Update documentation to clarify passing parameters to test cases

[aaronfranke]

I am using NUnit 3.13.3 and NUnit3TestAdapter 4.2.1 with .NET Framework 4.7.2 and Visual Studio 2019.

I am not able to pass arrays or lists. I have the method return type set to an array/list, and so is the return value and the parameter type. When I try, I get one of these error messages:

MyMethodTest(MyType) Failed	60 ms
System.ArgumentException : Object of type 'MyType' cannot be converted to type 'MyType[]'.

MyMethodTest(MyType) Failed	60 ms
System.ArgumentException : Object of type 'MyType' cannot be converted to type 'System.Collections.Generic.List`1[MyType]'.

I also tried setting the return type to TestCaseData and wrapping in new TestCaseData(retval) but that doesn't work either. The only solution I have found is to set the return type to TestCaseData[] and wrap like this:

return new TestCaseData[] { new TestCaseData(retval) };

This issue seems to be the same as #1327 and it looks like there has been two rounds of attempted fixes but it still doesn't work.

[mikkelbu]

@aaronfranke Can you provide a more complete example ?

The following gives me 4 test cases (whereof 2 fails)

    public class Tests
    {
        [TestCaseSource(nameof(GetData))]
        public void TestArrayLengthLargerThanTwo(int[] myArray)
        {
            Assert.That(myArray.Length, Is.GreaterThan(2));
        }

        public static IEnumerable<int[]> GetData()
        {
            yield return new[] { 1, 2, 3 };
            yield return new[] { 1 };
            yield return new[] { 2, 4, 6 };
            yield return new[] { 3 };
        }
    }

[aaronfranke]

@mikkelbu Here are two examples that replicate the problem I'm facing:

using NUnit.Framework;

class ExampleClass
{
    public ExampleClass() { }
}

[TestFixture]
class ExampleClassTest
{
    private static ExampleClass[] ExampleTestData()
    {
        ExampleClass[] array = new ExampleClass[2];
        array[0] = new ExampleClass();
        array[1] = new ExampleClass();
        return array;
    }

    [Test]
    [TestCaseSource(nameof(ExampleTestData))]
    public void ExampleTest(ExampleClass[] data)
    {
        Assert.That(true);
    }
}

Test	Duration	Traits	Error Message
ExampleTest(ExampleClass) Failed	61 ms		System.ArgumentException : Object of type 'ExampleClass' cannot be converted to type 'ExampleClass[]'.

using NUnit.Framework;
using System.Collections.Generic;

class ExampleClass
{
    public ExampleClass() { }
}

[TestFixture]
class ExampleClassTest
{
    private static List<ExampleClass> ExampleTestData()
    {
        List<ExampleClass> list = new List<ExampleClass>();
        list.Add(new ExampleClass());
        list.Add(new ExampleClass());
        return list;
    }

    [Test]
    [TestCaseSource(nameof(ExampleTestData))]
    public void ExampleTest(List<ExampleClass> data)
    {
        Assert.That(true);
    }
}

Test	Duration	Traits	Error Message
ExampleTest(ExampleClass) Failed	90 ms		System.ArgumentException : Object of type 'ExampleClass' cannot be converted to type 'System.Collections.Generic.List`1[ExampleClass]'.

[aaronfranke]

Based on your example I think I see what's wrong. NUnit is pulling out each element of the returned List and using that as test case data. The second example I posted works if I change the test data method to this:

    private static IEnumerable<List<ExampleClass>> ExampleTestData() {
        List<ExampleClass> list = new List<ExampleClass>();
        list.Add(new ExampleClass());
        list.Add(new ExampleClass());
        return new List<List<ExampleClass>> { list };
    }

The documentation for TestCaseSource needs to be updated. Currently it just says Indicates the source to be used to provide test fixture instances for a test class. and does not mention anything about the return type being an IEnumerable containing the actual data that's passed to the test method.

[mikkelbu]

Documentation PRs are welcome. The online documentation specifies all the requirements https://docs.nunit.org/articles/nunit/writing-tests/attributes/testcasesource.html

The second argument is a string representing the name of the source used to provide test cases. It has the following characteristics:

• It may be a field, property or method in the test class.
• It must be static. This is a change from NUnit 2.x.
• It must return an IEnumerable or a type that implements IEnumerable. For fields an array is generally used. For properties and methods, you may return an array or implement your own iterator.
• The individual items returned by the enumerator must be compatible with the signature of the method on which the attribute appears. See the Test Case Construction section below for details.