Setting the maximum length of a singleline/multiline text box

How can I set the maximum length of a singleline/multiline textbox in a webform?

Restricting the length of text on a Single line/Multi line textbox is not supported by the standard ASP.NET Web control. This can be done by adding the following Javascript function in your ASPX page.

1. Add a javascript:
At the top of your ASPX page you will see the following tag . Add the following code under this tag:

function Count(text,long)
{
var maxlength = new Number(long); // Change number to your max length.
if (text.value.length > maxlength){
text.value = text.value.substring(0,maxlength);
alert(" Only " + long + " characters");
}
}

2. Change your text controls on your ASPX page
On every multi-line text box where you would like to control the length, add the following code to the asp text box control.
onKeyUp="Count(this,300)" onChange="Count(this,300)"

Your original text control before adding this code:
<asp:TextBox ID="TESTDATA" runat="server" TextMode="MultiLine">asp:TextBox>

After adding the code:
<asp:TextBox ID="TextBox1" runat="server" onKeyUp="Count(this,300)" onChange="Count(this,300)" TextMode="MultiLine">asp:TextBox>