Press Ctrl-A to select all in a textbox

NOTE textbox must be multiline.

private void txtSQL_KeyDown(object sender, KeyEventArgs e)
{
    KeyPressSelectAll(txtSQL, e);
}

private void txtResult_KeyDown(object sender, KeyEventArgs e)
{
    KeyPressSelectAll(txtResult, e);
}

private void KeyPressSelectAll(TextBoxBase t, KeyEventArgs e)
{
    if (e.Control && !e.Alt && e.Modifiers != Keys.RMenu)
    {
        if (e.KeyCode == Keys.A)
        {
            //Select all text
            t.SelectAll();
            //t.Focus(); <-- Optional. See note below.
            e.Handled = true;
        }
    }
}

regarding t.Focus()

If the KeyPressSelectAll method can be triggerd by a different control, e.g. a button or menu, then you might also want to set focus to the textbox, so that the selection is made visible.