FluentValidation and MVC - From Server to Client

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We do a lot of interesting stuff with FluentValidation at work and more than a few times I’ve had to give the whiteboard presentation of how a server-side FluentValidation validator makes it to jQuery validation rules on the client and back. I figured it was probably time to just write it up so I can refer folks as needed.

Let’s start out with a simple model we want to validate. We’ll carry these examples with us in the walkthrough so you have something concrete to tie back to.

public class MyModel
  public string Name { get; set; }
  public int Age { get; set; }

On the server, we’d validate that model using FluentValidation by implementing FluentValidation.AbstractValidator<T> like this:

public class MyModelValidator : AbstractValidator<MyModel>
  public MyModelValidator()
    RuleFor(m => m.Name)
      .WithMessage("Please provide a name.");

    RuleFor(m => m.Age)
      .WithMessage("You must be over 21 to access this site.");

First let’s look at the way rules are set up in FluentValidation. You’ll see that each call to RuleFor points to a property we want to validate. Calling RuleFor sort of starts a “stack” of validators that are associated with that property. Each method that adds a validation (NotEmpty, GreaterThan) adds that validator to the “stack” and then sets it as “active” so other extensions that modify behavior (WithMessage) will know which validator they’re modifying.

A more complex setup might look like this:

RuleFor(m => m.Name)                        // Start a new validation "rule" with a set of validators attached
  .NotEmpty()                               // Add a NotEmptyValidator to the stack and make it "active"
  .WithMessage("Please provide a name.")    // Set the message for the NotEmptyValidator
  .Matches("[a-zA-Z]+")                     // Add a RegularExpressionValidator to the stack and make it "active"
  .WithMessage("Please use only letters."); // Set the message for the RegularExpressionValidator

In a server-side only scenario, when you validate an object each “rule” gets iterated through and each validator associated with that “rule” gets executed. More or less. There’s a bit of complexity to it, but that’s a good way to explain it without getting into the weeds.

ASP.NET MVC ships with an abstraction around model validation that starts with a ModelValidatorProvider. Out of the box, MVC has support for DataAnnotations attributes. (Rachel Appel has a great walkthrough of how that works.) From the name, you can guess that what this does is provide validators for your models. When MVC wants to validate your model (or determine how it’s validated), it asks the set of registered ModelValidatorProvider objects and they return the validators. It’s dumb when I say it out loud, but the class names start getting really long and a little confusing, so I just wanted to bring this up now: if you start getting confused, really stop to read the name of the class you’re confused about. It’ll help, trust me, because they all start sounding the same after a while.

FluentValidation has an associated FluentValidation.Mvc library that has the MVC adapter components. In there is a FluentValidationModelValidatorProvider. To get this hooked in, you need to register that FluentValidationModelValidatorProvider with MVC at application startup. The easiest way to do this is by calling FluentValidationModelValidatorProvider.Configure(), which automatically adds the provider to the list of available providers and also lets you do some additional configuration as needed.

Things you can do in FluentValidationModelValidatorProvider.Configure:

  • Specify the validator factory to use on the server to retrieve FluentValidation validators corresponding to models.
  • Add mappings for custom validators that map between the server-side FluentValidation validator and the MVC client-side validation logic.

Right now we’ll talk about the validator factory piece. I’ll get to the custom validator mappings later.

As mentioned, when you run FluentValidationModelValidatorProvider.Configure(), you can tell it which FluentValidation.IValidatorFactoryto use for mapping server-side validators (like the MyModelValidator) to models (like MyModel). Out of the box, FluentValidation ships with the FluentValidation.Attributes.AttributedValidatorFactory. This factory type lets you attach validators to your models with attributes, like this:

public class MyModel
  // ...

That’s one way to go, but I find that to be somewhat inflexible. I also don’t like my code too closely tied together like that, so instead, in MVC, I like to use the DependencyResolver to get my model-validator mappings. FluentValidation doesn’t ship with a factory that uses DependencyResolver, but it’s pretty easy to implement:

public class DependencyResolverModelValidatorFactory : IValidatorFactory
  public IValidator GetValidator(Type type)
    if (type == null)
      throw new ArgumentNullException("type");
    return DependencyResolver.Current.GetService(typeof(IValidator<>).MakeGenericType(type)) as IValidator;

  public IValidator<T> GetValidator<T>()
    return DependencyResolver.Current.GetService<IValidator<T>>();

Using that validator factory, you need to register your validators with your chosen DependencyResolver so that they’re exposed as IValidator<T> (like IValidator<MyModel>). Luckily AbstractValidator<T> implements that interface, so you’re set. In Autofac (my IoC container of choice) the validator registration during container setup would look like…


To register the model validator factory with FluentValidation, we’d do that during FluentValidationModelValidatorProvider.Configure() at application startup, like this:

  provider =>
    provider.ValidatorFactory = new DependencyResolverModelValidatorFactory();

Let’s checkpoint. Up to now we have:

  • A custom validator for our model.
  • A validator factory for FluentValidation that will tie our validator to our model using dependency resolution.
  • FluentValidation configured in MVC to be a source of model validations.

