuesQuery,
    IRouteDispatcher dispatcher)
    if (request == null)
        throw new ArgumentNullException("request");
    if (routeValuesQuery == null)
        throw new ArgumentNullException("routeValuesQuery");
    if (dispatcher == null)
        throw new ArgumentNullException("dispatcher");
    this.request = request;
    this.valuesQuery = routeValuesQuery;
    this.dispatcher = dispatcher;

Notice that it follows Nikola Malovic's 4th law of IoC that Injection Constructors should be simple.

Although not strictly required in order to make a library DI-friendly, expose every injected dependency as an Inspection Property - it will make the library easier to use when composed in one place, but used in another place. Again, Hyprlinkr does that:

public IRouteValuesQuery RouteValuesQuery
    get { return this.valuesQuery; }

and so on for its other dependencies, too.

Consider an Abstract Factory for short-lived objects #

Sometimes, your library must create short-lived objects in order to do its work. Other times, the library can only create a required object at run-time, because only at run-time is all required information available. You can use an Abstract Factory for that.

The Abstract Factory doesn't always have to be named XyzFactory; in fact, Hyprlinkr's IRouteDispatcher interface is an Abstract Factory, although it's in disguise because it has a different name.

public interface IRouteDispatcher
    Rouple Dispatch(
        MethodCallExpression method,
        IDictionary<stringobject> routeValues);

Notice that the return value of an Abstract Factory doesn't have to be another interface instance; in this case, it's an instance of the concrete class Rouple, which is a data structure without behaviour.

Consider a Facade #

If some objects are difficult to construct, because their classes have complex constructors, consider supplying a Facade with a good default combination of appropriate dependencies. Often, a simple alternative to a Facade is Constructor Chaining:

public RouteLinker(HttpRequestMessage request)
    : this(request, new DefaultRouteDispatcher())
public RouteLinker(HttpRequestMessage request, IRouteValuesQuery routeValuesQuery)
    : this(request, routeValuesQuery, new DefaultRouteDispatcher())
public RouteLinker(HttpRequestMessage request, IRouteDispatcher dispatcher)
    : this(request, new ScalarRouteValuesQuery(), dispatcher)
public RouteLinker(
    HttpRequestMessage request,
    IRouteValuesQuery routeValuesQuery,
    IRouteDispatcher dispatcher)
    if (request == null)
        throw new ArgumentNullException("request");
    if (routeValuesQuery == null)
        throw new ArgumentNullException("routeValuesQuery");
    if (dispatcher == null)
        throw new ArgumentNullException("dispatcher");
    this.request = request;
    this.valuesQuery = routeValuesQuery;
    this.dispatcher = dispatcher;

Notice how the Routelinker class provides appropriate default values for those dependencies it can.

However, a Library with a more complicated API could potentially benefit from a proper Facade. One way to make the API's extensibility points discoverable is by implementing the Facade as a Fluent Builder. The following RouteLinkerBuilder isn't part of Hyprlinkr, because I consider the Constructor Chaining alternative simpler, but it could look like this:

public class RouteLinkerBuilder
    private readonly IRouteValuesQuery valuesQuery;
    private readonly IRouteDispatcher dispatcher;
    public RouteLinkerBuilder()
        : this(new ScalarRouteValuesQuery(), new DefaultRouteDispatcher())
    private RouteLinkerBuilder(
        IRouteValuesQuery valuesQuery,
        IRouteDispatcher dispatcher)
        this.valuesQuery = valuesQuery;
        this.dispatcher = dispatcher;
    public RouteLinkerBuilder WithValuesQuery(IRouteValuesQuery newValuesQuery)
        return new RouteLinkerBuilder(newValuesQuery, this.dispatcher);
    public RouteLinkerBuilder WithDispatcher(IRouteDispatcher newDispatcher)
        return new RouteLinkerBuilder(this.valuesQuery, newDispatcher);
    public RouteLinker Create(HttpRequestMessage request)
        return new RouteLinker(request, this.valuesQuery, this.dispatcher);
    public IRouteValuesQuery ValuesQuery
        get { return this.valuesQuery; }
    public IRouteDispatcher Dispatcher
        get { return this.dispatcher; }

This has the advantage that it's easy to get started with the library:

var linker = new RouteLinkerBuilder().Create(request);

This API is also discoverable, because Intellisense helps users discover how to deviate from the default values:

Intellisense and Fluent Builder combined enhances discoverability

It enables users to override only those values they care about:

var linker =
    new RouteLinkerBuilder().WithDispatcher(customDispatcher).Create(request);

If I had wanted to force users of Hyprlinkr to use the (hypothetical) RouteLinkerBuilder, I could make the RouteLinker constructor internal, but I don't much care for that option; I prefer to empower my users, not constrain them.

Composition #

Any application that uses your library can compose objects from it in its Composition Root. Here's a hand-coded example from one of Grean's code bases:

private static RouteLinker CreateDefaultRouteLinker(HttpRequestMessage request)
    return new RouteLinker(
        new ModelFilterRouteDispatcher(
            new DefaultRouteDispatcher()

This example is just a small helper method in the Composition Root, but as you can see, it composes a RouteLinker instance using our custom ModelFilterRouteDispatcher class as a Decorator for Hyprlinkr's built-in DefaultRouteDispatcher.

However, it would also be easy to configure a DI Container to do this instead.

Summary #

If you follow SOLID, and normal rules for encapsulation, your library is likely to be DI-friendly. No special infrastructure is required to add 'DI support' to a library.


Maris Krivtezs #

I found great library for in process messaging made by Jimmy Bogard - MediatR, but it uses service locator. Implemented mediator uses service locator to lookup for handlers matching message type registered in container. Source.

What would be best approach to eliminate service locator in this case? Would it be better to pass all handler instances in mediator constructor and then lookup for matching one?

2014-06-02 20:10 UTC

Maris, thank you for writing. Hopefully, this article answers your question.

2014-06-03 9:20 UTC

Conforming Container

Monday, 19 May 2014 07:54:00 UTC

A Dependency Injection anti-pattern.

Once in a while, someone comes up with the idea that it would be great to introduce a common abstraction over various DI Containers in .NET. My guess is that part of the reason for this is that there are so many DI Containers to choose from on .NET:

... and these are just the major ones; there are many more! Hiding all these different libraries behind a common interface sounds like a smashing idea, but isn't.

General form #

At its core, a Conforming Container introduces a central interface, often called IContainer, IServiceLocator, IServiceProvider, ITypeActivator, IServiceFactory, or something in that vein. The interface defines one or more methods called Resolve, Create, GetInstance, or similar:

public interface IContainer
    object Resolve(Type type);
    object Resolve(Type type, params object[] arguments);
    T Resolve<T>();
    T Resolve<T>(params object[] arguments);
    IEnumerable<T> ResolveAll<T>();
    // etc.

Sometimes, the interface defines only a single of those methods; sometimes, it defines even more variations of methods to create objects based on a Type.

Some Conforming Containers stop at this point, so that the interface only exposes Queries, which means that they only cover the Resolve phase of the Register Resolve Release pattern. Other efforts attempt to address Register phase too:

public interface IContainer
    void AddService(Type serviceType, Type implementationType);
    void AddService<TService, TImplementation>();
    // etc.

