ASP.NET Core Web API Validation Error Handling

Question

I'm trying to return a custom response object when a model fails validation in my Web API project. I have attached attributes to the model like this:

  public class DateLessThanAttribute : ValidationAttribute
  {
    private readonly string _comparisonProperty;

    public DateLessThanAttribute(string comparisonProperty)
    {
      _comparisonProperty = comparisonProperty;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
      ErrorMessage = ErrorMessageString;
      var currentValue = (DateTime)value;

      var property = validationContext.ObjectType.GetProperty(_comparisonProperty);

      if (property == null)
        throw new ArgumentException("Property with this name not found");

      var comparisonValue = (DateTime)property.GetValue(validationContext.ObjectInstance);

      if (currentValue > comparisonValue)
        return new ValidationResult(ErrorMessage);

      return ValidationResult.Success;
    }
  }

On the model:

[DateLessThan("EndDate", ErrorMessage = "StartDate must be less than EndDate")]
public DateTime StartDate { get; set; }

And the controller:

public void PostCostingStandard(CostStandardRequest request)
{
  CostResult costResult;
  if (ModelState.IsValid)
  {
    // work
  }
  else
  {
    // return bad costResult object
  }
}

However, the model never reaches inside the controller to hit ModelState.IsValid. I've tried creating an ActionFilterAttribute and attaching it to my controller action as outlined here, but when setting a breakpoint, the ActionFilterAttribute never runs because the DateLessThanAttribute returns a response first. Is there an order to these filters that I've missed or have I simply implemented something incorrectly?

What exactly do you mean by the model never reaches inside the controller? How exactly are you testing your application? — Alfredo A.
– Alfredo A., Commented Nov 6, 2018 at 17:38

Danijel · Accepted Answer · 2018-11-06 17:45:06Z

8

You have to disable automatic model state validation. You can do so by adding the following code to your startup.cs

services.Configure<ApiBehaviorOptions>(options =>
{
    options.SuppressModelStateInvalidFilter = true;
});

answered Nov 6, 2018 at 17:45

Danijel

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7 Comments

Alfredo A. Over a year ago

I think you are refering to ASP .Net Core. It seems to me the question is related to ASP .Net

Danijel Over a year ago

It's possible. I will delete the answer if this is the case.

Ben Over a year ago

This is a .Net Core app, sorry I didn't state that clearly! Adding this worked perfectly for me, thanks. Just wish I would've found it stated more explicitly somewhere..

Voodoo Over a year ago

@Danijel You are disabling the default behaviour just to make your code work which will end up with duplication in each ActionMethod.

Danijel Over a year ago

@Voodoo It's not duplication because, in my particular case, we are handling the "error" a little differently. We are returning a more complex JSON object describing what's wrong with custom code + custom validation message + logging the error/sending email + other business logic depending on the case. This default behavior is meant for the most basic use-case while most more "serious" API's often need the control over handling bad requests.

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