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ASP.NET MVC comes with an error-handling feature that enables you to show nice error Views to your users instead of the dreaded Yellow Screen of Death. The most prominently described technique involves attributing either you Controller or a single Controller action, like we see in the AccountController created by the Visual Studio project template.

[HandleError]
public class AccountController : Controller { }

That sure is easy, but I don't like it for a number of reasons:

• Even though you can derive from HandleErrorAttribute, it's impossible to inject any dependencies into Attributes because they must be fully constructed at compile-time. That makes it really difficult to log errors to an interface.
• It violates the DRY principle. Although it can be applied at the Controller level, I still must remember to attribute all of my Controllers with the HandleErrorAttribute.

Another approach is to override Controller.OnException. That solves the DI problem, but not the DRY problem.

Attentive readers may now point out that I can define a base Controller that implements the proper error handling, and require that all my Controllers derive from this base Controller, but that doesn't satisfy me:

First of all, it still violates the DRY principle. Whether I have to remember to apply a [HandleError] attribute or derive from a : MyBaseController amounts to the same thing. I (and all my team members) have to remember this always. It's unnecessary project friction.

The second thing that bugs me about this approach is that I really do favor composition over inheritance. Error handling is a cross-cutting concern, so why should all my Controllers have to derive from a base controller to enable this? That is absurd.

Fortunately, ASP.NET MVC offers a third way.

The key lies in the Controller base class and how it deals with IExceptionFilters (which HandleErrorAttribute implements). When a Controller action is invoked, this actually happens through an IActionInvoker. The default ControllerActionInvoker is responsible for gathering the list of IExceptionFilters, so the first thing we can do is to derive from that and override its GetFilters method:

public class ErrorHandlingActionInvoker :
    ControllerActionInvoker
{
    private readonly IExceptionFilter filter;
 
    public ErrorHandlingActionInvoker(
        IExceptionFilter filter)
    {
        if (filter == null)
        {
            throw new ArgumentNullException("filter");
        }
 
        this.filter = filter;
    }
 
    protected override FilterInfo GetFilters(
        ControllerContext controllerContext,
        ActionDescriptor actionDescriptor)
    {
        var filterInfo = 
            base.GetFilters(controllerContext, 
            actionDescriptor);
        filterInfo.ExceptionFilters.Add(this.filter);
        return filterInfo;
    }
}

Here I'm simply injecting an IExceptionFilter instance into this class, so I'm not tightly binding it to any particular implementation - it will work with any IExceptionFilter we give it, so it simply serves the purpose of adding the filter at the right stage so that it can deal with otherwise unhandled exceptions. It's pure infrastructure code.

Notice that I first invoke base.GetFilters and then add the injected filter to the returned list of filters. This allows the normal ASP.NET MVC IExceptionFilters to attempt to deal with the error first.

This ErrorHandlingActionInvoker is only valuable if we can assign it to each and every Controller in the application. This can be done using a custom ControllerFactory:

public class ErrorHandlingControllerFactory : 
    DefaultControllerFactory
{
    public override IController CreateController(
        RequestContext requestContext,
        string controllerName)
    {
        var controller = 
            base.CreateController(requestContext,
            controllerName);
 
        var c = controller as Controller;
        if (c != null)
        {
            c.ActionInvoker = 
                new ErrorHandlingActionInvoker(
                    new HandleErrorAttribute());
        }
 
        return controller;
    }
}

Notice that I'm simply deriving from the DefaultControllerFactory and overriding the CreateController method to assign the ErrorHandlingActionInvoker to the created Controller.

In this example, I have chosen to inject the HandleErrorAttribute into the ErrorHandlingActionInvoker instance. This seems weird, but it's the HandleErrorAttribute that implements the default error handling logic in ASP.NET MVC, and since it implements IExceptionFilter, it works like a charm.

The only thing left to do is to wire up the custom ErrorHandlingControllerFactory in global.asax like this:

ControllerBuilder.Current.SetControllerFactory(
    new ErrorHandlingControllerFactory());

Now any Controller action that throws an exception is gracefully handled by the injected IExceptionFilter. Since the filter is added as the last in the list of ExceptionFilters, any normal [HandleError] attribute and Controller.OnException override gets a chance to deal with the exception first, so this doesn't even change behavior of existing code.

If you are trying this out on your own development machine, remember that you must modify your web.config like so:

<customErrors mode="On" />

If you run in the RemoteOnly mode, you will still see the Yellow Screen of Death on your local box.

