What is the parameter type for a generic function parameter when called from a base class with 'this'?

Question

Confusing question, I know. Given the following:

    class Test
    {
        public static void GenericFunc<T>(T SomeType)
        {
            System.Console.WriteLine("typeof(T): " + typeof(T).Name);
            System.Console.WriteLine("SomeType.GetType(): " + SomeType.GetType().Name);
        }
    }

    public class BaseType
    {
        public void RunTest() { Test.GenericFunc(this); }
    }

    public class DerivedType : BaseType { }

The following code produces interesting output:

    DerivedType Derived = new DerivedType();
    Derived.RunTest();

    // output:
    // typeof(T): BaseType
    // SomeType.GetType(): DerivedType

However, this behaves as I would expect:

    Test.GenericFunc(new Derived());

    // output:
    // typeof(T): DerivedType
    // SomeType.GetType(): DerivedType

Can anyone help me understand the mechanism here that's causing T to be evaluated as BaseType in the first case?

Thanks in advance!

Answer 1

Because this:

public class BaseType
{
    public void RunTest() { Test.GenericFunc(this); }
}

Is basically equivalent to:

public class BaseType
{
    public void RunTest() { Test.GenericFunc<BaseType>(this); }
}

Therefore the GenericFunc gets instatiated at compile time with T = BaseType. However the this object you are passing in at runtime is the derived type which you get by SomeType.GetType().

In the second case the compiler infers the type as DerivedType directly from the usage and therefore GenericFunc gets instatiated with T = DerivedType.