How can I set the maximum length of a singleline/multiline textbox in a webform?
Restricting the length of text on a Single line/Multiline textbox is not supported by the standard ASP.NET Web control. This can be done by adding the following the 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. }
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.
  1. 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>