What's the difference between Assert.AreNotEqual and Assert.AreNotSame?

Question

In C#, what\'s the difference between

Assert.AreNotEqual

and

Assert.AreNotSame

Answer 1

Two things can be equal, but different objects. AreNotEqual checks the objects values via the equality test, while AreNotSame checks that they are not the same exact object.

It is obvious why we would want to test that things AreNotEqual (we care about the values being tested); what about AreNotSame? The usefulness of this in testing is found when you have passed references around and want to make sure that after your shuffling is done that two references are still the same object.

In a real world case, we use a lot of caching objects to mitigate round trips to the database. After an object has been handed off to the cache system, our unit tests ensure that in some cases we get back the same object (cache was valid) and in other cases we get back a fresh object (cache was invalidated). Note that AreNotEqual would not necessary suffice in this case. If the object had a new timestamp in the database, yet the data was not "different enough" to fail an equality test, AreNotEqual wouldn't notice that we refreshed the object.