A tip on using ASP.NET validation controls

Executive summary:

• ValidationSummary controls look at the ErrorMessage field to figure out what to display, so always use ErrorMessage in a verbose enough way that it will be helpful from a ValidationSummary control.
• If you need a shorter message to display inline (i.e., where the validation control is on the form, as opposed to the ValidationSummary) use the body of the control to define it.

In the past, I've used RequiredFieldValidator controls on my web forms to remind users that certain fields are required. I would set the ErrorMessage to something vanilla like, "This field is required", or even something simpler like "*" (an asterisk) if I didn't have much room on the form to display more prose for an error.

A friend was recently testing a new feature that I'd built for our sales team and she had a hard time seeing the little red asterisks that were showing up next to required fields. It felt to her as though she was pushing the submit button on the form but nothing was happening. It was clear that a ValidationSummary control would be helpful, especially if placed close to the submit button for the form.

I've been a bit lazy in the past about using ValidationSummary controls, partially because most of my forms are simple enough that they feel a bit redundant. But on a more complicated form, they can be very helpful to guide users back to the places on the form where there's problems.

So I threw one of those puppies on the form and immediately saw that there was a problem - my error message was set to "*", which meant that my validation summary was pretty useless - it just displayed a bunch of red asterisks! And in places where I'd used the prose, "This field is required", well that was pretty useless as an error message in the summary.

After a bit of research and experimentation, I discovered that the ValidationSummary control looks at the ErrorMessage property on each validation control in order to figure out what to display in the summary. So it's important to use ErrorMessage with a summary in mind! Don't use text like "*" or "This field is required". Be more specific so the user can find her way up to the problem field, as in, "PostalCode is required".

But if you make ErrorMessage verbose so that it's helpful in a summary, it may make your form really ugly when displayed inline next to the control being validated. The trick is to use the body of the validation control element to specify the inline error message. Then you end up with two messages: a verbose one that's used in your summary, and a more localized, brief message that shows up right next to the control being validated. Note the asterisk that's in the body of the RequiredFieldValidator below:

<asp:RequiredFieldValidator
      ErrorMessage="Zip/postal code is required"
      ControlToValidate='txtPostalCode'
      ValidationGroup='BasicInfo'
      Display="Dynamic"
      runat='server'>*</asp:RequiredFieldValidator>

I've learned a lesson from all of this. In the future when I use validation controls I'll always provide a summary-friendly message in the ErrorMessage field, and if I need something different (typically shorter) to display inline, I'll put it in the body of the validation control element.

Hope this helps!