protected override void WndProc(ref Message m)
{
    const int WM_CLOSE = 0x0010;

    if (m.Msg == WM_CLOSE)
    {
        // I don't want 'X' to destroy the window - just hide it.
        // related: "Minimizing a window to the System Tray on a Close event" at
        // http://www.pixvillage.com/blogs/devblog/archive/2005/03/26/174.aspx
        this.Hide();
        return;
    }

    base.WndProc(ref m);
}

protected override void WndProc(ref Message m)
      {
         const int WM_CLOSE = 0x0010;

         if (m.Msg == WM_CLOSE)
         {
            // I don't want 'X' to destroy the window - just hide it.
            // related: "Minimizing a window to the System Tray on a Close event" at
            // http://www.pixvillage.com/blogs/devblog/archive/2005/03/26/174.aspx
            this.Hide();
            return;
         }

         base.WndProc(ref m);
      }

http://bytes.com/topic/c-sharp/answers/258899-how-do-i-intercept-close-event

Typical scenario: app pops up a dialog, user clicks 'X' and the dialog disposes. If the app attempts to access the dialog then runtime errors occur. This approach allows the app to retain access to the dialog to re-show the dialog.

An alternative would be for the app to check the disposed state of the dialog but this  Close-->Hide approach is simpler and more transparent.

http://www.pixvillage.com/blogs/devblog/archive/2005/03/26/174.aspx

Source contains a simple WndProc that handles WM_CLOSE by calling Hide().

C# WinForm, cause 'close' ('X') to hide instead of disposing by handling the WM_CLOSE message in WndProc.

C#