Let's say you have a Text Box = TextBox1 (textMode=MultiLine etc...) and you want to validate/Limit the number of character entered by a user

Do this:

1. Add a customField Validator.
2. In the customValidator properties: ErrorMessage = "Your Error Message" ControlToValidate ="control you want to validate"
3. Select the Event button or double click the CustomValidator on the page.
4. In the code behind Enter the following:

protected void CustomValidator1_ServerValidate
(object source, ServerValidateEventArgs args)

    {

        if (TextBox1.Text.Length > 10)

        {

            args.IsValid = false;

        }

        else

        {

            args.IsValid = true;

        }

    }

To avoid making a trip to the server and have instant validation when tabbing or moving to another field do this.

1. Return to the CustomValidator control properties and add under Behavior:
   ClientValidationFunction = validateComments (or whatever text you want to call it)
   
2. In your aspx file add a ScriptManager from the Ajax Extensions.
   Then add a new Item and select JScript File
3. In the script file add the following function.

Voila! if you using a master page add the SciptManager in your master page. Hope this helped.

function validateComments(sender, args) 

{

    if (args.Value.length > 10)

        args.IsValid = false;

    else

        args.IsValid = true;

}

Also, Reference the file in your script manager(see below after the function).

<asp:ScriptManager ID="ScriptManager1" runat="server">

    <Scripts>

    <asp:ScriptReference Path="~/validateComments.js" />

    </Scripts>

    </asp:ScriptManager>