The intent is to enable configuration of the container using some sort of metadata. Sometimes, the methods have more advanced configuration parameters that also enable you to specify the lifestyle of the service, etc.

Finally, a part of a typical Conforming Container ecosystem is various published Adapters to concrete DI Containers. A hypothetical Confainer project may publish the following Adapter packages:

  • Confainer.Autofac
  • Confainer.Windsor
  • Confainer.Ninject
  • Confainer.Unity
Notice that in this example, not all major .NET DI Containers are listed. This is a typical situation. Obviously, since the entire effort is to define an interface, contributors are often invited to provide Adapters for missing DI Containers.

Symptoms and consequences #

A Conforming Container is an anti-pattern, because it's

a commonly occurring solution to a problem that generates decidedly negative consequences,
such as:
  • Calls to the Conforming Container are likely to be sprinkled liberally over an entire code base.
  • It pushes novice users towards the Service Locator anti-pattern. Most people encountering Dependency Injection for the first time mistake it for the Service Locator anti-pattern, despite the entirely opposite natures of these two approaches to loose coupling.
  • It attempts to relieve symptoms of bad design, instead of addressing the underlying problem. Too many 'loosely coupled' designs attempt to rely on the Service Locator anti-pattern, which, by default, introduces a dependency to a concrete Service Locator throughout a code base. However, exclusively using the Constructor Injection and Composition Root design patterns eliminate the problem altogether, resulting in a simpler design with fewer moving parts.
  • It pulls in the direction of the lowest common denominator.
  • It stifles innovation, because new, creative, but radical ideas may not fit into the narrow view of the world a Conforming Container defines.
  • It makes it more difficult to avoid using a DI Container. A DI Container can be useful in certain scenarios, but often, hand-coded composition is better than using a DI Container. However, if a library or framework depends on a Conforming Container, it may be difficult to harvest the benefits of hand-coded composition.
  • It may introduce versioning hell. Imagine that you need to use a library that depends on Confainer 1.3.7 in an application that also uses a framework that depends on Confainer 2.1.7. Since a Conforming Container is intended as an infrastructure component, this is likely to happen, and to cause much grief.
  • A Conforming Container is often a product of Speculative Generality, instead of a product of need. As such, the API is likely to be poorly suited to address real-world scenarios, be difficult to extent, and may exhibit churn in the form of frequent breaking changes.
  • If Adapters are supplied by contributors (often the DI Container maintainers themselves), the Adapters may have varying quality levels, and may not support the latest version of the Conforming Container.

A code base using a Conforming Container may have code like this all over the place:

var foo = container.Resolve<IFoo>();
// ... use foo for something...
var bar = container.Resolve<IBar>();
// ... use bar for something else...
var baz = container.Resolve<IBaz>();
// ... use baz for something else again...

This breaks encapsulation, because it's impossible to identify a class' collaborators without reading its entire code base.

Additionally, concrete DI Containers have distinct feature sets. Although likely to be out of date by now, this feature comparison chart from my book illustrate this point:

Castle WindsorStructureMapSpring.NETAutofacUnityMEF
Code as Configuration x x x x
Auto-registration x x x
XML configuration x x x x x
Modular configuration x x x x x x
Custom lifetimes x x (x) x
Decommissioning x x (x) x
Interception x x x

This is only a simple chart that plots the most common features of DI Containers. Each DI Container has dozens of features - many of them unique to that particular DI Container. A Conforming Container can either support an intersection or union of all those features.

Intersection and union of containers

A Conforming Container that targets only the intersection of all features will be able to support only a small fraction of all available features, diminishing the value of the Conforming Container to the point where it becomes gratuitous.

A Conforming Container that targets the union of all features is guaranteed to consist mostly of a multitude of NotImlementedExceptions, or, put in another way, massively breaking the Liskov Substitution Principle.

Typical causes #

The typical causes of the Conforming Container anti-pattern are:

  • Lack of understanding of Dependency Injection. Dependency Injection is a set of patterns driven by the Dependency Inversion Principle. A DI Container is an optional library, not a required part.
  • A fear of letting an entire code base depend on a concrete DI Container, if that container turns out to be a poor choice. Few programmers have thouroughly surveyed all available DI Containers before picking one for a project, so architects desire to have the ability to replace e.g. StructureMap with Ninject.
  • Library designers mistakenly thinking that Dependency Injection support involves defining a Conforming Container.
  • Framework designers mistakenly thinking that Dependency Injection support involves defining a Conforming Container.
The root cause is always a lack of awareness of a simpler solution.

Known exceptions #

There are no cases known to me where a Conforming Container is a good solution to the problem at hand. There's always a better and simpler solution.

Refactored solution #

Instead of relying on the Service Locator anti-pattern, all collaborating classes should rely on the Constructor Injection pattern:

public class CorrectClient
    private readonly IFoo foo;
    private readonly IBar bar;
    private readonly IBaz baz;
    public CorrectClient(IFoo foo, IBar bar, IBaz baz)
        this.foo = foo;
        this.bar = bar;
        this.baz = baz;
    public void DoSomething()
        // ... use this.foo for something...
        // ... use this.bar for something else...
        // ... use this.baz for something else again...

This leaves all options open for any code consuming the CorrectClient class. The only exception to relying on Constructor Injection is when you need to compose all these collaborating classes. The Composition Root has the single responsibility of composing all the objects into a working object graph:

public class CompositionRoot
    public CorrectClient ComposeClient()
        return new CorrectClient(
            new RealFoo(),
            new RealBar(),
            new RealBaz());

In this example, the final graph is rather shallow, but it can be as complex and deep as necessary. This Composition Root uses hand-coded composition, but if you want to use a DI Container, the Composition Root is where you put it:

public class WindsorCompositionRoot
    private readonly WindsorContainer container;
    public WindsorCompositionRoot()
        this.container = new WindsorContainer();
        // Configure the container here,
        // or better yet: use a WindsorInstaller
    public CorrectClient ComposeClient()
        return this.container.Resolve<CorrectClient>();

This class (and perhaps a few auxiliary classes, such as a Windsor Installer) is the only class that uses a concrete DI Container. This is the Hollywood Principle in action. There's no reason to hide the DI Container behind an interface, because it has no clients. The DI Containers knows about the application; the application knows nothing about the DI Container.

In all but the most trivial of applications, the Composition Root is only an extremely small part of the entire application.

A Composition Root is only a small part of an application

(The above picture is meant to illustrate an arbitrary application architecture; it could be layered, onion, hexagonal, or something else - it doesn't really matter.) If you want to replace one DI Container with another DI Container, you only replace the Composition Root; the rest of the application will never notice the difference.

Notice that only applications should have Composition Roots. Libraries and frameworks should not.

  • Library classes should be defined with Constructor Injection throughout. If the library object model is very complex, a few Facades can be supplied to make it easier for library users to get started. See my article on DI-friendly libraries for more details.
  • Frameworks should have appropriate hooks built in. These hooks should not be designed as Service Locators, but rather as Abstract Factories. See my article on DI-friendly frameworks for more details.
These solutions are better than a Conforming Container because they are simpler, have fewer moving parts, are easier to understand, and easier to reason about.

Variations #

Sometimes the Conforming Container only defines a Service Locator-like API, and sometimes it also defines a configuration API. That configuration API may include various axes of configurability, most notably lifetime management and decommisioning.

