字典鍵值對的鍵是唯一的，如果添加了相同鍵的項就會拋異常，可以通過索引的方式進行重新賦值

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

namespace 字典集合Dictionary
{
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<int, string
 
> dic = new Dictionary<int, string>();
            dic.Add(1, "張三");
            dic.Add(2, "asd");
            dic.Add(3, "fa");
            dic.Add(4, "asd");
            dic.Add(5, "張三");
            //dic.Add(1, "asd");

            dic[1] = "新來的";//字典裡的鍵必須為int型別，通過索引的方式去重新賦值
            foreach
 
 (KeyValuePair<int,string> kv in dic)//kv 就是keyvalue，既代表鍵又代表值
            {
                Console.WriteLine("{0}----{1}", kv.Key, kv.Value);
            }
            Console.WriteLine("============================================");

            foreach (var item in dic.Keys)
            {
                Console.WriteLine(
 
"{0}----{1}", item, dic[item]);
            }
            Console.ReadKey();
        }
    }
}