I tried to fix the maximum length in multiline textbox by using the property MaxLength. But it didn't work in case of multiline textbox. In this blog i will explain how to get rid of this problem by using jquery.

$(function() {
 $('#ctl00_ContentPlaceHolder1_txtTest').keyup(function() {
 limitChars('ctl00_ContentPlaceHolder1_txtTest', 100, 'counter');
 })
 });

 function limitChars(textid, limit, infodiv) {
 var text = $('#' + textid).val();
 var textlength = text.length;
 if (textlength > limit) {
 $('#' + infodiv).html('You cannot write more then ' + limit + ' characters!');
 $('#' + textid).val(text.substr(0, limit));
 return false;
 }
 else {
 $('#' + infodiv).html('You have ' + (limit - textlength) + ' characters left.');
 return true;
 }
 }
 

 <asp:TextBox  id="txtTest" runat="server" TextMode="MultiLine"></asp:TextBox >
 <div id="counter">Max 100 characters.</div>