static void Main(string[] args)
        {
            //定義一個list
            List<string> list = new List<string> { "1", "2", "3", "4" };
            System.Diagnostics.Stopwatch wacth = new System.Diagnostics.Stopwatch();
            wacth.Start();
            //多執行緒的foreach，會開多個執行緒同時執行Handle方法
 

            Parallel.ForEach(list, p => Handle(p));
            wacth.Stop();
            Console.WriteLine(wacth.ElapsedMilliseconds);
            System.Diagnostics.Stopwatch wacth1 = new System.Diagnostics.Stopwatch();
            wacth1.Start();
            //一個執行緒執行Handle方法
            foreach
 
 (var p in list)
            {
                Handle(p);
            }
            wacth1.Stop();
            Console.WriteLine(wacth1.ElapsedMilliseconds);  
        }
        static void Handle(string str)
        {
            Console.WriteLine(str);
            Thread.Sleep(1000);
        }

得到的結果

發現多執行緒執行完之後輸出的順序不再是1，2，3，4