Unit Testing Microsoft.Composition

When using composition heavily in an application it can be useful to verify that composition succeeds via unit tests.

The challenge of unit testing composition code is to ensure that parts don't execute inappropriately during this testing. Unit tests for the parts themselves verify their behavior using mocks and other standard techniques that isolate the parts at test time.

The goal of composition testing is to ensure that a part can be created using the exports provided by other parts at run-time. For each part of interest, we want to verify that its imports can be satisfied by other parts, that there are no circular dependencies, and that no contracts are over-supplied.

This page provides some guidance and sample code for writing these kinds of tests.

The CompositionAssert class

The CompositionAssert class below provides two overloads of the CanExportSingle() assertion.

public static class CompositionAssert
    public static void CanExportSingle(CompositionContext context, Type contractType, string contractName = null, IDictionary<string, object> metadataConstraints = null)
        var lazyType = typeof(Lazy<>).MakeGenericType(contractType);
        var lazyContract = new CompositionContract(lazyType, contractName, metadataConstraints);

    public static void CanExportSingle<TContract>(CompositionContext context, string contractName = null, IDictionary<string, object> metadataConstraints = null)
        CanExportSingle(context, typeof(TContract), contractName, metadataConstraints);

These assertions are used like so:

public void WorkItemsControllerCanBeComposed()
    var host = CreateCompositionHost();

The CanExportSingle() method takes a contract type and checks that it can be composed. It does this by transforming the contract T into Lazy<T> under the hood before requesting an instance from the container. When lazy dependencies are used, the container won't actually create an instance of the requested export, so in the example, no WorkItemsController is created for the test. However, the container does validate lazy dependencies, so the test will only pass if WorkItemsController and all of its dependencies are correctly configured.

In this example CreateCompositionHost() stubs out whatever is necessary to create the container in exactly the state that is used at runtime.

Which contracts should I test?

CanExportSingle() does a deep check of dependencies, traversing all relationships including Lazy<T>, ExportFactory<T> and [ImportMany].

Therefore, it is only necessary to check exported contracts that are requested through either:
  • Direct calls to CompositionContext.GetExport(), CompositionContext.GetExports() and CompositionContext.TryGetExport(), or
  • Imported properties on objects passed to CompositionContext.SatisfyImports()

These are the Composition Roots of the application. They will generally be few, for example, the controllers in an ASP.NET MVC or Web API application, or ViewModels in an MVVM application.

What problems are not caught?

Currently, this technique will not reveal lifetime problems, for example requesting an instance of a part outside of a sharing boundary that it is constrained by.

Last edited Jun 26, 2012 at 1:21 AM by nblumhardt, version 3


JamieClayton Jul 30, 2012 at 10:49 AM 
In VS2012 RC with DotNet Framework 4.0 I get compile errors.
Had to modify CompositionContext to CompositionContainer
Unable to find reference to CompositionContract