What steps will reproduce the problem?

If you create those classes :

class Foo that implements IEquatable<Foo> interface IBar with method DoSomething(Foo foo)

You then create two different Foo instances : foo1 and foo2. According to your IEquatable<Foo> implementation, foo1 is equal to foo2.

What is the expected output? What do you see instead?

if you mock interface IBar and call DoSomething(foo1) and then ask the mock to verify if DoSomething was called with foo2, the verify will fail.

I expected it to pass but instead I had to override Object.Equals as well.

Is this by design?

What version of the product are you using?

Moq 4.0.10827.0

This bothers me a lot. NUnit works perfectly fine with IEquatable but sadly Moq not yet :(