NUnit: How to pass TestCaseData from a non-static method?

🧟‍♀️ Zombie response, but better late than never. 🧟

Another way to accomplish this is to have your test case data source return a function object that accepts the non-static members you need as its parameter(s). Then your test calls that to create the data that you wish NUnit could pass into you.

In your case, that looks something like:

private static IEnumerable<TestCaseData> GetTestDataA()
{
    yield return new TestCaseData(72.5,   new Func<Qv_ges, double>( qvGesQuer => qvGesQuer.FL.Value ), MAX_DELTA);
    yield return new TestCaseData(169.17, new Func<Qv_ges, double>( qvGesQuer => qvGesQuer.RL.Value ), MAX_DELTA);
    yield return new TestCaseData(241.67, new Func<Qv_ges, double>( qvGesQuer => qvGesQuer.NL.Value ), MAX_DELTA);
    yield return new TestCaseData(314.17, new Func<Qv_ges, double>( qvGesQuer => qvGesQuer.IL.Value ), MAX_DELTA);
}

[TestCaseSource( nameof(GetTestDataA) )]
public void MethodA( double expected, Func<Qv_ges, double> getValue, double latitude)
{ 
    Assert.AreEqual( expected, getValue( Qv_ges_Quer ), latitude );
}

If you need multiple parameters, add them to the functor's and lambda's parameters or consider passing in this instead. If you need multiple return values, make the function object return a tuple:

new Func<Qv_ges, (double, double)>( qvGesQuer => (qvGesQuer.RL.Value, qvGesQuer.IL.Value) )

A different approach is to pass in nameof() strings as the test params and use reflection to get the values of those parameters.

By design, the method, property or field used by the TestCaseSourceAttribute must be static. This is intended to avoid the need to instantiate the fixture class at the time the tests are loaded. Your fixture is only instantiated when we start the run - in the case of the GUI, each time we start the run - and its lifetime is only as long as it takes to run the fixture.

In your case, you have appear to have discovered that you can use a static method. That's best, if possible.

The only way to use instance methods here is to use the constructor TestCaseSourceAttribute(Type sourceType) where sourceType implements IEnumerable and returns your test case data directly. If you use this, I recommend using a different class from your TestFixture. It's not absolutely necessary. If you use the same class, different instances will be created at load time and run time, which have no connection whatsoever with one another. Many developers end up getting confused by this and try to leave state behind at load time for use by the tests. That won't work.