How to set Maxlength for multiline textbox in ASP.Net

Hi

I used below code for textbox that users can enter 30character in text box

                 <asp:TextBox ID="txttopic" runat="server"  MaxLength="30"></asp:TextBox>

It worked correctly

and now I want use MaxLenght for textbox with TextMode="MultiLine" like below

                 <asp:TextBox ID="txtmatn" runat="server" TextMode="MultiLine"  MaxLength="100"></asp:TextBox>

but it didn't worke for above TextBox

What should I do for this Textbox ?

Best Regards

Neda