using System;
 
public interface IFriendKey { object Id {get; set;} }
 
class Friend<TFriend>
{
	protected void FriendAssert(IFriendKey key)
	{
		if ( key == null || key.Id == null || key.Id.GetType() != typeof(TFriend) )
			throw new Exception("No right to execute the called method.");
	}
}
 
class A : Friend<B>
{
	public void f(IFriendKey key)
	{
		FriendAssert(key);
		Console.WriteLine("ONLY class B can execute this method successfully, even though it is declared public.");
	}
}
 
class B
{
	private class AFriendKey : IFriendKey 
	{
		public object Id {get; set;}
	}
 
	IFriendKey Key { get { return new AFriendKey() {Id = this}; } }
 
	public void g()
	{
		new A().f(this.Key);
	}
}
 
public class Test
{
	public static void Main()
	{
		new B().g();
	}
}

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