Convert KeyDown keys to one string C#

Rather than adding to a list why not build up the string:

private string input;

private bool shiftPressed;

private void Window_KeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.LeftShift || e.Key == Key.RightShift)
    {
        shiftPressed = true;
    }
    else
    {
        if (e.Key >= Key.D0 && e.Key <= Key.D9)
        {
            // Number keys pressed so need to so special processing
            // also check if shift pressed
        }
        else
        {
            input += e.Key.ToString();
        }
    }
}

private void Window_KeyUp(object sender, KeyEventArgs e)
{
    if (e.Key == Key.LeftShift || e.Key == Key.RightShift)
    {
        shiftPressed = false;
    }
}

Obviously you need to reset input to string.Empty when you start the next transaction.