用c#设计秒表代码程序

private DateTime TimeStart = new DateTime();
private Boolean IsFirst = true;
public Form1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
if (IsFirst)
{
TimeStart = DateTime.Now;
timer1.Enabled = true;
IsFirst = false;
}
else
{
timer1.Enabled = !timer1.Enabled;
}
}
private void button3_Click(object sender, EventArgs e)
{
TimeStart = DateTime.Now;
timer1.Enabled = false;
label1.Text = (DateTime.Now - TimeStart).ToString();
}

private void timer1_Tick(object sender, EventArgs e)
{
label1.Text = (DateTi