Rename `ModelMetadata.IsCollectionType`; perhaps provide real `IsCollectionType` as well??

[dougbu]

The ModelMetadata.IsCollectionType property actually checks whether ModelType implements IEnumerable. Since this property is new in MVC 6, we could rename it to match the actual meaning -- IsEnumerableType.

Product code uses ClosedGenericMatcher.ExtractGenericInterface() to find T for ICollection<T> in a few places where ModelMetadata is available. These cases could be handled using ModelMetadata.ElementMetadata and a "real" ModelMetadata.IsCollectionType or ModelMetadata.IsGenericCollectionType.

Final step along these lines would be to add ModelMetadata.TaskTypeParameter or ModelMetadata.TaskTypeParameterMetadata. This could replace uses of ClosedGenericMatcher.ExtractGenericInterface() to find T for Task<T> -- another repeated gesture.

These changes would make ModelMetadata more usable and broadly useful. Would also avoid lower-level helper methods and repeated code -- in our code and in customer code.

[rynowak]

I think it would be fine to pivot on IEnumerable vs ICollection and have both. I'd say if this particular change is good cleanup, go ahead and do it.

Overall though, I'm skeptical about our ability to remove all uses of ClosedGenericMatcher or to move them "behind the scenes". The suggested changes for Task<T> seem like a bad fit in general because tasks are rarely used as/in model types.

[dougbu]

d8d0a1a