Set the Maximum Length of a Singleline/ Multiline Textbox in a Web Form

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>