C#用QueryPerformanceCounter()实现精确定时器

很简单的原理，可是还是搞了半天。希望有大虾们指点指点。

        //调用API函数
        [DllImport("kernel32.dll")]
        extern static short QueryPerformanceCounter(ref long x);
        [DllImport("kernel32.dll")]
        extern static short QueryPerformanceFrequency(ref long x);
        //定义延迟函数
        public void delay(long delay_Time)
        {
           
            long stop_Value = 0;
            long start_Value = 0;
            long freq = 0;
            long n =0;
           
            QueryPerformanceFrequency(ref freq);  //获取CPU频率
            long count = delay_Time * freq / 1000;
            QueryPerformanceCounter(ref start_Value); //获取初始前值
           
            while ( n<count) //不能精确判定
            {
                QueryPerformanceCounter(ref stop_Value);//获取终止变量值
                n = stop_Value - start_Value;               
            }