c# - How to use method hiding (new) with generic constrained class

Question

I have a container class that has a generic parameter which is constrained to some base class. The type supplied to the generic is a sub of the base class constraint. The sub class uses method hiding (new) to change the behavior of a method from the base class (no, I can't make it virtual as it is not my code). My problem is that the 'new' methods do not get called, the compiler seems to consider the supplied type to be the base class, not the sub, as if I had upcast it to the base.

Clearly I am misunderstanding something fundamental here. I thought that the generic where T: xxx was a constraint, not an upcast type.

This sample code basically demonstrates what I'm talking about.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace GenericPartialTest
{
    class ContextBase
    {
        public string GetValue()
        {
            return "I am Context Base: " + this.GetType().Name;
        }

        public string GetOtherValue()
        {
            return "I am Context Base: " + this.GetType().Name;
        }

    }

    partial class ContextSub : ContextBase
    {
        public new string GetValue()
        {
            return "I am Context Sub: " + this.GetType().Name;
        }
    }

    partial class ContextSub
    {
        public new string GetOtherValue()
        {
            return "I am Context Sub: " + this.GetType().Name;
        }
    }

    class Container<T> where T: ContextBase, new()
    {
        private T _context = new T();

        public string GetValue()
        {
            return this._context.GetValue();
        }

        public string GetOtherValue()
        {
            return this._context.GetOtherValue();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Simple");
            ContextBase myBase = new ContextBase();
            ContextSub mySub = new ContextSub();

            Console.WriteLine(myBase.GetValue());
            Console.WriteLine(myBase.GetOtherValue());
            Console.WriteLine(mySub.GetValue());
            Console.WriteLine(mySub.GetOtherValue());

            Console.WriteLine("Generic Container");
            Container<ContextBase> myContainerBase = new Container<ContextBase>();
            Container<ContextSub> myContainerSub = new Container<ContextSub>();

            Console.WriteLine(myContainerBase.GetValue());
            Console.WriteLine(myContainerBase.GetOtherValue());
            Console.WriteLine(myContainerSub.GetValue());
            Console.WriteLine(myContainerSub.GetOtherValue());


            Console.ReadKey();
        }
    }
}

Edit:

I guess my confusion comes from that one can do this

class SomeClass<T> where T: AnotherType, new()
{
    T foo = new T();     
}

And I expected T to be T even though I understand the compiler would view T as having AnotherType's interface. I assumed the typing of T would happen at run-time even if the interface of T was set at compile time. The T foo declaration seems misleading here because it is really doing

AnotherType foo = new T();

Once I understand that it is not really declaring foo as type T, it is understandable why the new method hiding wouldn't work.

And that's all I have to say about that.

Answer 1

Methods declared new have no relation (from the compiler's perspective) to methods with the same name/signature in the base class. This is simply the compiler's way of allowing you to define different methods in derived classes that share a signature with a method in their base class heirarchy.

Now, with regard to your specific case, realize that generics have to compile to a single set of bytecode regardless of the types that are supplied as generic parameters. As a result, the compiler only knows about the method and properties that are defined on the generic type T - that would be the base type you specify in the generic constraint. The compiler knows nothing about the new methods in your derived type, even if you create an instance of a generic type with the derived type as the parameter. Therefore calls in the generic class will always go to the methods of the base type.

There's a lot of confusion about new/virtual/override; take a look at this SO question - Jason and Eric's answers are excellent. Jon Skeet's answer to a similar question may also help you understand why your implementation behaves the way it does.

There are two possible ways for you to work around this issue:

1. Perform a conditional cast (based on runtime type information) to the derived type (or an interface) in your generic class. This breaks encapsulation and adds undesirable coupling. It's also fragile if implemented poorly.
2. Define an interface that you use in your generic constraint that exposes the methods you care about. This may not be possible if the code you are deriving from is not something you can change.