Parameterized Auto Factory for the Unity IoC Container

A UnityContainer extension inspired by Autofac's Parameterized Instantiation.

What is it for?

It lets you supply certain parameters of the target type's constructor by passing them to the auto-generated factory. While taking advantage of the container to resolve and inject all the other parameters that you don't want to supply manually.

In other words. Unity generates a factory Func for constructor parameters of the form Func<IDependency>.

public class TargetType
{
    // Unity will generate createDependency for us.
    public TargetType(Func<IDependency> createDependency) { /* ... */ }
}

This extension takes it one step further. It automatically uses parameter overrides when

• it sees a dependency of the form Func<IDependencyOfDependency, IDependency>
• and the concrete class implementing IDependency needs a parameter of type IDependencyOfDependency.

This only kicks in if Func<IDependencyOfDependency, IDependency> is not explicitely registered.

Here is a quick example presented as an xUnit test.

public interface IDialogBoxService { /* ... */ }
public interface IUserListDataSource { /* ... */ }

public class UsersGridWindow
{
    public UsersGridWindow(string windowTitle, 
                           IUserListDataSource userListDataSource, 
                           IDialogBoxService dialogBoxService)
    {
        WindowTitle = windowTitle;
        UserListDataSource = userListDataSource;
        DialogBoxService = dialogBoxService;

        /* ... */
    }

    public string WindowTitle { get; }
    public IUserListDataSource UserListDataSource { get; }
    public IDialogBoxService DialogBoxService { get; }

    /* ... */
}

public class CachedUserListDataSource : IUserListDataSource
{
    public void WarmUp() { /* ... */ }
}

[Fact]
public void Factory_parameters_overrides_matching_constructor_parameters()
{
    // Setup the container
    var container = new UnityContainer()
        .AddNewExtension<UnityParameterizedAutoFactoryExtension>();

    // Here the extension kicks in and generates 
    // a parameterized factory of type Func<string, IUserListDataSource, UsersGridWindow>.
    // Of course, in a real app Func<string, IUserListDataSource, UsersGridWindow> would 
    // likely have been a constructor parameter.
    var createUsersGridWindow = container
        .Resolve<Func<string, IUserListDataSource, UsersGridWindow>>();

    // Now, let's try to show a scenario which illustrates why
    // a parameterized auto-factory can be useful.

    // Create and warm up a cached data source.
    // We can re-use the warmed up cache for any number of UsersGridWindow instances
    // or other windows which depend on IUserListDataSource.
    var cachedUserListDataSource = new CachedUserListDataSource();
    cachedUserListDataSource.WarmUp();

    // Pick window title for this particular window instance.
    // We can pick another title for another window instance.
    const string windowTitle = "Registered users";

    // Create the window.
    var usersGridWindow = createUsersGridWindow(windowTitle, cachedUserListDataSource);

    // Let's make sure the parameters were overridden as expected.
    Assert.Equal(usersGridWindow.WindowTitle, windowTitle); // We overrode this one.
    Assert.Same(usersGridWindow.UserListDataSource, cachedUserListDataSource); // And this one too.

    Assert.NotNull(usersGridWindow.DialogBoxService); // We didn't override DialogBoxService,
                                                      // and so it was resolved from the container.
}

Download and install

Unity v4.0.1

In case Unity v4.0.1 is used in your project, install ParameterizedAutoFactory.Unity4 version of this extension.

To install it run the following command in the NuGet Package Manager Console.

Install-Package ParameterizedAutoFactory.Unity4

Unity v5.x

In case Unity v5.x is used in your project, install ParameterizedAutoFactory.Unity5 version of this extension.

To install it run the following command in the NuGet Package Manager Console.

Install-Package ParameterizedAutoFactory.Unity5

This will download all the binaries, and add necessary references to your project.

How to use it?

One way to register the extension in container is to use the Unity.UnityContainerExtensions.AddNewExtension extension method.

var container = new UnityContainer()
    .AddNewExtension<UnityParameterizedAutoFactoryExtension>();

A call to Unity.UnityContainerExtensions.AddNewExtension parameterized by UnityParameterizedAutoFactoryExtension is approximately equivalent to executing the snippet below.

var extension = (UnityParameterizedAutoFactoryExtension)container.Resolve(
    typeof(UnityParameterizedAutoFactoryExtension));
container.AddExtension(extension);

Thank you!

• This extension is inspired by Autofac's Parameterized Instantiation.
• UnityAutoMoq gave a great example of extension hooking into Unity's dependencies instantiation/resolution pipeline.

License

The extension is licensed under the MIT license.