Handling Validation Errors in MVC (a different approach)

I just got done reading Scott Guthrie‘s and Phil Haack’s posts on the MVC Preview 4 release and there was one little piece in the “What’s Next” section that really peeked my interest. As it turns out the MVC guys are figuring out how to tackle the problem with handling validation errors and reporting them back to the user. How great is that? I don’t know about you but validation errors are one of the worst things to have to deal with when it comes to a web based application.

As we all know you should never rely on client side validation and you should always validate input (especially user input) prior to calling any business methods or database calls. My problem has always been with trying to get the validation messages back to the client. I am always determined to have my validation routines in my business layer. But then comes time to report validation errors back to the client and I end up sticking them in the code behind. Sure I might (emphasis on might) create a utility or helper class but then the routines become more and more specific and less and less common and I am right back to sticking them in the code behind.

Where else can you put them since you not only have access to the control but you also have all the tools you need to render the output?

While I have been playing with MVC I have found myself once again struggling with sending validation messages back to the user. This time, however, I decided to try a few different techniques and one has proven to work well (for me at least).

I first started off creating a class to store my validation messages. Since my primary focus is capturing form related validation errors I decided to add a PropertyInfo property that will give me some more context later on if I need it.

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So now that I have a class that contains a validation message with some extra info layered in I turn my attention to how to store the errors. At first I decided to make a single collection that would be used as the return type of my validation routines but quickly found it to be too restrictive. I decided to leverage the HttpContext.Items collection and the CallContext class instead. The HttpContext.Items collection for web based usage and the CallContext for unit testing. To isolate myself from these details I created a little wrapper class to manage the switching between web usage and unit testing.

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This class uses generic methods so I can strongly type the objects being stored.
With that out of the way I created another class specific to my validation storage which does nothing more than make calls to the ThreadContext class.

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Finally I create a class that handles the addition and retrieval of the validation messages.
Let’s first take a look at the constructor for this class.

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As you can see I first create a new collection to store the validation messages. Next I check to see if a ValidationStorage class already exists and if so I set the existing instance to the Parent property (more on this later). Finally I make the instance the Current object in the ValidationStorage class. I could have easily used a Stack of Validator but rather than popping and pushing I decided to take this approach instead.

Now let’s turn our attention to the Dispose method.

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As you can see here when I dispose the Validator object I check to see if the instance has a Parent and if it does I take it’s Details and add them to Parent.Details.

Now on to the properties. Nothing much going on here but in order to understand what the methods are doing you will need to know the properties that are available.

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The Current and Exists properties make calls to the ValidationStorage class. The Failed property will return “false” if a Validator does not exist in the ValidationStorage. Later on you get a better understand of why these properties are static.

On to the instance methods.

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The HasMessages method allows me to check to see if I have any ValidationDetail for a given property by name. And if there are messages then I can call GetMessage or GetMessages.

Finally the static methods. These methods are what will be accessed from within the validation routines.

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Thee methods add the ValidationDetail object to the Details collection. Prior to adding it, however, I check to see if there is an existing Validator available to use and if not I new up a new instance. The Error and Exception methods will also pass in a boolean to the AddDetail method to indicate that validation has failed.

So now let’s take a look at how to use this. For simplicity assume that there is a controller that performs some basic validation routines against a simple model. Let’s first take a look at the model.

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The controller then needs to have a few validation routines so let’s look at those now.

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As you can see here when a validation error occurs I simply call the static method Error and that’s it. But how does it convey the messages back to the client? This can bee seen in the following code.

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After the controller calls the validation routines it makes a call to the static Failed property of the Validator class which will indicate whether or not validation has failed (obviously). If it has failed validation then I simply render the Edit view again. Within the Edit view a check is made to see if a Validator exists using the Exists property and if so the the ValidatioDetail objects can be blindly displayed or a check against per property can be made for a more contextual error messages.

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One key thing that I think really makes this work well is the ability that this gives to call the Validator from almost anywhere in my code. Validation routines can now reside within the business layer like they should. And since the ThreadContext is in place web services can use and implement the same checks that are done in the controller.

While I think this does the job I also feel that there can be some things improved upon or added to really make it shine. For instance:

  • Adding a severity property which would indicate if the client has entered something really harmful and allow for an exception page to be rendered
  • Changing the Details collection to a Dictionary to easily store multiple validation errors per property or data element
  • Add an Html extension method to deal with the checking and displaying of validation messages

Any ideas or comments are welcomed.

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