Decommissioning is often designed around the concept of a disposable 'context' scope, but as I explain in my book, that's not an extensible pattern.

Known examples #

There are various known examples of Conforming Containers for .NET:

Additionally, it looks like the new Dependency Injection support for ASP.NET is taking this route as well, although hopefully, it's not too late to change that.

Configuring Azure Web Jobs

Friday, 16 May 2014 17:58:00 UTC

It's easy to configure Azure Web Jobs written in .NET.

Azure Web Jobs is a nice feature for Azure Web Sites, because it enables you to bundle a background worker, scheduled batch job, etc. together with your Web Site. It turns out that this feature works pretty well, but it's not particularly well-documented, so I wanted to share a few nice features I've discovered while using them.

You can write a Web Job as a simple Command Line executable, but if you can supply command-line arguments to it, I have yet to discover how to do that. A good alternative is an app.config file with configuration settings, but it can be a hassle to deal with various configuration settings across different deployment environments. There's a simple solution to that.

CloudConfigurationManager #

If you use CloudConfigurationManager.GetSetting, configuration settings are read using various fallback mechanisms. The CloudConfigurationManager class is poorly documented, and I couldn't find documentation for the current version, but one documentation page about a deprecated version sums it up well enough:

"The GetSetting method reads the configuration setting value from the appropriate configuration store. If the application is running as a .NET Web application, the GetSetting method will return the setting value from the Web.config or app.config file. If the application is running in Windows Azure Cloud Service or in a Windows Azure Website, the GetSetting will return the setting value from the ServiceConfiguration.cscfg."
That is probably still true, but I've found that it actually does more than that. As far as I can tell, it attempts to read configuration settings in this prioritized order:
  1. Try to find the configuration value in the Web Site's online configuration (see below).
  2. Try to find the configuration value in the .cscfg file.
  3. Try to find the configuration value in the app.config file or web.config file.
By the Web Site's online configuration, I mean the configuration settings you can edit on a deployment-by-deployment basis using the Azure management portal, under the Configure 'tab':

Online configuration for an Azure Web Site

(It's possible that, under the hood, this UI actually maintains an auto-generated .cscfg file, in which case the first two bullet points above turn out to be one and the same.)

This is a really nice feature, because it means that you can push your deployments directly from your source control system (I use Git), and leave your configuration files empty in source control:

  <add key="timeout" value="0:01:00" />
  <add key="estimatedDuration" value="0:00:02" />
  <add key="toleranceFactor" value="2" />
  <add key="idleTime" value="0:00:05" />
  <add key="storageMode" value="files" />
  <add key="storageConnectionString" value="" />
  <add key="raygunKey" value="" />

Instead of having to figure out how to manage or merge those super-secret keys in the build system, you can simply shortcut the whole issue by not involving those keys in your build system; they're only stored in Azure - where you can't avoid having them anyway, because your system needs them in order to work.

Usage #

It's easy to use CloudConfigurationManager: instead of getting your configuration values with ConfigurationManager.AppSettings, you use CloudConfigurationManager.GetSetting:

let clientId = CloudConfigurationManager.GetSetting "clientId"

The CloudConfigurationManager class isn't part of the .NET Base Class Library, but you can easily add it from NuGet; it's called Microsoft.WindowsAzure.ConfigurationManager. The Azure SDK isn't required - it's just a stand-alone library with no dependencies, so I happily add it to my Composition Root when I know I'm going to deploy to an Azure Web Site.

Web Jobs #

Although I haven't found any documentation to that effect yet, a .NET console application running as an Azure Web Job will pick up configuration settings in the way described above. On other words, it shares configuration values with the web site that it's part of. That's darn useful.


Howard Camp #
Any secrets that I have must be in the app.config in order for me to run locally during development, correct? How do I keep them out of the build system?
2017-12-05 02:29 UTC

Howard, thank you for writing. You should always keep your secrets out of source control. In some projects, I've used web.config transforms for that purpose. Leave your checked-in .config files empty, and have local (not checked-in) .config files on development machines, production servers, and so on.

As far as I know, on most other platforms, people simply use environment variables instead of configuration files. To me, that sounds like a simple solution to the problem.

2017-12-05 08:16 UTC

Service Locator violates SOLID

Thursday, 15 May 2014 18:51:00 UTC

Yet another reason to avoid the Service Locator anti-pattern is that it violates the principles of Object-Oriented Design.

Years ago, I wrote an article about Service Locator. Regular readers of this blog may already know that I consider Service Locator an anti-pattern. That hasn't changed, but I recently realized that there's another way to explain why Service Locator is the inverse of good Object-Oriented Design (OOD). My original article didn't include that perspective at all, so perhaps this is a clearer way of explaining it.

In this article, I'll assume that you're familiar with the SOLID principles (also known as the Principles of OOD), and that you accept them as generally correct. It's not because I wish to argue by an appeal to authority, but rather because threre's already a large body of work that explains why these principles are beneficial to software design.

In short, Service Locator violates SOLID because it violates the Interface Segregation Principle (ISP). That's because a Service Locator effectively has infinitely many members.

Service Locator deconstructed #

In order to understand why a Service Locator has infinitely many members, you'll need to understand what a Service Locator is. Often, it's a class or interface with various members, but it all boils down to a single member:

T Create<T>();

Sometimes the method takes one or more parameters, but that doesn't change the conclusion, so I'm leaving out those input parameters to keep things simple.

A common variation is the untyped, non-generic variation:

object Create(Type type);

Since my overall argument relies on the generic version, first I'll need to show you why those two methods are equivalent. If you imagine that all you have is the non-generic version, you can easily write a generic extension method for it:

public static class ServiceLocatorEnvy
    public static T Create<T>(this IServiceLocator serviceLocator)
        return (T)serviceLocator.Create(typeof(T));

As you can see, this extension method has exactly the same signature as the generic version; you can always create a generic Service Locator based on a non-generic Service Locator. Thus, while my main argument (coming up next) is based on a generic Service Locator, it also applies to non-generic Service Locators.

Infinite methods #

From a client's perspective, there's no limit to how many variations of the Create method it can invoke:

var foo = serviceLocator.Create<IFoo>();
var bar = serviceLocator.Create<IBar>();
var baz = serviceLocator.Create<IBaz>();
// etc.

Again, from the client's perspective, that's equivalent to multiple method definitions like:

IFoo CreateFoo();
IBar CreateBar();
IBaz CreateBaz();
// etc.

However, the client can keep coming up with new types to request, so effectively, the number of Create methods is infinite!

Relation to the Interface Segregation Principle #

By now, you understand that a Service Locator is an interface or class with effectively an infinite number of methods. That violates the ISP, which states:

Clients should not be forced to depend on methods they do not use.
However, since a Service Locator exposes an infinite number of methods, any client using it is forced to depend on infinitely many methods it doesn't use.

Quod Erat Demonstrandum #

The Service Locator anti-pattern violates the ISP, and thus it also violates SOLID as a whole. SOLID is also known as the Principles of OOD. Therefore, Service Locator is bad Objected-Oriented Design.

Update 2015-10-26: The fundamental problem with Service Locator is that it violates encapsulation.


