Yesterday I wanted to upgrade the NUnit testing framework we use in our project to the latest stable version. We used 2.5.10 and it had reached 2.6.0. I simply removed the old version and replaced it with the new. Some of the tests failed.

Investigating revealed all tests had something in common: they were testing if two collections are not equal (meaning not the same instance) then that the collections are not equivalent (meaning none of the items in one collection is found in the other), yet that the values in the items are the same. Practically it was a test that checked if a cloning operation was successful. And it failed because from this version on, the two collections were considered Equal and Equivalent.

That is at least strange and so I searched the release notes for some information about this and found this passage: EqualConstraint now recognizes and uses IEquatable<T> if it is implemented on either the actual or the expected value. The interface is used in preference to any override of Object.Equals(), so long as the other argument is of Type T. Note that this applies to all equality tests performed by NUnit.

Indeed, checking the failing tests I realized that the collections contained IEquatable types.


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