C# 3.0 Func/OrderBy type inference

Question

So odd situation that I ran into today with OrderBy:

Func<SomeClass, int> orderByNumber = 
  currentClass => 
   currentClass.SomeNumber;

Then:

someCollection.OrderBy(orderByNumber);

This is fine, but I was going to create a method instead because it might be usable somewhere else other than an orderBy.

private int ReturnNumber(SomeClass currentClass)
{
  return currentClass.SomeNumber;
}

Now when I try to plug that into the OrderBy:

someCollection.OrderBy(ReturnNumber);

It can't infer the type like it can if I use a Func. Seems like to me they should be the same since the method itself is "strongly typed" like the Func.

Side Note: I realize I can do this:

Func<SomeClass, int> orderByNumber = ReturnNumber;

Answer 1

This could also be related to "return-type type inference" not working on Method Groups .

Essentially, in cases (like Where 's predicate) where the generic parameters are only in input positions, method group conversion works fine. But in cases where the generic parameter is a return type (like Select or OrderBy projections), the compiler won't infer the appropriate delegate conversion.

Answer 2

ReturnNumber is not a method - instead, it represents a method group containing all methods with the name ReturnNumber but with potentially different arity-and-type signatures. There are some technical issues with figuring out which method in that method group you actually want in a very generic and works-every-time way. Obviously, the compiler could figure it out some, even most, of the time, but a decision was made that putting an algorithm into the compiler which would work only half the time was a bad idea.

The following works, however:

someCollection.OrderBy(new Func<SomeClass, int>(ReturnNumber))