Nelson LaQuet #
Doesn't this argument also apply to any method that takes a generic parameter? Meaning any use of generic methods also violates ISP?
2014-05-15 21:06 UTC

Nelson, thank you for writing. First, it's important to realize that this overall argument applies to methods with 'free' generic type parameters; that is, a method where the type in itself isn't generic, but the method is. One example of the difference I mean is that a generic Abstract Factory is benign, whereas a Server Locator isn't.

Second, that still leaves the case where you may have a generic parameter that determines the return type of the method. The LINQ Select method is an example of such a method. These tend not to be problematic, but I had some trouble explaining why that is until James Jensen explained it to me.

2014-05-16 11:33 UTC

Dear Mark,

I totally agree with your advise on Service Locator being an anti-pattern, specifically if it is used within the core logic. However, I don't think that your argumentation in this post is correct. I think that you apply Object-Oriented Programming Principles to Metaprogramming, which should not be done, but I'm not quite sure if my argument is completely reasonable.

All .NET DI Containers that I know of use the Reflection API to solve the problem of dynamically composing an object graph. The very essence of this API is it's ability to inspect and call members of any .NET type, even the ones that the code was not compiled against. Thus you do not use Strong Object-Oriented Typing any longer, but access the members of a e.g. a class indirectly using a model that relies on the Type class and its associated types. And this is the gist of it: code is treated as data, this is Metaprogramming, as we all know.

Without these capabilities, DI containers wouldn't be able to do their job because they couldn't e.g. analyze the arguments of a class's constructor to further instantiate other objects needed. Thus we can say that DI containers are just an abstraction over the Metaprogramming API of .NET. And of course, these containers offer an API that is shaped by their Metaprogramming foundation. This can be seen in your post: although you discuss the generic variation T Create<T>(), this is just syntactic sugar for the actual important method: object Create(Type type).

Metaprogramming in C# is totally resolved at runtime, and therefore one shouldn't apply the Interface Segregation Principle to APIs that are formed by it. These are designed to help you improve the Object-Oriented APIs which particularly incorporate Static Typing enforced by the compiler. A DI container does not have an unlimited number of Create methods, it has a single one and it receives a Type argument - the generic version just creates the Type object for you. And the parameter has to be as "weak" as Type, because we cannot use Static Typing - this technically allows the client to pass in types that the container is not configured for, but you cannot prevent this using the compiler because of the dynamic nature of the Reflection API.

What is your opinion on that?

2016-01-06 21:55 UTC

Kenny, thank you for writing. The point that this post is making is mainly that Service Locator violates the Interface Segregation Principle (ISP). The appropriate perspective on ISP (and LSP and DIP as well) is from a client. The client of a Service Locator effectively sees an API with infinitely many methods. That's where the damage is done.

How the Service Locator is implemented isn't important to ISP, LSP, or DIP. (The SRP and OCP, on the other hand, relate to implementations.) You may notice that this article doesn't use the word container a single time.

2016-01-07 19:16 UTC

Dear Mark,

I get the point that you are talking from the client's perspective - but even so, a client programming against a Service Locator should be aware that it is programming against an abstraction of a Metaprogramming API (and not an Object-Oriented API). If you think about the call to the Create method, then you basically say "Give me an object graph with the specified type as the object graph root" as a client - how do you implement this with the possibilities that OOP provides? You can't model this with classes, interfaces, and the means of Procedural and Structural Programming that are integrated in OOP - because these techniques do not allow you to treat code as data.

And again, your argument is based on the generic version of the Create method, but that shouldn't be the point of focus. It is the non-generic version object Create (Type type) which clearly indicates that it is a Metaprogramming API because of the Type parameter and the object return type - Type is the entry point to .NET Reflection and object is the only type the Service Locator can guarantee as the object graph is dynamically resolved at runtime - no Strong Typing involved. The existence of the generic Create variation is merely justified because software developers are lazy - they don't want to manually downcast the returned object to the type they actually need. Well, one could argue that this comforts the Single Point of Truth / Don't repeat yourself principles, too, as all information to create the object graph and to downcast the root object are derived from the generic type argument, but that doesn't change the fact that Service Locator is a Metaprogramming API.

And that's why I solely used the term DI container throughout my previous comment, because Service Locator is just the part of the API of a DI container that is concerned with resolving object graphs (and the Metainformation to create these object graphs was registered beforehand). Sure, you can implement Service Locators as a hard-wired registry of Singleton objects (or even Factories that create objects on the fly) to circumvent the use of the Reflection API (although one probably had to use some sort of Type ID in this case, maybe in form of a string or GUID). But honestly, these are half-baked solutions that do not solve the problem in a reusable way. A reusable Service Locator must treat code as data, especially if you want additional features like lifetime management.

Another point: take for example the MembershipProvider class - this polymorphic abstraction is truly a violation of the Interface Segregation Principle because it offers way too many members that a client probably won't need. But notice that each of these members has a different meaning, which is not the case with the Create methods of the Service Locator. The generic Create method is just another abstraction over the non-generic version to simplify the access to the Service Locator.

Long story short: Service Locator is a Metaprogramming API, the SOLID principles target Object-Oriented APIs, thus the latter shouldn't be used to assess the former. There's is no real way to hide the fact that clients need to be aware that they are calling a Metaprogramming API if they directly reference a Service Locator (which shouldn't be done in core logic).

2016-01-08 09:40 UTC

Kenny, thank you for writing. While I don't agree with everything you wrote, your arguments are well made, and I have no problems following them. If we disagree, I think we disagree about semantics, because the way I read your comment, I think it eventually leads you to the same conclusions that I have arrived at.

Ultimately, you also state that Service Locator isn't an Object-Oriented Design, and in that, I entirely agree. The SOLID principles are also known as the principles of Object-Oriented Design, so when I'm stating that I think that Service Locator violates SOLID, my more general point is that Service Locator isn't Object-Oriented because it violates a fundamental principle of OOD. You seem to have arrived at the same conclusion, although via a different route. I always like when that happens, because it confirms that the conclusion may be true.

To be honest, though, I don't consider the arguments I put forth in the present article as my strongest ever. Sometimes, I write articles on topics that I've thought about for years, but I also often write articles that are half-baked ideas; I put these articles out in order to start a discussion, so I appreciate your comments.

I'm much happier with the article that argues that Service Locator violates Encapsulation.

2016-01-08 10:55 UTC

AutoFixture conventions with Albedo

Thursday, 01 May 2014 21:40:00 UTC

You can use Albedo with AutoFixture to build custom conventions.

In a question to one of my previous posts, Jeff Soper asks about using custom, string-based conventions for AutoFixture:

"I always wince when testing for the ParameterType.Name value [...] It seems like it makes a test that would use this implementation very brittle."
Jeff's concern is that when you're explicitly looking for a parameter (or property or field) with a particular name (like "currencyCode"), the unit test suite may become brittle, because if you change the parameter name, the string may retain the old name, and the Customization no longer works.

Jeff goes on to say:

"This makes me think that I shouldn't need to be doing this, and that a design refactoring of my SUT would be a better option."
His concerns can be addressed on several different levels, but in this post, I'll show you how you can leverage Albedo to address some of them.

