Asp.Net MVC3: Set custom IServiceProvider in ValidationContext so validators can resolve services

Question

Update 18th December 2012

Since this question seems to be getting quite a few views, I should point out that the accepted answer is not

Answer 1

On MVC 5.2, you can leveragesteal @Andras's answer and the MVC source and:

1. Derive a DataAnnotationsModelValidatorEx from DataAnnotationsModelValidator

namespace System.Web.Mvc
{
    // From https://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Web.Mvc/DataAnnotationsModelValidator.cs
    // commit 5fa60ca38b58, Apr 02, 2015
    // Only diff is adding of secton guarded by THERE_IS_A_BETTER_EXTENSION_POINT
    public class DataAnnotationsModelValidatorEx : DataAnnotationsModelValidator
    {
        readonly bool _shouldHotwireValidationContextServiceProviderToDependencyResolver;

        public DataAnnotationsModelValidatorEx(
            ModelMetadata metadata, ControllerContext context, ValidationAttribute attribute,
            bool shouldHotwireValidationContextServiceProviderToDependencyResolver=false)
            : base(metadata, context, attribute)
        {
           _shouldHotwireValidationContextServiceProviderToDependencyResolver =
                shouldHotwireValidationContextServiceProviderToDependencyResolver;
        }
    }
}

2. Clone the base impl of public override IEnumerable<ModelValidationResult> Validate(object container)

3. Apply the hack Render the elegant incision after Validate creates the context:-

public override IEnumerable Validate(object container) { // Per the WCF RIA Services team, instance can never be null (if you have // no parent, you pass yourself for the "instance" parameter). string memberName = Metadata.PropertyName ?? Metadata.ModelType.Name; ValidationContext context = new ValidationContext(container ?? Metadata.Model) { DisplayName = Metadata.GetDisplayName(), MemberName = memberName };

#if !THERE_IS_A_BETTER_EXTENSION_POINT
   if(_shouldHotwireValidationContextServiceProviderToDependencyResolver 
       && Attribute.RequiresValidationContext)
       context.InitializeServiceProvider(DependencyResolver.Current.GetService);
#endif

   ValidationResult result = Attribute.GetValidationResult(Metadata.Model, context);
    if (result != ValidationResult.Success)
    {
        // ModelValidationResult.MemberName is used by invoking validators (such as ModelValidator) to
        // construct the ModelKey for ModelStateDictionary. When validating at type level we want to append the
        // returned MemberNames if specified (e.g. person.Address.FirstName). For property validation, the
        // ModelKey can be constructed using the ModelMetadata and we should ignore MemberName (we don't want
        // (person.Name.Name). However the invoking validator does not have a way to distinguish between these two
        // cases. Consequently we'll only set MemberName if this validation returns a MemberName that is different
        // from the property being validated.

       string errorMemberName = result.MemberNames.FirstOrDefault();
        if (String.Equals(errorMemberName, memberName, StringComparison.Ordinal))
        {
            errorMemberName = null;
        }

       var validationResult = new ModelValidationResult
        {
            Message = result.ErrorMessage,
            MemberName = errorMemberName
        };

       return new ModelValidationResult[] { validationResult };
    }

   return Enumerable.Empty<ModelValidationResult>();
}

4. Tell MVC about the new DataAnnotationsModelValidatorProvider in town

after your Global.asax does DependencyResolver.SetResolver(new AutofacDependencyResolver(container)) :-

DataAnnotationsModelValidatorProvider.RegisterAdapterFactory(
    typeof(ValidatorServiceAttribute),
    (metadata, context, attribute) => new DataAnnotationsModelValidatorEx(metadata, context, attribute, true));

5. Use your imagination to abuse your new Service Locator consume using ctor injection via GetService in your ValidationAttribute, for example:

public class ValidatorServiceAttribute : ValidationAttribute
{
    readonly Type _serviceType;

    public ValidatorServiceAttribute(Type serviceType)
    {
        _serviceType = serviceType;
    }

    protected override ValidationResult IsValid(
        object value, 
        ValidationContext validationContext)
    {
        var validator = CreateValidatorService(validationContext);
        var instance = validationContext.ObjectInstance;
        var resultOrValidationResultEmpty = validator.Validate(instance, value);
        if (resultOrValidationResultEmpty == ValidationResult.Success)
            return resultOrValidationResultEmpty;
        if (resultOrValidationResultEmpty.ErrorMessage == string.Empty)
            return new ValidationResult(ErrorMessage);
        return resultOrValidationResultEmpty;
    }

    IModelValidator CreateValidatorService(ValidationContext validationContext)
    {
        return (IModelValidator)validationContext.GetService(_serviceType);
    }
}