Next let’s talk about how the FluentValidation validators get their results into the MVC ModelState for server-side validation.

ASP.NET MVC has a class called ModelValidator that is used to abstract away the concept of model validation. It’s responsible for generating the validation errors that end up in ModelState as well as the set of client validation rules that define how the browser-side behavior gets wired up. The ModelValidator class has two methods, each of which corresponds to one of these responsibilities.

  • Validate: Executes server-side validation of the model and returns the list of results.
  • GetClientValidationRules: Gets metadata in the form of ModelClientValidationRule objects to send to the client so script can do client-side validation.

For DataAnnotations, there are implementations of ModelValidator corresponding to each attribute. For FluentValidation, there are ModelValidator implementations that correspond to each server-side validation type. For example, looking at our MyModel.Name property, we have a NotEmptyValidator attached to it. That NotEmptyValidator maps to a FluentValidation.Mvc.RequiredFluentValidationPropertyValidator. These mappings are maintained by the FluentValidationModelValidatorProvider. (Remember I said you could add custom mappings during the call to Configure? This is what I was talking about.)

When MVC needs the set of ModelValidator implementations associated with a model, the basic process is this:

  • The ModelValidatorProviders.Providers.GetValidators method is called. This iterates through the registered providers to get all the validators available. Eventually the FluentValidationModelValidatorProvider is queried.
  • The FluentValidationModelValidatorProvider uses the registered IValidatorFactory (DependencyResolverModelValidatorFactory) to get the FluentValidation validator (MyModelValidator) associated with the model (MyModel).
  • The FluentValidationModelValidatorProvider then looks at the rules associated with the property being validated (MyModel.Name) and converts each validator in the rule (NotEmptyValidator) into its mapped MVC ModelValidator type (RequiredFluentValidationPropertyValidator).
  • The list of mapped ModelValidator instances is returned to MVC for execution.

Here’s where it starts coming together.

When you do an MVC HtmlHelper call to render a data entry field for a model property, it looks something like this:

@Html.TextBoxFor(m => m.Name)

Internally, during the textbox generation, a bit more happens under the covers:

  • The input field HTML generation process calls ModelValidatorProviders.Providers.GetValidators for the field. (This is the process mentioned earlier.)
  • The ModelValidator(s) returned each have GetClientValidationRules called to get the set of ModelClientValidationRule data defining the client-side validation info.
  • The ModelClientValidationRule objects get converted to data-val- attributes that will be attached to the input field.

Using our example, if we did that Html.TextBoxFor(m => m.Name) call…

  • The FluentValidationModelValidatorProvider would, through the process mentioned earlier, yield a RequiredFluentValidationPropertyValidator corresponding to the NotEmptyValidator we’re using on the Name field.
  • That RequiredFluentValidationPropertyValidator would have GetClientValidationRules called to get the client-side validation information. I happen to know that the RequiredFluentValidationPropertyValidator returns a ModelValidationRequiredRule.
  • Each of the ModelClientValidationRule objects (in this case, just the ModelValidationRequiredRule) would be converted to data-val- attributes on the textbox.

What’s in a ModelClientValidationRule and how does it translate to attributes?

Basically each ModelClientValidationRule corresponds to a jQuery validation type. The ModelValidationRequiredRule is basically just a pre-populated ModelClientValidationRule derivative that looks like this:

new ModelClientValidationRule
  ValidationType = "required",
  ErrorMessage = "Please provide a name."

The ValidationType corresponds to the name of a jQuery validation type and the error message is the one we specified way back when we defined our FluentValidation validator.

When that gets converted into data-val- attributes, it ends up looking like this:

<input type="text" ... data-val-required="Please provide a name." />

That’s how the validation information gets from the server to the client. Once it’s there, it’s time for script to pick it up.

MVC uses jQuery validation to execute the client-side validation. It takes advantage of a library jQuery Unobtrusive Validation which is used to parse the data-val- attributes into client-side logic. The attributes fairly well correspond one-to-one with existing jQuery validation rules. For example, data-val-required corresponds to the required() validation method. That jQuery Unobtrusive Validation library reads the attributes and maps them (and their values) into jQuery validation actions that are attached to the form. Standard jQuery validation takes it from there.

The whole process for validating a submitted form is:

  • The Html.TextBoxFor call gets the set of validators for the field and attaches the data-val- attributes to it using the process described earlier.
  • jQuery Unobtrusive Validation parses the attributes on the client and sets up the client-side validation.
  • When the user submits the form, jQuery validation executes. Assuming it passes, the form gets submitted to the server.
  • During model binding, the MVC DefaultModelBinder gets the list of ModelValidator instances for the model using the process described earlier.
  • Each ModelValidator gets its Validate method called. Any failures will get added to the ModelState by the DefaultModelBinder. In our case, the RequiredFluentValidationPropertyValidator will pass through to our originally configured NotEmptyValidator on the server side and the results of the NotEmptyValidator will be translated into ModelValidationResult objects for use by the DefaultModelBinder.

Whew! That’s a lot of moving pieces. But that should explain how it comes together so you at least know what you’re looking at.

When you want to add a custom server-side validator, you have to hook into that process. Knowing the steps outlined above, you can see you have a few things to do (and a few places where things can potentially break down).