If you often find yourself in a situation where you're writing an AutoFixture Customization based on string matching of parameters, properties or fields, you should ask yourself if you're targeting one specific class, or if you're writing a convention? If you often target individual specific classes, you probably need to rethink your strategy, but you can easily run into situations where you need to introduce true conventions in your code base. This can be beneficial, because it'll make your code more consistent.

Here's an example from the code base in which I'm currently working. It's a REST service written in F#. To model the JSON going in and out, I've defined some Data Transfer Records, and some of them contain dates. However, JSON doesn't deal particularly well with dates, so they're treated as strings. Here's a JSON representation of a comment:

    "author": {
        "id": "1234",
        "name": "Mark Seemann",
        "email": "1234@ploeh.dk"
    "createdDate": "2014-04-30T18:14:08.1051775+00:00",
    "text": "Is this a comment?"

The record is defined like this:

type CommentRendition = {
    Author : PersonRendition
    CreatedDate : string
    Text : string }

This is a problem for AutoFixture, because it sees CreatedDate as a string, and populates it with an anonymous string. However, much of the code base expects the CreatedDate to be a proper date and time value, which can be parsed into a DateTimeOffset value. This would cause many tests to fail if I didn't change the behaviour.

Instead of explicitly targeting the CreatedDate property on the CommentRendition record, I defined a conventions: any parameter, field, or property that ends with "date" and has the type string, should be populated with a valid string representation of a date and time.

This is easy to write as a one-off Customization, but then it turned out that I needed an almost similar Customization for IDs: any parameter, field, or property that ends with "id" and has the type string, should be populated with a valid GUID string formatted in a certain way.

Because ParameterInfo, PropertyInfo, and FieldInfo share little polymorphic behaviour, it's time to pull out Albedo, which was created for situations like this. Here's a reusable convention which can check any parameter, proeprty, or field for a given name suffix:

type TextEndsWithConvention(value, found) =
    inherit ReflectionVisitor<bool>()
    let proceed x =
        TextEndsWithConvention (value, x || found) :> IReflectionVisitor<bool>
    let isMatch t (name : string) =
        t = typeof<string>
        && name.EndsWith(value, StringComparison.OrdinalIgnoreCase)
    override this.Value = found
    override this.Visit (pie : ParameterInfoElement) =
        let pi = pie.ParameterInfo
        isMatch pi.ParameterType pi.Name |> proceed
    override this.Visit (pie : PropertyInfoElement) =
        let pi = pie.PropertyInfo
        isMatch pi.PropertyType pi.Name |> proceed
    override this.Visit (fie : FieldInfoElement) =
        let fi = fie.FieldInfo
        isMatch fi.FieldType fi.Name |> proceed
    static member Matches value request =
        let refraction =
                    ParameterInfoElementRefraction<obj>() :> IReflectionElementRefraction<obj>
                    PropertyInfoElementRefraction<obj>()  :> IReflectionElementRefraction<obj>
                    FieldInfoElementRefraction<obj>()     :> IReflectionElementRefraction<obj>
        let r = refraction.Refract [request]
        r.Accept(TextEndsWithConvention(value, false)).Value

It simply aggregates a boolean value (found), based on the name and type of various properties, fields, and parameters that comes its way. If there's a match, found will be true; otherwise, it'll be false.

The date convention is now trivial:

type DateStringCustomization() =
    let builder = {
        new ISpecimenBuilder with
            member this.Create(request, context) =
                if request |> TextEndsWithConvention.Matches "date"
                then box ((context.Resolve typeof<DateTimeOffset>).ToString())
                else NoSpecimen request |> box }
    interface ICustomization with
        member this.Customize fixture = fixture.Customizations.Add builder

The ID convention is very similar:

type IdStringCustomization() =
    let builder = {
        new ISpecimenBuilder with
            member this.Create(request, context) =
                if request |> TextEndsWithConvention.Matches "id"
                then box ((context.Resolve typeof<Guid> :?> Guid).ToString "N")
                else NoSpecimen request |> box }
    interface ICustomization with
        member this.Customize fixture = fixture.Customizations.Add builder

With these conventions in place in my entire test suite, I can simply follow them and get correct values. What happens if I refactor one of my fields so that they no longer have the correct suffix? That's likely to break my tests, but that's a good thing, because it alerts me that I deviated from the conventions, and (inadvertently, I should hope) made the production code less consistent.

Single Writer Web Jobs on Azure

Wednesday, 30 April 2014 06:39:00 UTC

How to ensure a Single Writer in load-balanced Azure deployments

In my Functional Architecture with F# Pluralsight course, I describe how using the Actor model (F# Agents) can make a concurrent system much simpler to implement, because the Agent can ensure that the system only has a Single Writer. Having a Single Writer eliminates much complexity, because while the writer decides what to write (if at all), nothing changes. Multiple readers can still read data, but as long as the Single Writer can keep up with input, this is a much simpler way to deal with concurrency than the alternatives.

However, the problem is that while F# Agents work well on a single machine, they don't (currently) scale. This is particularly notable on Azure, because in order get the guaranteed SLA, you'll need to deploy your application to two or more nodes. If you have an F# Agent running on both nodes, obviously you no longer have a Single Writer, and everything just becomes much more difficult. If only there was a way to ensure a Single Writer in a distributed environment...

Fortunately, it looks like the (in-preview) Azure feature Web Jobs (inadvertently) solves this major problem for us. Web Jobs come in three flavours:

  • On demand
  • Continuously running
  • Scheduled
If you were to implement your writer as a web Job, the On demand option isn't particularly helpful, because you want the writer to run continuously, or at least very often. How about Continuously running, then?

That turns out not to be a particularly useful option as well, because

"If your website runs on more than one instance, a continuously running task will run on all of your instances."
On the other hand
"On demand and scheduled tasks run on a single instance selected for load balancing by Microsoft Azure."
It sounds like Scheduled Web Jobs is just what we need!

There's just one concern that we need to address: what happens if a Scheduled Web Job is taking too long running, in such a way that it hasn't completed when it's time to start it again. For example, what if you run a Scheduled Web Job every minute, but it sometimes takes 90 seconds to complete? If a new process starts executing while the first one is running, you would no longer have a Single Writer.

Reading the documentation, I couldn't find any information about how Azure handles this scenario, so I decided to perform some tests.

The Qaiain email micro-service proved to be a fine tool for the experiment. I slightly modified the code to wait for 90 seconds before exiting:

let main argv = 
    match queue |> AzureQ.dequeue with
    | Some(msg) ->
        msg.AsString |> Mail.deserializeMailData |> send
        queue.DeleteMessage msg
    | _ -> ()
    Async.Sleep 90000 |> Async.RunSynchronously
    match queue |> AzureQ.dequeue with
    | Some(msg) ->
        msg.AsString |> Mail.deserializeMailData |> send
        queue.DeleteMessage msg
    | _ -> ()
    0 // return an integer exit code

In addition to that, I also changed how the subject of the email that I would receive would look, in order to capture the process ID of the running application, as well as the time it sent the email:

smtpMsg.Subject <-
        "Process ID: %i, Time: %O"

My hypothesis was that if Scheduled Web Jobs are well-behaved, a new job wouldn't start if an existing job was already running. Here are the results:


This looks great, but it's easier to see if I visualize it:

Scheduled delayed Web Job observations

As you can see, processes do not overlap in time. This is a highly desirable result, because it seems to guarantee that we can have a Single Writer running in a distributed, load-balanced system.

