Mocking ICollection property in entity?

You can't mock the Count method you are using, because it is an extension method. It isn't a method defined on ICollection.

There's no need to mock the collection when you can simply verify whether the exception was thrown using the real collection rather than a mock. As you're using MsTest rather than NUnit, you can't simply add an ExpectedException attribute to verify that the exception is thrown, but you can do something like the following.

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