Maxlength of Multilne Textbox in ASP.Net

<script language="javascript" type="text/javascript">

function textboxMultilineMaxNumber(txt, maxLen)

{

    try {

        if ((txt.value.length) > (maxLen-1 ))

        return false;

        }

 catch (e)

    {

    }

}

    </script>

<asp:TextBox ID="TextBox1" runat="server" Width="200px" TextMode="MultiLine" Height="100px"  onkeypress="return textboxMultilineMaxNumber(this,5)"></asp:TextBox>