The basic steps for adding a new custom FluentValidation validator in MVC are:

  • Create your server-side validator. This means implementing FluentValidation.Validators.IPropertyValidator. It’s what gets used on the server for validation but doesn’t do anything on the client. I’d look at existing validators to get ideas on how to do this, since it’s a little different based on whether, say, you’re comparing two properties on the same object or comparing a property with a fixed value. I’d recommend starting by looking at a validator that does something similar to what you want to do. The beauty of open source.
  • Add an extension method that allows your new validator type to participate in the fluent grammar. The standard ones are in FluentValidation.DefaultValidatorExtensions. This is how you get nice syntax like RuleFor(m => m.Name).MyCustomValidator(); for your custom validator.
  • Create a FluentValidation.Mvc.FluentValidationPropertyValidator corresponding to your server-side validator. This will be responsible for creating the appropriate ModelClientValidationRule objects to tie your server-side validator to the client and for executing the corresponding server-side logic. (Really the work is in the client-side part of things. The base FluentValidationPropertyValidator already passes-through logic to your custom server-side validator so you don’t have to really do anything to get that to happen.)
  • Add a mapping between your server-side validator and your FluentValidationPropertyValidator. This takes place in FluentValidationModelValidatorProvider.Configure. Let’s talk about that.

Remember earlier I said we’d talk about adding custom validator mappings to the FluentValidationModelValidatorProviderduring configuration time? You need to do that if you write your own custom validator. When you want to map a server-side validator to a client-side validator, it looks like this:

  provider =>
    provider.ValidatorFactory = new DependencyResolverModelValidatorFactory();
      (metadata, context, rule, validator) => new MyCustomFluentValidationPropertyValidator(metadata, context, rule, validator));

There are mappings already for the built-in validators. You just have to add your custom ones. Unfortunately, the actual list of mappings itself isn’t documented anywhere I can find, so you’ll need to look at the static constructor on FluentValidationModelValidatorProvider to see what maps to what by default.

I won’t walk through the full creation of an end-to-end custom validator here. That’s probably another article since this is already way, way too long without enough pictures to break up the monotony. Instead, I’ll just mention a couple of gotchas.


Say you have a custom email validator. You decide you want to shortcut a few things by just deriving from the original email validator and overriding a couple of things. Sounds fine, right?

public class MyCustomEmailValidator : EmailValidator

Or maybe you don’t do that, but you do decide to implement the standard FluentValidation email validator interface:

public class MyCustomEmailValidator : PropertyValidator, IEmailValidator

The gotcha here is that server-side-to-client-side mapping that I showed you earlier. FluentValidation maps by type and already ships with mappings for the out-of-the-box validators. If you implement your custom validator like this, chances are you’re going to end up with the default client-side logic rather than your custom one. The default was registered first, right? It’ll never get to your override… and there’s not really a way to remove mappings.

You also don’t want to override the mapping like this:

  (metadata, context, rule, validator) => new MyCustomFluentValidationPropertyValidator(metadata, context, rule, validator));

You don’t want to do that because then when you use the out-of-the-box email validator, it’ll end up with your custom logic.

This is really painful to debug. Sometimes this is actually what you want - reuse of client (or server) logic but not both… but generally, you want a unique custom validator from end to end.

Recommendation: Don’t implement the standard interfaces unless you plan on replacing the standard behavior across the board and not using the out-of-the-box validator of that type. (That is, if you want to totally replace everything about client and server validation for IEmailValidator, then implement IEmailValidator on your server-side validator and don’t use the standard email validator anymore.)


Say you don’t want to implement a whole custom validator and would rather use the FluentValidation lambda syntax like this:

RuleFor(m => m.Age).Must(val => val - 2 > 6);

That lambda will only run on the server. Maybe that’s OK for a quick one-off, but if you want corresponding client-side validation you actually need to implement the end-to-end solution. There’s no automated functionality for translating the lambdas into script, nor is there an automatic facility for converting this into an AJAX remote validation call. (But that’s a pretty neat idea.)


If you dive deep enough, you’ll notice that certain server-side FluentValidation validators boil down to the same ModelClientValidationRule values when it’s time to emit the data-val- attributes. For example, the FluentValidation GreaterThanValidator and LessThaValidator both end up with ModelClientValidationRule values that say the validation type is “range” (not “min” or “max” as you might expect). What that means is that on the server side, this looks sweet and works:

RuleFor(m => m.Age).GreaterThan(5).LessThan(10);

But when you try to render the corresponding textbox, you’re going to get an exception telling you that you can’t put “range” validation twice on the same field. Problem.

What it means is that when you create custom validations, you have to be mindful of which jQuery rule you’re going to associate it with on the client. It also means you have to be creative, sometimes, about how you set up MVC validations on the server to make sure you don’t have problems when it comes to adapting things to the client-side. Stuff that works on the server won’t necessarily work in MVC.

For reference, here’s a SUPER ABBREVIATED SEQUENCE DIAGRAM of how it comes together. Again, there are several steps omitted here, but this should at least jog your memory and help you visualize all the stuff I mentioned above.