using System;

using System.Diagnostics;

​

public class Program

{

    public static void Main()

    {

        var a = new GenericTypeClass<TestType>();

        var b = a as Formal;

        if (b == null)

            Console.WriteLine("'a as Formal' Is NULL");

        try

        {

            var c = (Formal)a;

        }

        catch (Exception ex)

        {

            Console.WriteLine("'(Formal)a' gives this error: " + ex.Message);

        }

    }

}

​

public class Formal : GenericTypeClass<TestType>

{

}

​

public class GenericTypeClass<T>

    where T : class, IType

{

}

​

public class TestType : IType

{

}

​

public interface IType

{

}