Non-static TestCaseSource/ValueSource

[manfred-brands]

At the moment we require the members that are passed into the TestCaseSource and ValueSource attributes to be static.

However, if the name is null we assume the TestFixture itself is an IEnumerable and we actually instantiate the fixture to return the test parameters.

What is the reason, not to allow instance fields for the other source members?
Code wise the change wouldn't be too difficult.

object instance = member.IsStatic ? null : Reflect.Construct(sourceType, null)

Then update all further code to pass instance instead of null?

[stevenaw]

@manfred-brands I found a bit of a write up about the history of this here: #3532 (comment)

That comment mentions avoiding multiple construction of objects. My (perhaps incorrect) assumption had been that it also could encourage more deterministic behavior by avoiding referencing instance data when generating test cases.

I admit, I haven't read the whole thread just yet. And, of course, decisions can always be revisited.

@jnm2 At one point you were advocating for going further towards statics in #3537. I'm unsure if that was simply for API consistency or if you had other design considerations in mind too at the time?