Azure Web Jobs are currently in preview, so let's hope the Azure team preserve this functionality in the final version. If you care about this, please let the team know.

Composed assertions with Unquote

Friday, 21 March 2014 08:52:00 UTC

With F# and Unquote, you can write customized, composable assertions.

Yesterday, I wrote this unit test:

[<Theory; UnitTestConventions>]
let PostReturnsCorrectResult
    (sut : TasksController)
    (task : TaskRendition) =
    let result : IHttpActionResult = sut.Post task
    verify <@ result :? Results.StatusCodeResult @>
    verify <@ HttpStatusCode.Accepted =
        (result :?> Results.StatusCodeResult).StatusCode @>

For the record, here's the SUT:

type TasksController() =
    inherit ApiController()
    member this.Post(task : TaskRendition) =
        this.StatusCode HttpStatusCode.Accepted :> IHttpActionResult

There's not much to look at yet, because at that time, I was just getting started, and as always, I was using Test-Driven Development. The TasksController class is an ASP.NET Web API 2 Controller. In this incarnation, it merely accepts an HTTP POST, ignores the input, and returns 202 (Accepted).

The unit test uses AutoFixture.Xunit to create an instance of the SUT and a DTO record, but that's not important in this context. It also uses Unquote for assertions, although I've aliased the test function to verify. Although Unquote is an extremely versatile assertion module, I wasn't happy with the assertions I wrote.

What's the problem? #

The problem is the duplication of logic. First, it verifies that result is, indeed, an instance of StatusCodeResult. Second, if that's the case, it casts result to StatusCodeResult in order to access its concrete StatusCode property; it feels like I'm almost doing the same thing twice.

You may say that this isn't a big deal in a test like this, but in my experience, this is a smell. The example looks innocuous, but soon, I'll find myself writing slightly more complicated assertions, where I need to type check and cast more than once. This can rapidly lead to Assertion Roulette.

The xUnit.net approach #

For a minute there, I caught myself missing xUnit.net's Assert.IsAssignableFrom<T> method, because it returns a value of type T if the conversion is possible. That would have enabled me to write something like:

let scr = Assert.IsAssignableFrom<Results.StatusCodeResult> result
Assert.Equal(HttpStatusCode.Accepted, scr.StatusCode)

It seems a little nicer, although in my experience, this quickly turns to spaghetti, too. Still, I found myself wondering if I could do something similar with Unquote.

A design digression #

At this point, you are welcome to pull GOOS at me and quote: listen to your tests! If the tests are difficult to write, you should reconsider your design; I agree, but I can't change the API of ASP.NET Web API. In Web API 1, my preferred return type for Controller actions were HttpResponseMessage, but it was actually a bit inconvenient to work with in unit tests. Web API 2 introduces various IHttpActionResult implementations that are easier to unit test. Perhaps this could be better, but it seems like a step in the right direction.

In any case, I can't change the API, so coming up with a better way to express the above assertion is warranted.

Composed assertions #

To overcome this little obstacle, I wrote this function:

let convertsTo<'a> candidate =
    match box candidate with
    | :? 'a as converted -> Some converted
    | _ -> None

(You have to love a language that let's you write match box! There's also a hint of such nice over Some converted...)

The convertsTo function takes any object as input, and returns an Option containing the converted value, if the conversion is possible; otherwise, it returns None. In other words, the signature of the convertsTo function is obj -> 'a option.

This enables me to write the following Unquote assertion:

[<Theory; UnitTestConventions>]
let PostReturnsCorrectResult
    (sut : TasksController)
    (task : TaskRendition) =
    let result : IHttpActionResult = sut.Post task
    verify <@
            |> convertsTo<Results.StatusCodeResult>
            |> Option.map (fun x -> x.StatusCode)
            |> Option.exists ((=) HttpStatusCode.Accepted) @>

While this looks more verbose than my two original assertions, this approach is more composable.

The really beautiful part of this is that Unquote can still tell me what goes wrong, if the test doesn't pass. As an example, if I change the SUT to:

type TasksController() =
    inherit ApiController()
    member this.Post(task : TaskRendition) =
        this.Ok() :> IHttpActionResult

The assertion message is:

System.Web.Http.Results.OkResult |> Dsl.convertsTo |> Option.map (fun x -> x.StatusCode) |> Option.exists ((=) Accepted)
None |> Option.map (fun x -> x.StatusCode) |> Option.exists ((=) Accepted)
None |> Option.exists ((=) Accepted)

Notice how, in a series of reductions, Unquote breaks down for me exactly what went wrong. The top line is my original expression. The next line shows me the result of evaluating System.Web.Http.Results.OkResult |> Dsl.convertsTo; the result is None. Already at this point, it should be quite evident what the problem is, but in the next line again, it shows the result of evaluating None |> Option.map (fun x -> x.StatusCode); again, the result is None. Finally, it shows the result of evaluating None |> Option.exists ((=) Accepted), which is false.

Here's another example. Assume that I change the SUT to this:

type TasksController() =
    inherit ApiController()
    member this.Post(task : TaskRendition) =
        this.StatusCode HttpStatusCode.OK :> IHttpActionResult

In this example, instead of returning the wrong implementation of IHttpActionResult, the SUT does return a StatusCodeResult instance, but with the wrong status code. Unquote is still very helpful:

System.Web.Http.Results.StatusCodeResult |> Dsl.convertsTo |> Option.map (fun x -> x.StatusCode) |> Option.exists ((=) Accepted)
Some System.Web.Http.Results.StatusCodeResult |> Option.map (fun x -> x.StatusCode) |> Option.exists ((=) Accepted)
Some OK |> Option.exists ((=) Accepted)

Notice that it still uses a series of reductions to show how it arrives at its conclusion. Again, the first line is the original expression. The next line shows the result of evaluating System.Web.Http.Results.StatusCodeResult |> Dsl.convertsTo, which is Some System.Web.Http.Results.StatusCodeResult. So far so good; this is as required. The third line shows the result of evaluating Some System.Web.Http.Results.StatusCodeResult |> Option.map (fun x -> x.StatusCode), which is Some OK. Still good. Finally, it shows the result of evaluating Some OK |> Option.exists ((=) Accepted), which is false. The value in the option was HttpStatusCode.OK, but should have been HttpStatusCode.Accepted.

Summary #

Unquote is a delight to work with. As the project site explains, it's not an API or a DSL. It just evaluates and reports on the expressions you write. If you already know F#, you already know how to use Unquote, and you can write your assertion expressions as expressive and complex as you want.


Friday, 14 March 2014 14:50:00 UTC

Announcing Exude, an extension to xUnit.net providing test cases as first-class, programmatic citizens.

Sometimes, when writing Parameterized Tests with xUnit.net, you need to provide parameters that don't lend themselves easily to be defined as constants in [InlineData] attributes.

In Grean, we've let ourselves inspire by Mauricio Scheffer's blog post First-class tests in MbUnit, but ported the concept to xUnit.net and released it as open source.

It's called Exude and is available on GitHub and on NuGet.

