禁用 Alt+F4 但允许通过代码关闭表单，CloseReason.UserClosing 没有帮助

I want that the form will not close by doing Alt + F4 but if Application.Exit() or this.Close is called from the same Form, it should be closed.

I tried CloseReason.UserClosing但仍然没有帮助。

If you need to filter out Alt + F4 event only (leaving clicking of close box, this.Close() and Application.Exit() to behave as usual) then I can suggest the following:

1. 设置表格的KeyPreview http://msdn.microsoft.com/en-us/library/system.windows.forms.form.keypreview.aspx财产给true;

2. 连接表格FormClosing http://msdn.microsoft.com/en-us/library/system.windows.forms.form.formclosing.aspx and KeyDown http://msdn.microsoft.com/en-us/library/system.windows.forms.control.keydown.aspx events:

   private void Form1_FormClosing(object sender, FormClosingEventArgs e)
   {
       if (_altF4Pressed)
       {
           if (e.CloseReason == CloseReason.UserClosing)
               e.Cancel = true;
           _altF4Pressed = false;
       }
   }
   
   private bool _altF4Pressed;
   private void Form1_KeyDown(object sender, KeyEventArgs e)
   {
       if (e.Alt && e.KeyCode == Keys.F4)
           _altF4Pressed = true;
   }