protected override void OnClosing(CancelEventArgs e)
{
  if(this.Visible)
  {
    e.Cancel=true;
　　　//
　　 // WHATE TODO 
　　 //
  }
}

protected override void WndProc(ref Message m)
{
const int WM_SYSCOMMAND = 0x0112;
const int SC_CLOSE = 0xF060;
if (m.Msg == WM_SYSCOMMAND && (int) m.WParam == SC_CLOSE)
{
// User clicked close button
this.WindowState = FormWindowState.Minimized;
return;
}
base.WndProc(ref m);
}

private void richTextBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
if(e.Control && e.KeyCode==Keys.V)
e.Handled = true;
}

#region 单实例运行
/// <summary>
/// 单实例运行
/// </summary>
/// <param name="frm">所要运行的主窗体</param>
public static void SingleRun(Form frm)
{
  System.Threading.Mutex mutex = new System.Threading.Mutex(false,"SINGLE_INSTANCE_MUTEX");
  if (!mutex.WaitOne(0, false))
  {
 mutex.Close();
 mutex = null;
  }
  if (mutex != null)
  {
 Application.Run(frm);
  }
  else
  {
 MessageBox.Show("应用程序已启动","信息提示",MessageBoxButtons.OK,MessageBoxIcon.Information);
  }
}
#endregion

 #region 将控件转换为圆形
 [System.Runtime.InteropServices.DllImport("gdi32")]
 private static extern IntPtr BeginPath(IntPtr hdc);
 [System.Runtime.InteropServices.DllImport("gdi32")]
 private static extern int SetBkMode(IntPtr hdc,int nBkMode); 
 const int TRANSPARENT=1;
 [System.Runtime.InteropServices.DllImport("gdi32")]   
 private static extern IntPtr EndPath(IntPtr hdc);
 [System.Runtime.InteropServices.DllImport("gdi32")]
 private static extern IntPtr PathToRegion(IntPtr hdc);
 [System.Runtime.InteropServices.DllImport("gdi32")]
 private static extern int Ellipse(IntPtr hdc,int x1,int y1,int x2,int y2);
 [System.Runtime.InteropServices.DllImport("user32")]
 private static extern IntPtr SetWindowRgn(IntPtr hwnd,IntPtr hRgn,bool bRedraw);
 [System.Runtime.InteropServices.DllImport("user32")]
 private static extern IntPtr GetDC(IntPtr hwnd);
 /// <summary>
 /// 将控件转为圆形
 /// </summary>
 /// <param name="control">控件名</param> 
 public static void MakeControlToCircle(Control[] control)
 {
  IntPtr dc;
  IntPtr region;
  for(int i=0;i<control.Length;i++)
  {
  dc=GetDC(control[i].Handle);
  BeginPath(dc);
  SetBkMode(dc,TRANSPARENT);
  Ellipse(dc,0,0,control[i].Width-3,control[i].Height-2);
  EndPath(dc);
  region=PathToRegion(dc);
  SetWindowRgn(control[i].Handle,region,false);
  }  
 }
  #endregion

/// <summary>
/// 在同一应用程序中同一窗体只打开一个
/// </summary>
/// <param name="frmOpen">要打开的窗体实例</param>
/// <returns></returns>
public static void OpenSeamForm(Form frmOpen)
{
  foreach (Form frm in Application.OpenForms)
  {
 if (frm.Text == frmOpen.Text)
 {
   frm.Visible = true;
   frm.WindowState = FormWindowState.Normal;
   frm.Activate();
   return;
 }
  }
  frmOpen.Show();
}