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,
    1. function Count(text, long)
    2. {
    3. var maxlength = new Number(long); // Change number to your max length.
    4. if (text.value.length > maxlength) {
    5. text.value = text.value.substring(0, maxlength);
    6. alert(" Only " + long + " characters");
    7. }
    8. }
  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,
    1. <asp:TextBox ID="TESTDATA" runat="server" TextMode="MultiLine">asp:TextBox>
    After adding the code
    1. <asp:TextBox ID="TextBox1" runat="server" onKeyUp="Count(this,300)" onChange="Count(this,300)" TextMode="MultiLine">asp:TextBox>