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XUnit–Improve type safety

While doing a code review, I discovered a feature in XUnit I didn't know it existed. Let me share what I discovered. 

I've been used to specify data for my parameterised tests either using the [InlineData] attribute or through the [MemberData] or [ClassData] attributes.

When using [MemberData] or [ClassData], XUnit expects that you return an IEnumerable<object[]> as far I as I know.

Here is an example:

If I try to use a typed alternative, it results in a compiler error:

However it turns out that there is a type safe alternative available through TheoryData<>.

The TheoryData<> types provide a series of abstractions around the IEnumerable<object[]> required by theory tests. It consists of a TheoryData base class, and a number of generic derived classes TheoryData<>. It can be used in combination with both the [MemberData] or [ClassData] attributes while enforcing type safety.

Here is our original example rewritten to use TheoryData<>:

Nice!

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