The TextBox control does not support the CTRL+A shortcut key when the Multiline property value is true.

你必须自己实现.

这样的事情应该有效：

private void textBox1_KeyDown(object sender,KeyEventArgs e)
    {
        if (e.Control & e.KeyCode == Keys.A)
        {
            textBox1.SelectAll();
        }
        else if (e.Control & e.KeyCode == Keys.Back)
        {
            SendKeys.SendWait("^+{LEFT}{BACKSPACE}");
        }
    }