How to express an expected call on a Moq mock in the memberdata for an XUnit Theory.

In XUnit you can use the [Theory] attribute to run the same test against a set of different parameters. Moq allows you to specify expectations on which methods of a mocked interface are called, with the Verify method. Recently, I wanted to combine these two techniques to be able to convey the expectation along with the other test parameters to the XUnit Theory. It took me a little thought to get this right. The trick is in casting the expected call (usually an Action<T>) to an Expression<Action<T>>. That is done in the gist below.