Character limit on a multi line text box

//Write code on page load event

TextBox1.Attributes.Add("onkeydown", "return checkcharacter(this,50)");

//write javascript function between head section on html page

<script language="javascript">
function checkcharacter(txt,maxLen){
try{
if(txt.value.length > (maxLen-1))
return false;
else if( txt.value.length == (maxLen-1))
alert('you can enter only 50 character !!!!!!!! ');
}
catch(e){
}}
</script>