Month: March 2011

Making sense on many levels – ASP.NET MVC 2 and Model-Level Error Reporting

markegilbert4 Comments

In the previous episode of “Mark and the Chartreuse-Field Project”, Mark was working to get site-wide error reporting up and running.  Today, Mark tackles model-level error reporting.  Let’s tune in and see how he’s doing.

***

Early on in this project I learned how to associate a custom error message with a specific form field:

ViewData.ModelState.AddModelError("MyField", "My custom error message here")

That allowed the out-of-the-box validation messaging to highlight the offending field and display the message right next to it:

<%=Html.ValidationMessageFor(Function(model) model.MyField)%>

However, today I ran into a situation where I needed to be able to display a custom message that applied to the entire page, not a specific field.  There was a validation summary control at the top of the page already:

<%= Html.ValidationSummary(True) %>

My first thought was to associate the custom message with that control so it would appear there.  Following the pattern to associate a message with a specific field, my first attempt at code-roulette looked like this:

ViewData.ModelState.AddModelError("", "My custom error message here")

I then threw a dummy exception in the middle of one of my controller actions to force an error to appear.  No dice – the error was simply swallowed by the page.  I did some searching and came across a post (http://stackoverflow.com/questions/4017827/manually-adding-text-to-html-validationsummary) that mentioned using an asterisk as the field name to get the message to show up in the validation summary:

ViewData.ModelState.AddModelError("*", "My custom error message here")

Still no dice.  Ok, time to back up a minute.  Was my custom error even making it into the ModelState object?  I put a breakpoint on the line immediately after AddModelError, and inspected the ViewData.ModelState.Keys property:

(0): "id"
(1): "CurrentEvent.Title"
…
(17): "*"

So, if the error is getting added to the ModelState correctly, then why isn’t it showing up?  The answer came in two parts.  First, I was passing a “True” to Html.ValidationSummary.  This was configuring the control to not display messages that were tied to a field (the actual parameter name here is “excludePropertyErrors”).  My asterisk was being treated like another field name – one that didn’t match a field-level validation control – and was therefore being excluded by the validation summary control.  Second, a post from the ASP.NET forums (http://forums.asp.net/p/1628537/4193163.aspx) suggested that displaying model-level errors were not possible in MVC 2, and apparently were in MVC 3.  Upgrading wasn’t an option for me, so I needed to find another way.

What if I were to create a new model property called ErrorMessage, and then associate the custom messages with THAT field?  Then, I would just need to add a validation control for that field to my view.  I added the property, “ErrorMessage”, to my model and modified my AddModelError call like so:

ViewData.ModelState.AddModelError("ErrorMessage", "My custom error message here")

Then I added a Html.ValidationMessageFor control to my view, and tied it to this new property:

<%=Html.ValidationMessageFor(Function(model) model.ErrorMessage)%>

And voila!  Model-level errors in MVC 2!