How can I validate multiple action parameters with ValidationAttribute in ASP.NET Core 3.0

Question

Consider the following controller action signatures:

[HttpGet]
public IEnumerable<Whatever> Get(DateTime from, DateTime to)

Or

[HttpGet]
public Whatever Get(int amount, SomeUnit unit)

I'd like to be able to run a validation on parameters supplied by the caller. In both cases what passes as valid input in the first parameter depends on the value of second one.

So far I have failed to find an explanation or example of how this can be achieved with validation attributes in ASP.NET Core 3.0

I see that for example CustomValidationAttribute allows using it on methods. I can give it a method that IsValid will delegate to and provide object and validation context:

IsValid(Object, ValidationContext)

I checked documentation of both IsValid and ValidationContext and I cannot find any hint on how would I access the parameters passed to the validated action.

While googling for hints I found an excerpt from "Pro ASP.NET Web API" HTTP Web Services in ASP.NET" where they implement something similar. The excerpt ends however before the code is shown and I don't have that book (didn't find a corresponding book but on ASP.NET Core).

svstnv · Accepted Answer · 2020-02-04 16:10:34Z

3

CustomValidationAttribute is used for properties and parameters. For an action validation you should write your own filter. Here is an example:

public class MyValidationAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if(context.ModelState.IsValid)
        {
            var dateFrom = (DateTime)context.ActionArguments["from"];
            var dateTo = (DateTime)context.ActionArguments["to"];

            if(dateFrom <= dateTo)
            {
                // continue the flow in pipeline
                return;
            }
        }

        context.Result = new BadRequestResult();
    }
}

And then you can use it in your controller:

[HttpGet]
[MyValidation]
public IEnumerable<Whatever> Get(DateTime from, DateTime to)
{
    // Here ModelState.IsValid is true
}

edited Feb 4, 2020 at 16:10

answered Feb 4, 2020 at 16:01

svstnv

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3 Comments

Emil Over a year ago

Thank you for answering. I see this will work, but I was hoping asp.net core would offer a more elegant solution. You see, I already have ActionFilter (applied globally to all controllers) which checks if ModelState is not valid and then returns my custom error reponse with all the validation errors. With your approach, I am not sure how much control do I have over which action filter acts first. If there are already some validation errors on the model, and my error-returning filter would act first, I would return potentially incomplete number of validation errors to the caller.

svstnv Over a year ago

@Emil you can set a filter's Order property

Yarek T Over a year ago

Is there any way to use existing validators? [StringLength(1)] etc?

Elijah Sink · Accepted Answer · 2025-02-05 15:15:05Z

You can put your action's parameters into an object that defines validation logic using the IValidatableObject interface. For example:

using System.ComponentModel.DataAnnotations;
using Microsoft.AspNetCore.Mvc;


public class WhateverRequest : IValidatableObject
{
    public DateTime From { get; set; }

    public DateTime To { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        // Sample validations that use both properties:
        if (From > To)
        {
            yield return new ValidationResult(
                "The 'from' date must be earlier than or equal to the 'to' date.",
                [nameof(From), nameof(To)]
            );
        }

        if ((To - From).TotalDays > 365)
        {
            yield return new ValidationResult(
                "The date range cannot exceed 1 year.",
                [nameof(From), nameof(To)]
            );
        }
    }
}

Used like so:

[HttpGet]
public IActionResult Get([FromQuery] WhateverRequest request)
{
    // Your filter can handle the validation state, or you could do it here:
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    return Ok(/* your response */);
}

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