Comments

In unit testing an important step is to exercise the SUT. The member you want to invoke is often a method that takes one or more parameters, but in some test cases you don't care about the values of those parameters - you just want to invoke the method.

You can always make up one or more Dummy parameters and pass them to the method in question, but you could also use one of AutoFixture's Do convenience methods. There are several overloads that all take delegates that specify the action in question while providing you with Dummies of any parameters you don't care about.

A good example is WPF's ICommand interface. The most prevalent method is the Execute method that takes a single parameter parameter:

void Execute(object parameter);

Most of the time we don't really care about the parameter because we only care that the command was invoked. However, we still need to supply a value for the parameter when we unit test our ICommand implementations. Obviously, we could just pass in a new System.Object every time, but why not let AutoFixture take care of that for us?

(You may think that new'ing up a System.Object is something you can easily do yourself, but imagine other APIs that require much more complex input parameters, and you should begin to see the potential.)

Here's a state-based unit test that verifies that the Message property of the MyViewModel class has the correct value after the FooCommand has been invoked:

[TestMethod]
public void FooWillUpdateMessage()
{
    // Fixture setup
    var fixture = new Fixture();
    var sut = fixture.CreateAnonymous<MyViewModel>();
    // Exercise system
    fixture.Do((object parameter) => 
        sut.FooCommand.Execute(parameter));
    // Verify outcome
    Assert.AreEqual("Foo", sut.Message, "Message");
    // Teardown
}

Notice how the Do method takes an Action<object> to specify the method to invoke. AutoFixture automatically supplies an instance for the parameter parameter using the same engine to create Anonymous variables that it uses for everything else.

The Do method in question is really a generic method with this signature:

public void Do<T>(Action<T> action);

There are also overloads that take two, three or four input parameters, corresponding to the available Action types available in the BCL.

These methods are simply convenience methods that allow you to express your test code more succinctly than you would otherwise have been able to do.

Daniel Frost has published a podcast where he discusses Dependency Injection with me. It's about half an hour long and in Danish. Hear it here.

When using Castle Windsor in web applications you would want to register many of your components with a lifestyle that is associated with a single request. This is the purpose of the PerWebRequest lifestyle.

If you try that with ASP.NET MVC on IIS7, you are likely to receive the following error message:

Looks like you forgot to register the http module Castle.MicroKernel.Lifestyle.PerWebRequestLifestyleModule
Add '<add name="PerRequestLifestyle" type="Castle.MicroKernel.Lifestyle.PerWebRequestLifestyleModule, Castle.MicroKernel" />' to the <httpModules> section on your web.config.

Unfortunately, following the instructions in the error message doesn't help. There's a discussion about this issue on the Castle Project Users forum, but the gist of it is that if you don't need to resolve components during application startup, this shouldn't be an issue, and indeed it isn't - it seems to be something else.

In my case I seem to have solved the problem by registering the HTTP module in configuration/system.webServer/modules instead of configuration/system.web/httpModules.

Although I haven't had the opportunity to dive into the technical details to understand why this works, this seems to solve the problem on both Windows Vista and Windows Server 2008.

Comments

The following feature suggestion was recently posted in the AutoFixture Issue Tracker board:

"We're using AutoFixture to create random rows of data in our DB. A lot of times though, it creates strings that are too long for the database columns. It would be nice if the .With<string> method had an overload that took in a min/max length. We want the random data, but capped at a limit.

"fixture.Build<MyObject>.With(x = x.MyString, 0, 100);

"As an aside, this is for a project that uses Nhibernate and Fluent Nhibernate, which has these lengths already defined. I would be nice if AutoFixture could automatically pick up on that somehow."

I think such an feature is an excellent idea, but I don't think I will include in AutoFixture. Why not?

So far, I have kept the AutoFixture API pretty clean and very generic, and it is my belief that this is one of the main reasons it is so expressive and flexible. There are no methods that only work on specific types (such as strings), and I am reluctant to introduce them now.

In the last six months, I have identified a lot of specialized usage idioms that I would love to package into a reusable library, but I think that they will pollute the core AutoFixture API, so I'm going to put those in one or more optional 'add-on' libraries.

The ability to define strings that are constrained on length could be one such feature, but rather than wait for a helper library, I will show you have you can implement such a method yourself.

