Stopwatch()测试运行时间

在文本框中输入数据进行排序，用stopwatch()测试的各种排序方法的运行时间，在程序运行后，第一次可以成功测出，但若将文本框中内容删除，再次输入数据，只能成功排序，并不能再次测试成功是为什么呢？

protected void compareButton_Click(object sender, EventArgs e)
{
//int[] test = new int[7];

        //记录时间
        //Stopwatch sw = new Stopwatch();
        sw.Reset();
        sw.Start();
        InsertionSort(aa);//
        sw.Stop();
        InsertionTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;
       // TimeSpan timespan = sw.Elapsed;
        //InsertionTime = timespan.TotalMilliseconds;
       // Label1.Text = "总耗时：" + InsertionTime.ToString() + "微秒\n";
        // Label1.Text = forShow(tt);

        sw.Reset();
        sw.Start();
        BubbleSort(bb);
        sw.Stop();
        BubbleTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;

        sw.Reset();
        sw.Start();
        ShellSort(cc);
        sw.Stop();
        ShellTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;

        sw.Reset();
        sw.Start();
        SelectionSort(dd);
        sw.Stop();
        SelectionTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;

        sw.Reset();
        sw.Start();
        QuickSort(ee,0,count-1);
        sw.Stop();
        QuickTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;

        sw.Reset();
        sw.Start();
        MergeSort(ff);
        sw.Stop();
        MergeTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;

        sw.Reset();
        sw.Start();
        HeapSort(gg);
        sw.Stop();
        HeapTime = 1000000.0 * sw.ElapsedTicks / Stopwatch.Frequency;