Comm comm = new Comm();
string message = null;
if (InputBox.Text == "say " + message)
            {
                OutputBox.AppendText(comm.do_say(message));
            }

class Comm
{
    public string do_say(string message)
    {
        return "You say: " + message + "n";
    }
}

(InputBox.Text == "say " + message)

(InputBox.Text == "say {0}", message)

if(InputBox.Text.StartsWith("Say "))
    OutputBox.Text += InputBox.Text.SubString(4);

if (InputBox.Text == "say " + message)

Match m = Regex.Match(InputBox.Text, @"^says+(.*)$", RegexOptions.IgnoreCase);
if (m.Success)
{
    OutputBox.AppendText(Comm.GetScreenoutput(m.Groups[1].Value));
}

static class Comm
{
    public static string GetScreenOutput(string message)
    {
        return "You say: " + message + "n";
    }
}

if (InputBox.Text.ToUpper().StartsWith("SAY "))
{
    OutputBox.AppendText(comm.do_say(message));
}