循环窗体上的TEXTBOX

private void button1_Click(object sender, System.EventArgs e)
  {
   OperateControls(this);
  }
  public void OperateControls(Control c)
  {
   if(c.Controls!=null)
   {
    foreach(Control txt in c.Controls)
    {
     if(txt is System.Windows.Forms.TextBox)
     {
      txt.Text =String.Empty;
     }
     else
     {
      OperateControls(txt);
     }
    }
   }
  }