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:

	public class CalculatorTests
	{
	[Theory]
	[MemberData(nameof(TestData))]
	public void CanAdd(int value1, int value2, int expected)
	{
	var calculator = new Calculator();
	var result = calculator.Add(value1, value2);
	Assert.Equal(expected, result);
	}
	public static IEnumerable<object[]> TestData =>
	new List<object[]>
	{
	new object[] { 1, 2, 3 },
	new object[] { -1, -2, -3 }
	};
	}

view raw CalculatorTests.cs hosted with ❤ by GitHub

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<>:

	public class CalculatorTests
	{
	[Theory]
	[MemberData(nameof(TestData))]
	public void CanAdd(int value1, int value2, int expected)
	{
	var calculator = new Calculator();
	var result = calculator.Add(value1, value2);
	Assert.Equal(expected, result);
	}
	public static TheoryData<int, int, int> TestData =>
	new TheoryData<int, int, int>
	{
	{ 1, 2, 3 },
	{ -1, -2, -3 }
	};
	}

view raw CalculatorTests.cs hosted with ❤ by GitHub

Nice!