private void Log(string s , Color? c = null)
{
richTextBox.SelectionStart = richTextBox.TextLength;
richTextBox.SelectionLength = 0;
richTextBox.SelectionColor = c ?? Color.Black;
richTextBox.AppendText((richTextBox.Lines.Count() == 0 ? "" : Environment.NewLine) + DateTime.Now + "\t" + s);
richTextBox.SelectionColor = Color.Black;
}