The first thing we need is a method that can create anonymous strings given length constraints. One possible implementation is this ConstrainedStringGenerator class:

public class ConstrainedStringGenerator
{
    private readonly int minimumLength;
    private readonly int maximumLength;
 
    public ConstrainedStringGenerator(int minimumLength, 
        int maximumLength)
    {
        if (maximumLength < 0)
        {
            throw new ArgumentOutOfRangeException("...");
        }
        if (minimumLength > maximumLength)
        {
            throw new ArgumentOutOfRangeException("...");
        }
 
        this.minimumLength = minimumLength;
        this.maximumLength = maximumLength;
    }
 
    public string CreateaAnonymous(string seed)
    {
        var s = string.Empty;
        while (s.Length < this.minimumLength)
        {
            s += GuidStringGenerator.CreateAnonymous(seed);
        }
        if (s.Length > this.maximumLength)
        {
            s = s.Substring(0, this.maximumLength);
        }
        return s;
    }
}

The CreateAnonymous method uses AutoFixture's GuidStringGenerator class to create anonymous strings of the required length. For this implementation I chose a basic algorithm, but I'm sure you can create one that is more sophisticated if you need it.

The next thing we need to do is to implement the desired With method. That can be done with an extension method works on ObjectBuilder<T>:

public static class ObjectBuilderExtension
{
    public static ObjectBuilder<T> With<T>(
        this ObjectBuilder<T> ob,
        Expression<Func<T, string>> propertyPicker,
        int minimumLength,
        int maximumLength)
    {
        var me = (MemberExpression)propertyPicker.Body;
        var name = me.Member.Name;
        var generator =
            new ConstrainedStringGenerator(
                minimumLength, maximumLength);
        var value = generator.CreateaAnonymous(name);
        return ob.With(propertyPicker, value);
    }
}

The method takes the same input as ObjectBuilder<T>'s With method, plus the two integers that constrain the length. Note that the propertyPicker expression has been constrained to deal only with strings.

In this implementation, we use the ConstrainedStringGenerator class to generate the desired value for the property, where after we can use the existing With method to assign the value to the property in question.

This now allows us to write Build statements like the one originally requested:

var mc = fixture.Build<MyClass>()
    .With(x => x.SomeText, 0, 100)
    .CreateAnonymous();

The other part of the request, regarding NHibernate integration, I will leave to the interested reader - mostly because I have never used NHibernate, so I have no clue how to do it. I would, however, love to see a blog post with that addition.

This entire example is now part of the AutoFixture test suite, so if you are interested in looking at it in more detail, you can get it from the AutoFixture source code (available at CodePlex).

Comments

AutoFixture beta 1 is now available on the CodePlex site! We have been using AutoFixture quite intensively in Safewhere for almost half a year now, and the core of it has turned out to be stable and much more powerful than I originally imagined.

During that period, I have discovered and fixed a few bugs, but the most positive experience has been how extended usage has inspired us to come up with numerous ideas to new cool features. Some of these features are already implemented in the current release, and the rest are listed on the AutoFixture site.

The beta 1 release page has more details about this particular release.

A few months ago I described how you can use AutoFixture's With method to assign property values as part of building up an anonymous variable. In that scenario, the With method is used to explicitly assign a particular, pre-defined value to a property.

There's another overload of the With method that doesn't take an explicit value, but rather uses AutoFixture to create an anonymous value for the property. So what's the deal with that if AutoFixture's default behavior is to assign anonymous values to all writable properties?

In short it's an opt-in mechanism that only makes sense if you decide to opt out of the AutoProperties features.

As always, let's look at an example. This time, I've decided to show you a slightly more realistic example so that you can get an idea of how AutoFixture can be used to aid in unit testing. This also means that the example is going to be a little more complex than usual, but it's still simple.

Imagine that we wish to test that an ASP.NET MVC Controller Action returns the correct result. More specifically, we wish to test that the Profile method on MyController returns a ViewResult with the correct Model (the current user's name, to make things interesing).

Here's the entire test. It may look more complicated than it is - it's really only 10 lines of code, but I had to break them up to prevent unpleasant wrapping if your screen is narrow.

[TestMethod]
public void ProfileWillReturnResultWithCorrectUserName()
{
    // Fixture setup
    var fixture = new Fixture();
    fixture.Customize<ControllerContext>(ob => ob
        .OmitAutoProperties()
        .With(cc => cc.HttpContext));
 
    var expectedUserName = 
        fixture.CreateAnonymous("UserName");
 
    var httpCtxStub = new Mock<HttpContextBase>();
    httpCtxStub.SetupGet(x => x.User).Returns(
        new GenericPrincipal(
            new GenericIdentity(expectedUserName), null));
    fixture.Register(httpCtxStub.Object);
 
    var sut = fixture.Build<MyController>()
        .OmitAutoProperties()
        .With(c => c.ControllerContext)
        .CreateAnonymous();
    // Exercise system
    ViewResult result = sut.Profile();
    // Verify outcome
    var actual = result.ViewData.Model;
    Assert.AreEqual(expectedUserName, actual, "User");
    // Teardown
}

Apart from AutoFixture, I'm also making use of Moq to stub out HttpContextBase.

