Question

I’ve encountered something quite surprising when using generic constraints with inheritance. I have an overloaded methods Foo that differ with parameter – either base or derived class instance. In both cases it’s generally just passing the instance to the second pair of overloaded methods – Bar.

When I call Foo with base class instance, Bar overload for the base class is called. When I call Foo with derived class instance, Bar overload for the derived class is called. This is clear and expected.

But when I tried to merge Foo methods into single one GenericFoo that use generics and constraints, methods are resolved differently – T is resolved correctly, but only base-class overload of Bar is called.

public class Animal { }
public class Cat : Animal { }

public class AnimalProcessor
{
    public static void Foo(Animal obj)
    {
        Console.WriteLine("Foo(Animal)");
        Bar(obj);
    }

    public static void Foo(Cat obj)
    {
        Console.WriteLine("Foo(Cat)");
        Bar(obj);
    }

    // new generic method to replace the two above
    public static void GenericFoo<T>(T obj)
        where T : Animal
    {
        Console.WriteLine("Foo(generic)");
        Bar(obj);
    }

    public static void Bar(Animal obj)
    {
        Console.WriteLine("Bar(Animal)");
    }

    public static void Bar(Cat obj)
    {
        Console.WriteLine("Bar(Cat)");
    }
}

Testing code – two first cases for non-generic old methods, two last for new generic method.

Console.WriteLine("Animal()");
AnimalProcessor.Foo(new Animal());
Console.WriteLine();

Console.WriteLine("Cat()"); 
AnimalProcessor.Foo(new Cat());
Console.WriteLine();

Console.WriteLine("Animal()");
AnimalProcessor.GenericFoo(new Animal());
Console.WriteLine();

Console.WriteLine("Cat()"); 
AnimalProcessor.GenericFoo(new Cat());
Console.ReadLine();

And the result – note the difference in type resolved in Bar:

Animal()
Foo(Animal)
Bar(Animal)

Cat()
Foo(Cat)
Bar(Cat)

Animal()
Foo(generic)
Bar(Animal)

Cat()
Foo(generic)
Bar(Animal)

It looks like the compiler binds all calls from GenericFoo to the least specific overload, even if all more specific-typed calls are known at compile time. Why is that, what is the reason for such behaviour? Which part of specs defines this?

Answer 1

Per OP’s request, comment re-posted as answer:

Generics are not templates. Generic methods are compiled once and their behavior is for the ‘most generic’ case (in this case, Animal.) This is different from C++ style templating, where the template is compiled separately for each specialization by type.