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RFC 002- Framework Extensibility for Custom Assertions


This details the MSTest V2 framework extensibility for extending the Assert class to add custom assertions.


Often times, the default set of assertion APIs are not sufficient to satisfy a wide range of requirements for unit test writers. In most of these situations users end up having utility methods to address this need reducing discoverability in a test suite. If the test framework provides an extensibility in the assertion infrastructure itself, custom assertion functionality can be

  • Easily accessed
  • Easily organized and
  • Possibly shared with the community.

Detailed Design


  1. Custom assertions should be easily pluggable into the test frameworks assertion infrastructure.
  2. Users of custom assertions should be able to acquire and use them with ease.

Proposed solution

Here is a solution that is both easily pluggable and acquirable:

The test frameworks Assertion class should be a non-static singleton with a C# Property('That') for accessing the instance:

public class Assert
    public static Assert That
            // ...

Extension writers can then add C# extension methods for the Assertion class like below:

public static class SampleAssertExtensions
    public static void IsOfType<T>(this Assert assert, object obj)
        if(obj is T)
        throw new AssertFailedException("Type does not match");

And consumers of this extension can consume it in their test code with the below simple syntax:

using SampleAssertExtensionsNamespace;

public void TestMethod
    // ...

Benefits for custom assertion writers

  1. Leverages the default C# constructs - No new interfaces/objects to understand and extend.
  2. Extensions can be organized under a verb. For instance assertions expecting exceptions can be organized under the Throws verb like
  1. Ability to create a chain of assertions in a single assert. For instance

Benefits for custom assertion consumers

  1. Easily discoverable - Intellisense shows up in most IDEs ensuring discoverability for these custom assertions since they are all rooted under the in-box Assert class.
  2. Readable - Using linq type expressions enhances readability.

Open questions

  1. How important are combined asserts in a single Assert statement (Assert.That.Something.And.Something) and how much of this should be available in-box?
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