Allows you to slap it on your model:-

class MyModel 
{
    ...
    [Required, StringLength(42)]
    [ValidatorService(typeof(MyDiDependentValidator), 
        ErrorMessage = "It's simply unacceptable")]
    public string MyProperty { get; set; }
    ....
}

which wires it to a:

public class MyDiDependentValidator : Validator<MyModel>
{
    readonly IUnitOfWork _iLoveWrappingStuff;

    public MyDiDependentValidator(IUnitOfWork iLoveWrappingStuff)
    {
        _iLoveWrappingStuff = iLoveWrappingStuff;
    }

    protected override bool IsValid(MyModel instance, object value)
    {
        var attempted = (string)value;
        return _iLoveWrappingStuff.SaysCanHazCheez(instance, attempted);
    }
}

The preceding two are connected by:

interface IModelValidator
{
    ValidationResult Validate(object instance, object value);
}

public abstract class Validator<T> : IModelValidator
{
    protected virtual bool IsValid(T instance, object value)
    {
        throw new NotImplementedException(
            "TODO: implement bool IsValid(T instance, object value)" +
            " or ValidationResult Validate(T instance, object value)");
    }

    protected virtual ValidationResult Validate(T instance, object value)
    {
        return IsValid(instance, value) 
            ? ValidationResult.Success 
            : new ValidationResult("");
    }

    ValidationResult IModelValidator.Validate(object instance, object value)
    {
        return Validate((T)instance, value);
    }
}

I'm open to corrections, but most of all, ASP.NET team, would you be open to a PR to add a constructor with this facility to DataAnnotationsModelValidator?

Answer 2

Have you thought about creating a model validator, using a modelValidatorProvider, instead of using validation attributes? This way you're not dependant on ValidationAttribute but can create your own validation implementation (this will work in addition the existing DataAnnotations validation).

http://msdn.microsoft.com/en-us/library/system.web.mvc.modelvalidatorprovider.aspx

http://dotnetslackers.com/articles/aspnet/Experience-ASP-NET-MVC-3-Beta-the-New-Dependency-Injection-Support-Part2.aspx#s10-new-support-for-validator-provider

http://dotnetslackers.com/articles/aspnet/Customizing-ASP-NET-MVC-2-Metadata-and-Validation.aspx#s2-validation

Answer 3

Update

In addition to the class shown below, I've done a similar thing for IValidatableObject implementations as well (short notes towards the end of the answer instead of a full code sample because then the answer just gets too long) - I've added the code for that class as well in response to a comment - it does make the answer very long, but at least you'll have all the code you need.

Original

Since I'm targeting ValidationAttribute-based validation at the moment I researched where MVC creates the ValidationContext that gets fed to the GetValidationResult method of that class.

Turns out it's in the DataAnnotationsModelValidator's Validate method:

public override IEnumerable<ModelValidationResult> Validate(object container) {
  // Per the WCF RIA Services team, instance can never be null (if you have
  // no parent, you pass yourself for the "instance" parameter).
  ValidationContext context = new ValidationContext(
    container ?? Metadata.Model, null, null);
  context.DisplayName = Metadata.GetDisplayName();

  ValidationResult result = 
    Attribute.GetValidationResult(Metadata.Model, context);

  if (result != ValidationResult.Success) {
    yield return new ModelValidationResult {
      Message = result.ErrorMessage
    };
  }
}

(Copied and reformatted from MVC3 RTM Source)

So I figured some extensibility here would be in order:

public class DataAnnotationsModelValidatorEx : DataAnnotationsModelValidator
{
  public DataAnnotationsModelValidatorEx(
    ModelMetadata metadata, 
    ControllerContext context, 
    ValidationAttribute attribute)
    : base(metadata, context, attribute)
  {
  }

  public override IEnumerable<ModelValidationResult> Validate(object container)
  {
    ValidationContext context = CreateValidationContext(container);

    ValidationResult result = 
      Attribute.GetValidationResult(Metadata.Model, context);

    if (result != ValidationResult.Success)
    {
      yield return new ModelValidationResult
      {
        Message = result.ErrorMessage
      };
    }
  }

  // begin Extensibility

  protected virtual ValidationContext CreateValidationContext(object container)
  {
    IServiceProvider serviceProvider = CreateServiceProvider(container);
    //TODO: add virtual method perhaps for the third parameter?
    ValidationContext context = new ValidationContext(
      container ?? Metadata.Model, 
      serviceProvider, 
      null);
    context.DisplayName = Metadata.GetDisplayName();
    return context;
  }

  protected virtual IServiceProvider CreateServiceProvider(object container)
  {
    IServiceProvider serviceProvider = null;

    IDependant dependantController = 
      ControllerContext.Controller as IDependant;

    if (dependantController != null && dependantController.Resolver != null)
      serviceProvider = new ResolverServiceProviderWrapper
                        (dependantController.Resolver);
    else
      serviceProvider = ControllerContext.Controller as IServiceProvider;
    return serviceProvider;
  }
}

So I check first for my IDependant interface from the controller, in which case I create an instance of a wrapper class that acts as an adapter between my IDependencyResolver interface and System.IServiceProvider.