You can see the Anonymous With method in two different places: in the call to Customize and when the SUT is being built. In both cases you can see that the call to With follows a call to OmitAutoProperties. In other words: we are telling AutoFixture that we don't want any of the writable properties to be assigned a value except the one we identify.

Let me highlight some parts of the test.

fixture.Customize<ControllerContext>(ob => ob
    .OmitAutoProperties()
    .With(cc => cc.HttpContext));

This line of code instructs AutoFixture to always create a ControllerContext in a particular way: I don't want to use AutoProperties here, because ControllerContext has a lot of writable properties of abstract types, and that would require me to set up a lot of Test Doubles if I had to assign values to all of those. It's much easier to simply opt out of this mechanism. However, I would like to have the HttpContext property assigned, but I don't care about the value in this statement, so the With method simply states that AutoFixture should assign a value according to whatever rule it has for creating instances of HttpContextBase.

I can now set up a Stub that populates the User property of HttpContextBase:

var httpCtxStub = new Mock<HttpContextBase>();
httpCtxStub.SetupGet(x => x.User).Returns(
    new GenericPrincipal(
        new GenericIdentity(expectedUserName), null));
fixture.Register(httpCtxStub.Object);

This is registered with the fixture instance which closes the loop to the previous customization.

I can now create an instance of my SUT. Once more, I don't want to have to set up a lot of irrelevant properties on MyController, so I opt out of AutoProperties and then explicitly opt in on the ControllerContext. This will cause AutoFixture to automatically populate the ControllerContext with the HttpContext Stub:

var sut = fixture.Build<MyController>()
    .OmitAutoProperties()
    .With(c => c.ControllerContext)
    .CreateAnonymous();

For completeness' sake, here's the MyController class that I am testing:

public class MyController : Controller
{
    public ViewResult Profile()
    {
        object userName = this.User.Identity.Name;
        return this.View(userName);
    }
}

This test may seem complex, but it really accomplishes a lot in only 10 lines of code, considering that ASP.NET MVC Controllers with a working HttpContext are not particularly trivial to set up.

In summary, this With method overload lets you opt in on one or more explicitly identified properties when you have otherwise decided to omit AutoProperties. As all the other Builder methods, this method can also be chained.

Daniel Frost has published a podcast where he discusses WCF with me. It's about half an hour and in Danish. Hear it here.

For the last few months I've been writing a book for Manning tentatively titled Dependency Injection in .NET. The page about the book is now live at the Manning web site where you can read more about it and, if you would like, purchase an Early Access edition and read the chapters as they are being written.

If you have ever wanted to learn about Dependency Injection (DI) related to .NET, here's your chance!

At the moment I'm about a third of the way into the book, so there's still some way to go, but I hope to be done with it in 2010.

If you decide to purchase an Early Access edition, I'd love to receive your feedback in the online forum.

Comments

The SOLID principles of OOD as originally put forth by Robert C. Martin make for such a catchy acronym, although they seem to originally have been spelled SOLDI.

In any case I've lately been thinking a bit about these principles and it seems to me that the Single Responsibility Principle (SRP) and the Interface Segregation Principle (ISP) seem to be very much related. In essence you could say that the ISP is simply SRP applied to interfaces.

The notion underlying both is that a type should deal with only a single concept. Whether that applies to the public API or the internal implementation is less relevant because a corollary to the Liskov Substitution Principle (LSP) and Dependency Inversion Principle (DIP) is that we shouldn't really care about the internals (unless we are actually implementing, that is).

The API is what matters.

Although I do understand the subtle differences between SRP and ISP I think they are so closely related that one of them is really redundant. We can remove the ISP and still have a fairly good acronym: SOLD (although SOLID is still better).

There's one principle that I think is missing from this set: The principle about Command/Query Separation (CQS). In my opinion, this is a very important principle that should be highlighted more than is currently the case.

If we add CQS to SOLD, we are left with some less attractive acronyms:

• SCOLD
• COLDS
• CLODS

Not nearly as confidence-inspiring acronyms as SOLID, but nonetheless, I'm striving to write COLDS code.