Here's a small example:

public static IEnumerable<ITestCase> YieldFirstClassTests()
    yield return new TestCase(_ => Assert.Equal(1, 1));
    yield return new TestCase(_ => Assert.Equal(2, 2));
    yield return new TestCase(_ => Assert.Equal(3, 3));

More examples and information is available on the project site.

Arbitrary Version instances with FsCheck

Tuesday, 11 March 2014 10:01:00 UTC

This post explains how to configure FsCheck to create arbitrary Version values.

When I unit test generic classes or methods, I often like to use Version as one of the type arguments. The Version class is a great test type because

  • it's readily available, as it's defined in the System namespace in mscorlib
  • it overrides Equals so that it's easy to compare two values
  • it's a complex class, because it composes four integers, so it's a good complement to String, Int32, Object, Guid, and other primitive types

Recently, I've been picking up FsCheck to do Property-Based Testing, but out of the box it doesn't know how to create arbitrary Version instances.

It turns out that you can easily and elegantly tell FsCheck how to create arbitrary Version instances, but since I haven't seen it documented, I thought I'd share my solution:

type Generators =
    static member Version() =
        |> Gen.map int
        |> Gen.four
        |> Gen.map (fun (ma, mi, bu, re) -> Version(ma, mi, bu, re))
        |> Arb.fromGen

As the FsCheck documentation explains, you can create custom Generator by defining a static class that exposes members that return Arbitrary<'a> - in this case Arbitrary<Version>.

If you'd like me to walk you through what happens here, read on, and I'll break it down for you.

First, Arb.generate<byte> is a Generator of Byte values. While FsCheck doesn't know how to create arbitrary Version values, it does know how to create arbitrary values of various primitive types, such as Byte, Int32, String, and so on. The Version constructors expect components as Int32 values, so why did I select Byte values instead? Because Version doesn't accept negative numbers, and if I had kicked off my Generator with Arb.generate<int>, it would have created all sorts of integers, including negative values. While it's possible to filter or modify the Generator, I thought it was easier to simply kick off the Generator with Byte values, because they are never negative.

Second, Gen.map int converts the initial Gen<byte> to Gen<int> by invoking F#'s built-in int conversion function.

Third, Gen.four is a built-in FsCheck Generator Combinator that converts a Generator into a Generator of four-element tuples; in this case it converts Get<int> to Gen<int * int * int * int>: a Generator of a four-integer tuple.

Fourth, Gen.map (fun (ma, mi, bu, re) -> Version(ma, mi, bu, re)) converts Gen<int * int * int * int> to Gen<Version> by another application of Gen.map. The function supplied to Gen.map takes the four-element tuple of integers and invokes the Version constructor with the major, minor, build, and revision integer values.

Finally, Arb.fromGen converts Gen<Version> to Arbitrary<Version>, which is what the member must return.

To register the Generators custom class with FsCheck, I'm currently doing this:

do Arb.register<Generators>() |> ignore

You can see this entire code in context here.

SOLID: the next step is Functional

Monday, 10 March 2014 08:33:00 UTC

If you take the SOLID principles to their extremes, you arrive at something that makes Functional Programming look quite attractive.

You may have seen this one before, but bear with me :)

The venerable master Qc Na was walking with his student, Anton. Hoping to prompt the master into a discussion, Anton said "Master, I have heard that objects are a very good thing - is this true?" Qc Na looked pityingly at his student and replied, "Foolish pupil - objects are merely a poor man's closures."

Chastised, Anton took his leave from his master and returned to his cell, intent on studying closures. He carefully read the entire "Lambda: The Ultimate..." series of papers and its cousins, and implemented a small Scheme interpreter with a closure-based object system. He learned much, and looked forward to informing his master of his progress.

On his next walk with Qc Na, Anton attempted to impress his master by saying "Master, I have diligently studied the matter, and now understand that objects are truly a poor man's closures." Qc Na responded by hitting Anton with his stick, saying "When will you learn? Closures are a poor man's object." At that moment, Anton became enlightened.

- Anton van Straaten

While this is a lovely parable, it's not a new observation that objects and closures seem closely related, and there has been much discussion back and forth about this already. Still, in light of a recent question and answer about how to move from Object-Oriented Composition to Functional Composition, I'd still like to explain how, in my experience, the SOLID principles lead to a style of design that makes Functional Programming quite attractive.

A SOLID road map #

In a previous article, I've described how application of the Single Responsibility Principle (SRP) leads to many small classes. Furthermore, if you rigorously apply the Interface Segregation Principle (ISP), you'll understand that you should favour Role Interfaces over Header Interfaces.

If you keep driving your design towards smaller and smaller interfaces, you'll eventually arrive at the ultimate Role Interface: an interface with a single method. This happens to me a lot. Here's an example:

public interface IMessageQuery
    string Read(int id);

If you apply the SRP and ISP like that, you're likely to evolve a code base with many fine-grained classes that each have a single method. That has happened to me more than once; AutoFixture is an example of a big and complex code base that looks like that, but my other publicly available code bases tend to have the same tendency. In general, this works well; the most consistent problem is that it tends to be a bit verbose.

Objects as data with behaviour #

One way to characterise objects is that they are data with behaviour; that's a good description. In practice, when you have many fine-grained classes with a single method, you may have classes like this:

public class FileStore : IMessageQuery
    private readonly DirectoryInfo workingDirectory;

    public FileStore(DirectoryInfo workingDirectory)
        this.workingDirectory = workingDirectory;

    public string Read(int id)
        var path = Path.Combine(
            id + ".txt");            
        return File.ReadAllText(path);

This FileStore class is a simple example of data with behaviour.

  • The behaviour is the Read method, which figures out a file path for a given ID and returns the contents of the file.
  • The data (also sometimes known as the state) is the workingDirectory field.
In this example, the data is immutable and passed in via the constructor, but it could also have been a public, writeable property, or even a public field.

The workingDirectory field is a Concrete Dependency, but it could also have been a primitive value or an interface or abstract base class. In the last case, we would often call the pattern Constructor Injection.

Obviously, the data could be multiple values, instead of a single value.

The FileStore example class implements the IMessageQuery interface, so it's a very representative example of what happens when you take the SRP and ISP to their logical conclusions. It's a fine class, although a little verbose.

When designing like this, not only do you have to come up with a name for the interface itself, but also for the method, and for each concrete class you create to implement the interface. Naming is difficult, and in such cases, you have to name the same concept twice or more. This often leads to interfaces named IFooer with a method called Foo, IBarer with a method called Bar, etc. You get the picture. This is a smell (that also seems vaguely reminiscent of the Reused Abstractions Principle). There must be a better way.

Hold that thought.

Functions as pure behaviour #

As the introductory parable suggests, perhaps Functional Programming offers an alternative. Before you learn about Closures, though, you'll need to understand Functions. In Functional Programming, a Function is often defined as a Pure Function - that is: a deterministic operation without side-effects.

Since C# has some Functional language support, I'll first show you the FileStore.Read method as a Pure Function in C#:

Func<DirectoryInfointstring> read = (workingDirectory, id) =>
        var path = Path.Combine(workingDirectory.FullName, id + ".txt");
        return File.ReadAllText(path);

This Function does the same as the FileStore.Read method, but it has no data. You must pass in the working directory as a function argument just like the ID. This doesn't seem equivalent to an object.

