C# Generics - Interface declaration

I was challenged recently with a question about generics with constraints.

The claim was that it’s only a compile flaw that allows your to declare an interface for a generic type with a constraint. Namely that the syntax

public interface ISomething <T> where T: SomeClass

would pass compilation but would not be useful (runtime) because you can’t declare a variable

ISomething myVar = new ISomething<SomeClass>();

or something to that extent. I went home feeling a bit uneasy about the discussion, then coded this up the way I see it. While it is completely true that you can’t ‘new’ an interface, _using _an interface that has a generic type is completely possible and legal.

Here it is in all it’s g(l)ory.

using System;
using System.IO;
using System.Text;
/// Demo of generic interface declaration with constraint.
/// Showing compilation and runtime feasibility.
namespace NH.Demo
{
	class Demo
	{
		static void Main(string[] args)
		{
			ITryInstance o1 = new GenericInstanceA();
			ITry<MyDerivedType> o2 = new GenericInstanceB();
			Console.WriteLine("Generic instance 1 " + o1.ProperT.SomeField);
			Console.WriteLine("Generic instance 2 " + o2.ProperT.SomeField);
		}
	}

	interface ITry<T> where T : MyBaseType
	{
		T ProperT{ get; }
	}
	
	public class MyBaseType
	{
		public string SomeField;
	}
	
	public class MyDerivedType : MyBaseType
	{
		public MyDerivedType(string arg)
		{
			base.SomeField = arg;
		}

	}

	interface ITryInstance : ITry<MyDerivedType>
	{
	}

	/// <summary>
	/// this will fail. cosntraint violation
	/// "The type 'string' must be convertible 
	/// to 'NH.Demo.MyBaseType' in order to use it as 
	/// parameter 'T' in the generic type or 
	/// method 'NH.Demo.ITry<T>'"
	/// </summary>
	//interface IFail : ITry<string> { }
	public class GenericInstanceA : ITryInstance
	{
		MyDerivedType ITry<MyDerivedType>.ProperT
		{
			get { return new MyDerivedType("hi there!"); }
		}
	}

	public class GenericInstanceB : ITry<MyDerivedType>
	{
		MyDerivedType ITry<MyDerivedType>.ProperT
		{
			get { return new MyDerivedType("hi there! again"); }
		}
	}
}