Improve failure message for Assert.IsNull

[drieseng]

Summary

When a test failure is reported by Assert.IsNull(...), no information is provided on the actual value.

Background and Motivation

When you assert that a given "value" (object reference) is null, the failure that is reported when it isn't null could provide more information on the "value".

This is definitely useful if the type of value implements IFormattable or is a nullable value type for which the underlying type implements IFormattable.

To reproduce, run the following test class:

using Microsoft.VisualStudio.TestTools.UnitTesting;

[TestClass]
public class Foo
{
    [TestMethod]
    public void IsNull()
    {
        string actual = "XX";

        Assert.IsNull(actual);
    }
}

Expected behavior

The following test failure is reported:
Assert.IsNull failed. Expected:<null>. Actual:<XX>. 'value' expression: 'actual'.

Actual behavior

The following test failure is reported:
Assert.IsNull failed. 'value' expression: 'actual'.

[Evangelink]

This is covered by RFC 011 — Structured Assertion Messages, which redesigns all assertion failure messages to follow a structured multi-line format.

Specifically, the new Assert.IsNull failure message will include the actual value:

Assertion failed. Expected value to be null.

actual: "some value"

Assert.IsNull(actual)
   at ...

The RFC is currently under discussion. Once approved and implemented, this issue will be resolved.

[Evangelink]

Fixed by #8187