using System;
 
public class Repository: Key.CreateProxy{
 
    public void Add(int data){
 
        Key key = Create();
 
        // 以下データの保存処理とか
    }
}
 
 
public class Key{
 
	public int Value{get; private set;}
 
	// 外部からの生成禁止
	private Key(int i){
		Value = i;
	}
 
	// このプロキシを継承したクラスでしか生成できない
	internal class CreateProxy{
		static int s_Increment = 0;
 
		protected static Key Create(){
			return new Key(s_Increment++);
		}
	}
}

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