Closures as behaviour with data #

A Closure is an important concept in Functional Programming. In C# it looks like this:

var workingDirectory = new DirectoryInfo(Environment.CurrentDirectory);
Func<intstring> read = id =>
        var path = Path.Combine(workingDirectory.FullName, id + ".txt");
        return File.ReadAllText(path);

This is called a Closure because the Function closes over the Outer Variable workingDirectory. Effectively, the function captures the value of the Outer Variable.

What does that compile to?

Obviously, the above C# code compiles to IL, but if you reverse-engineer the IL back to C#, this is what it looks like:

private sealed class <>c__DisplayClass3
    public DirectoryInfo workingDirectory;

    public string <UseClosure>b__2(int id)
        return File.ReadAllText(
            Path.Combine(this.workingDirectory.FullName, id + ".txt"));

It's a class with a field and a method! Granted, the names look somewhat strange, and the field is a public, mutable field, but it's essentially identical to the FileStore class!

Closures are behaviour with data, whereas objects are data with behaviour. Hopefully, the opening parable makes sense to you now. This is an example of one of Erik Meijer's favourite design concepts called duality.

Partial Function Application #

Another way to close over data is called Partial Function Application, but the result is more or less the same. Given the original pure function:

Func<DirectoryInfointstring> read = (workingDirectory, id) =>
        var path = Path.Combine(workingDirectory.FullName, id + ".txt");
        return File.ReadAllText(path);

you can create a new Function from the first Function by only invoking it with some of the arguments:

var wd = new DirectoryInfo(Environment.CurrentDirectory);
Func<intstring> r = id => read(wd, id);

The r function also closes over the wd variable, and the compiled IL is very similar to before.

Just use F#, then! #

If SOLID leads you to many fine-grained classes with a single method, C# starts to be in the way. A class like the above FileStore class is proper Object-Oriented Code, but is quite verbose; the Closures and Partially Applied Functions compile, but are hardly idiomatic C# code.

On the other hand, in F#, the above Closure is simply written as:

let workingDirectory = DirectoryInfo(Environment.CurrentDirectory)
let read id = 
    let path = Path.Combine(workingDirectory.FullName, id.ToString() + ".txt")

The read value is a Function with the signature 'a -> string, which means that it takes a value of the generic type 'a (in C#, it would typically have been named T) and returns a string. This is just a more general version of the IMessageQuery.Read method. When 'a is int, it's the same signature, but in F#, I only had to bother naming the Function itself. Functions are anonymous interfaces, so these are also equivalent.

Likewise, if you have a Pure Function like this:

let read (workingDirectory : DirectoryInfo) id =
    let path = Path.Combine(workingDirectory.FullName, id.ToString() + ".txt")

the Partially Applied Function is written like this:

let wd = DirectoryInfo(Environment.CurrentDirectory)
let r = read wd

The r Function is another Function that takes an ID as input, and returns a string, but notice how much less ceremony is involved.

Summary #

SOLID, particularly the SRP and ISP, leads you towards code bases with many fine-grained classes with a single method. Such objects represent data with behaviour, but can also be modelled as behaviour with data: Closures. When that happens repeatedly, it's time to make the switch to a Functional Programming Language like F#.


Looks like we had similar thoughts at the same time - mine are here
It's surprising to me that we've not moved more to the functional paradigm as an industry, when so many pieces of evidence point to it working more effectively than OO.
It feels like people can't seem to break away from those curly braces, which is perhaps why Scala is doing so well on the JVM.
2014-03-10 11:51 UTC
I like where you're going with this post, but I just can't get my head round how you would consume the closure you've written. Most of the time you would consume the IMessageQuery by taking it in your class's constructor and letting your DI framework new it up for you:
public class MyService

	public MyService(IMessageQuery messageQuery)
How would you do this with a closure? Your function no longer has a type that we can use (it's just int -> string). Surely your service doesn't look like this?
type MyService (messageQuery: int -> string) = ...
How would you register the types for injection in this example?
2014-03-11 10:05 UTC
Great explanation and justification! I believe the story goes further beyond SOLID into many other patterns. I wrote a post about OOP patterns from Functional Perspective.
2014-03-11 13:18 UTC

Richard, thank you for writing. You ask "Surely your service doesn't look like this? type MyService (messageQuery: int -> string) = ..."

Probably not. Why even have a class? A client consuming the closure would just take it as a function argument:

let myClient f =
    let message = f 42
    // Do something else interesting...
    // Return a result...

Here, f is a function with the int -> string signature, and myClient is another function. Just as you can keep on composing classes using the Composite, Decorator, and Adapter patterns, you can keep on composing functions with other functions by taking functions as function arguments.

At the top level of your application, you may have to implement a class to fit into a framework. For an example of integrating with the ASP.NET Web API, see my A Functional Architecture with F# Pluralsight course.

When it comes to integrating with a DI Container, I tend to not care about that these days. I prefer composing the application with Poor Man's DI, and that works beautifully with F#.

2014-03-11 16:57 UTC

Excellent post!

Under "Partial Function Application", you state "Given the original pure function" - the file I/O would appear to make that impure. Similarly under "Just use F#, then!" with "Likewise, if you have a Pure Function like this".

2014-03-12 17:40 UTC

Bill, you are correct! I may have gotten a little carried away at that point. The method is side-effect-free, and deterministic (unless someone comes by and changes the file), but it does depend on state on disk. Thank you for pointing that out; I stand corrected. Hopefully, that mistake of mine doesn't detract from the overall message.

2014-03-12 19:26 UTC
Leif Battermann #

Hey Mark, obviously switching to F# is not always that easy. I currently have a very similar situation like the one you describe in this post. I refactored the code to using partial application and a functional programming style with C# which works fine. You are saying that the two approaches are actually more or less the same thing which I can see. I am wondering now what the benefit is from refactoring to a functional style with partial application? Does it make sense to do that using C#? The dependencies that I inject are repositories with DB access. So I don't get the true benefits of FP because of the state of the DB. Is it still reasonable to switch to the FP approach? Personally I just like the style and I think it is a littel bit cleaner to have no constructors and private fields. Any thoughts on that? Thanks, Leif.

2014-04-02 11:21 UTC

Leif, thank you for writing. Is there value in adopting a functional style in C#? Yes, I think so, but not (in my opinion) from closures or partial function application. While it's possible to do this in C#, the syntax is awkward compared to F#. It also goes somewhat against the grain of C#.

The main benefit from FP is immutable state, which makes it much easier to reason about the code and the state of the application. Once you understand how to model a problem around immutable data, even C# code becomes much easier to reason about, so I definitely think it makes sense to adopt patterns for working with immutable data in C#.

For years, I've written C# code like that. Not only is it possible, but I strongly prefer it over more 'normal' C# with mutable state. Still, there's a lot of boilerplate code you have to write in C#, such as constructors and read-only property pairs, copy-and-update methods, structural equality, etc. After having done that for a couple of years, I got tired of writing all that boilerplate code, when I get it for free in F#.

Like you, I still have a large body of C# code that I have to maintain, so while I choose F# for most new development, I write 'functional C#' in my C# code bases. Even if there are small pockets of mutable state here and there (like you describe), I still think it makes sense to keep as much as possible immutable.

2014-04-03 17:34 UTC

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