ConcurrentDictionary的ContainsKey 保证了多线程读取、写入没问题，线程安全。但是并不能保证Key 重复 。这里多线程的时候，Key可能重复导致Add失败，请优先使用 AddOrUpdate 。

 

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
using System.Diagnostics;
using System.Reflection;

namespace MulThreadTest
{

 //下面的示范是一个并发插入错误的写法。
///方案1：加锁--lock (ReaderWriterLockSlim)；方案2：使用TryAddOrUpdate
    class MainClass
    {
        public static void Main (string[] args)
        {

    
            var dic = new ConcurrentDictionary<int,int> ();


            for (int i = 0; i < 10; i++) {
                
                var th = new Thread (new ParameterizedThreadStart ((state) => {

                    var num = DoHelper.Instance.SayHello ();

                    if (dic.ContainsKey (num)) {
                        dic.AddOrUpdate (num, num, (m, n) => {
                            return n;
                        });
                        Console.WriteLine ("AddOrUpdate: " + num.ToString ());

                    } else {
                        var addRes = dic.TryAdd (num, num);

                        if (false == addRes) {
                            Console.WriteLine ("Error add. num exits");
                        }
                    }
                }));    


                th.Start (i);
            }

            Console.ReadKey ();

        }




    }



    public class DoHelper
    {

        public static DoHelper Instance = new DoHelper ();

        public int SayHello ()
        {
             
            var num = new Random (DateTime.Now.Millisecond).Next ();

            return num;
             
        }
    
    }
}

参考：

https://referencesource.microsoft.com/#mscorlib/system/Collections/Concurrent/ConcurrentDictionary.cs,4d0f4ac22dbeaf08