Set this event handler for your textbox to enable Ctrl-A shortcut and select all text:
private void anyTextBox_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
if (e.KeyChar == '\x1')
{
((TextBox)sender).SelectAll();
e.Handled = true;
}
}
Thanks to:
http://www.dzone.com/snippets/ctrl-shortcut-select-all-text