Question

In a web form there are different asp.net validation controls. Is it possible to check a particular validation control is valid ? For example on leaving focus of textbox, first I will check requiredFieldValidatorUserName is valid ? If it is valid then I will check on server using ajax that this user name is not booked already.

Edit:

Explaination: I want to check validity (that input was valid) of a validation control on client side.

Please guide.

Answer 1

The best way would be to use a CustomValidator with client side code, as this will display all the error messages, block form submission and also ensure that the validation is repeated at the server side – remember, just because you have client-side validation available, doesn’t mean the user’s seen it: Always validate your input at the server as well.

Your CustomValidator would then be coded to call the Ajax methods, and would show the error messages correctly to the client:

<asp:Label ID="UserNameLabel" AssociatedControlID="UserName" runat="server">
  UserName *:</asp:Label>
<asp:TextBox ID="UserName" runat="server" />
<asp:RequiredFieldValidator ID="UserNameRequired" runat="server" 
                            ControlToValidate="UserName" EnableClientScript="true"
                            ErrorMessage="You must supply a username!" />
<asp:CustomValidator ID="UserNameCustom" runat="server"
                     ControlToValidate="UserName" 
                     ClientValidationFunction="CheckExisting"
                     OnServerValidate="UserNameCustomValidate"
                     ErrorMessage="Username already taken" />

And your ClientValidationFunction should look something like:

<script type="text/javascript">
  function CheckExisting(source, arguments) {
    // Pass the arguments.Value to your AJAX call:
    if (ajaxCallUserNameTaken(arguments.Value)) {
      arguments.IsValid = false;
    }
  }
</script>

(Obviously, you’ll need to write the ajaxCallUserNameTaken method to call your page method/web service/etc.)

Doing it this way will ensure that the validation methods happen as expected; this will get called whenever the user tabs out of the textbox leaving a value (it won’t get called if the textbox is empty), and will ensure that the user can’t submit the page until they supply a unique value. You’ll also want to create the method referenced in OnServerValidate to ensure that the value’s good once it hits the server too – this should call the same code that the AJAX endpoint uses to reduce duplication of code, etc.

I was originally going to suggest that you could use the Page_Validators object on the client-side to do some checking in the onBlur event, but I don’t really think this is suitable here as it results in more pain:

1. It assumes that although there might be more than one validator on the page, there’s only the RequiredFieldValidator on the control we’re checking
2. The RequiredFieldValidator isn’t fired during OnBlur if a user moves out of a control without setting a value – only if they set and clear the value, so even if isvalid is true, you need to check for an empty string!