int a = 0;
System.Threading.Tasks.Parallel.For(0, 100000, (i) =>
{
    a++; 
});
Console.Write(a);

解决方案如下

int a = 0;
            object lockObj= new object();
            System.Threading.Tasks.Parallel.For(0, 100000, (i) =>
            {
                lock (lockObj)
                {
                    a++;
                }
            });
            Console.Write(a);

结果就正常了.