C# - Method inside generic class with additional constraints to T

Question

I have a generic class Foo<T> where T: SomeBaseType.

And in the specific case where T is SpecificDerivedType, I want my class to have an additional method.

Something like:

class Foo<T> where T: SomeBaseType
{
    //.... lots of regular methods taking T, outputting T, etc.

    //in pseudo code
    void SpecialMethod() visible if T: SpecificDerivedType
    {
        //...code...
    }
}

How can I achieve this?

Answer 1

Make an extension method for Foo<T>:

public static void SpecialMethod<T>(this Foo<T> foo)
    where T : SpecificDerivedType
{
}

Answer 2

How can I achieve this?

You can't. Create a specialized Foo.

class SpecialFoo<T> : Foo<T> where T: SpecificDerivedType
{
    void SpecialMethod()
    {
        //...code...
    }
}

Other suggest an extension method, which is obviously not the same as what you ask. It might be a workaround though.