I think I may be a bit confused on the syntax of the Moq Callback methods. When I try to do something like this:

IFilter filter = new Filter();
List<IFoo> objects = new List<IFoo> { new Foo(), new Foo() };  

IQueryable myFilteredFoos = null;
mockObject.Setup(m => m.GetByFilter(It.IsAny<IFilter>())).Callback( (IFilter filter) => myFilteredFoos = filter.FilterCollection(objects)).Returns(myFilteredFoos.Cast<IFooBar>());

This throws a exception because myFilteredFoos is null during the Cast<IFooBar>() call. Is this not working as I expect? I would think FilterCollection would be called and then myFilteredFoos would be non-null and allow for the cast.

FilterCollection is not capable of returning a null which draws me to the conclusion it is not being called. Also, when I declare myFilteredFoos like this:

Queryable myFilteredFoos;

The Return call complains that myFilteredFoos may be used before it is initialized.