控制对 Parallel.For 循环中变量的访问

using System;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static void Main( string[] args )
    {
        object sync = new object();
        int sum = 0;
        Parallel.For( 1, 1000, ( i ) => {
            lock( sync ) sum = sum + i; // lock is necessary

            // As a practical matter, ensure this `parallel for` executes
            // on multiple threads by simulating a lengthy operation.
            Thread.Sleep( 1 );
        } );
        Console.WriteLine( "Correct answer should be 499500.  sum is: {0}", sum );
    }
}

仅仅在没有锁定的情况下执行 sum = sum + i 是不够的，因为读取 - 修改 - 写入操作不是原子操作。线程将覆盖对 sum 的任何外部修改，这些修改在读取 sum 的当前值之后，但在将 sum + i 的修改值存储回 sum 之前发生。