>>206
これではどうだ。
richtb.SelectionChanged += new EventHandler(richtb_SelChanged);

private void richtb_SelChanged(object sender, EventArgs e)
{
string str = richtb.SelectedText;
int len = str.Length;
int start = richtb.SelectionStart;
Console.WriteLine("{0} {1}", start, len );
}