I thought I'd also handle cases where a controller itself is an IServiceProvider too (not that that applies in my case - but it's a more general solution).

Then I make the DataAnnotationsModelValidatorProvider use this validator by default, instead of the original:

//register the new factory over the top of the standard one.
DataAnnotationsModelValidatorProvider.RegisterDefaultAdapterFactory(
  (metadata, context, attribute) => 
    new DataAnnotationsModelValidatorEx(metadata, context, attribute));

Now 'normal' ValidationAttribute-based validators, can resolve services:

public class ExampleAttribute : ValidationAttribute
{
  protected override ValidationResult 
    IsValid(object value, ValidationContext validationContext)
  {
    ICardTypeService service = 
      (ICardTypeService)validationContext.GetService(typeof(ICardTypeService));
  }
}

This still leaves direct ModelValidator-derived needing to be reimplemented to support the same technique - although they already have access to the ControllerContext, so it's less of an issue.

Update

A similar thing has to be done if you want IValidatableObject-implementing types to be able to resolve services during the implementation of Validate without having to keep deriving your own adapters for each type.

• Derive a new class from ValidatableObjectAdapter, I called it ValidatableObjectAdapterEx
• from MVCs v3 RTM source, copy the Validate and ConvertResults private method of that class.
• Adjust the first method to remove references to internal MVC resources, and
• change how the ValidationContext is constructed

Update (in response to comment below)

Here's the code for the ValidatableObjectAdapterEx - and I'll point out hopefully more clearly that IDependant and ResolverServiceProviderWrapper used here and before are types that only apply to my environment - if you're using a global, statically-accessible DI container, however, then it should be trivial to re-implement these two classes' CreateServiceProvider methods appropriately.

public class ValidatableObjectAdapterEx : ValidatableObjectAdapter
{
  public ValidatableObjectAdapterEx(ModelMetadata metadata, 
                                    ControllerContext context)
   : base(metadata, context) { }

  public override IEnumerable<ModelValidationResult> Validate(object container)
  {
    object model = base.Metadata.Model;
    if (model != null)
    {
      IValidatableObject instance = model as IValidatableObject;
      if (instance == null)
      {
        //the base implementation will throw an exception after 
        //doing the same check - so let's retain that behaviour
        return base.Validate(container);
      }
      /* replacement for the core functionality */
      ValidationContext validationContext = CreateValidationContext(instance);
      return this.ConvertResults(instance.Validate(validationContext));
    }
    else
      return base.Validate(container);  /*base returns an empty set 
                                          of values for null. */
  }

  /// <summary>
  /// Called by the Validate method to create the ValidationContext
  /// </summary>
  /// <param name="instance"></param>
  /// <returns></returns>
  protected virtual ValidationContext CreateValidationContext(object instance)
  {
    IServiceProvider serviceProvider = CreateServiceProvider(instance);
    //TODO: add virtual method perhaps for the third parameter?
    ValidationContext context = new ValidationContext(
      instance ?? Metadata.Model,
      serviceProvider,
      null);
    return context;
  }

  /// <summary>
  /// Called by the CreateValidationContext method to create an IServiceProvider
  /// instance to be passed to the ValidationContext.
  /// </summary>
  /// <param name="container"></param>
  /// <returns></returns>
  protected virtual IServiceProvider CreateServiceProvider(object container)
  {
    IServiceProvider serviceProvider = null;

    IDependant dependantController = ControllerContext.Controller as IDependant;

    if (dependantController != null && dependantController.Resolver != null)
    {
      serviceProvider = 
        new ResolverServiceProviderWrapper(dependantController.Resolver);
    }
    else
      serviceProvider = ControllerContext.Controller as IServiceProvider;

    return serviceProvider;
  }

  //ripped from v3 RTM source
  private IEnumerable<ModelValidationResult> ConvertResults(
    IEnumerable<ValidationResult> results)
  {
    foreach (ValidationResult result in results)
    {
      if (result != ValidationResult.Success)
      {
        if (result.MemberNames == null || !result.MemberNames.Any())
        {
          yield return new ModelValidationResult { Message = result.ErrorMessage };
        }
        else
        {
          foreach (string memberName in result.MemberNames)
          {
            yield return new ModelValidationResult 
             { Message = result.ErrorMessage, MemberName = memberName };
          }
        }
      }
    }
  }
}

End Code

With that class in place, you can register this as the default adapter for IValidatableObject instances with the line:

DataAnnotationsModelValidatorProvider.
  RegisterDefaultValidatableObjectAdapterFactory(
    (metadata, context) => new ValidatableObjectAdapterEx(metadata, context)
  );