Thoughts on Software

Accessing Model validation errors from the Controller

with 16 comments

MVC performs automatic model validation whilst it is binding the model, so as soon as we hit the Controller action method that is being called, we have access to a validated model. In fact, as discussed in my last post, we actually have access to the results of model validation in the method’s ActionFilters, including the first ActionFilter: OnActionExecuting, which fires before we even reach the action method.

MVC Exposes the results of this validation through the Controller’s ViewData.ModelState property. This is directly accessible in the Controller by calling ModelState and is most often used to check whether a model passed validation:


if (ModelState.IsValid)
    // model passed validation, so do some work here


But if ModelState.IsValid returns false, ie the model failed validation, how can we see the collection of errors?

In addition to the IsValid property, ModelState also contains an IEnumerable of KeyValuePairs of type <string, ModelState>. The ModelState object has two properties Key and Value. Key is an id for the ModelState (and is often property name of the property which failed validation but this depends on the validator that trapped the validation failure) and Value is the ModelState object itself and contains a collection of zero or more ModelError objects. Got all that?!

Now that’s all very well and good but once we know that a model has failed validation we may want to do something like find the properties with errors and log them, for example. A bit of LINQ magic can help us here: we can use LINQ to give us a List of ModelState objects that have an error. The LINQ code?


    var errors = ModelState.Where(state => state.Values.Errors.Count > 0).ToList()


This will give us a generic List<ModelState> object called errors that contains only the properties that have errors to do with as we please.


Written by andrewlocatelliwoodcock

December 16, 2011 at 20:17

16 Responses

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