C# UnitTest - Assert.AreEqual() does not call Equals if the argument is null

Question

i recently stumbled upon a seemingly weird behavior that Google completely failed to explain.

using Microsoft.VisualStudio.TestTools.UnitTesting;

class TestClass
{
    public override bool Equals(object obj)
    {
        return true;
    }
}

[TestMethod]
public void TestMethod1()
{
    TestClass t = new TestClass ();
    Assert.AreEqual (t, null); // fails
    Assert.IsTrue (t.Equals (null)); // passes
}

I would expect this test to succeed. However, in Visual Studio 2008 / .NET 3.5 it fails. Is it intended to be like that or is it a bug?

Answer 1

Your TestClass violates the contract of Object.Equals. Assert.AreEqual is relying on that contract, quite reasonably.

The docs state (in the list of requirements):

• x.Equals(a null reference (Nothing in Visual Basic)) returns false.

Answer 2

When testing for nulls, do not use Assert.AreEqual.

You have to use Assert.IsNull() for that.