Mocking a method that takes an in parameter

[foxesknow]

I have a method that takes a large readonly struct as an in parameter. I'd like to setup a mock so that I can match against a certain property on the struct.

Because it's passed as in I need to use It.Ref. However, there's no option to attach a predicate. For example I'd like to say:

myMock1.Setup(x => x.Visit(ref It.Ref<LargeStruct>.Is(r = r.UserID == 10))

The only option available is to match any instance of LargeStruct which isn't a narrow enough condition.

Is there an alternative way to handle this scenario?

[stakx]

Unfortunately, something like It.Ref<T>.Is(predicate) cannot currently be implemented, because the C# compilers do not allow ref returns in LINQ expression trees. (If you try to do so, you'll get an error CS8153: An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference.) So as far as I can tell, what you're asking for is currently not possible.