Web API Validation not trigger with custom model binder

Question

I'm building Web Service with Web API 5. I'm implementing custom model binder by extending IModelBinder interface to map complex type as a parameter to action. The binding part is working fine. But Model validation does not occur. ModelState.IsValid is always true.

public class PagingParamsVM
{
        [Range(1, Int32.MaxValue, ErrorMessage = "Page must be at least 1")]
        public int? Page { get; set; }

        [Range(1, Int32.MaxValue, ErrorMessage = "Page size must be at least 1")]
        public int? PageSize { get; set; }
}

public class PaginationModelBinder : IModelBinder
{
        public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
        {
              var model = (PagingParamsVM)bindingContext.Model ?? new PagingParamsVM();
              //model population logic
              .....

              bindingContext.Model = model;
              return true;
        }
}

public IEnumerable<NewsItemVM> Get([ModelBinder(typeof(PaginationModelBinder))]PagingParamsVM pegination)
{
            //Validate(pegination); //if I call this explicitly ModelState.IsValid is set correctly.
            var valid = ModelState.IsValid; //this is always true
}

public class ModelStateValidationActionFilter : ActionFilterAttribute
{
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            var valid = actionContext.ModelState.IsValid //this is always true.
        }
}

If I call Validate() explicitly or use [FromUri] attribute, ModelState.IsValid is set correctly.

public IEnumerable<NewsItemVM> Get([FromUri]PagingParamsVM pegination)
{
            var valid = ModelState.IsValid;
}

Should I implement validation part inside model binder. If so how should I implement?

Possible duplicate of [SO answer](http://stackoverflow.com/questions/8668869/custom-model-binder-not-validating-model). — Mihail Stancescu, Dec 05 '16 at 07:49
@MihailStancescu I saw this question. It works fine with DataAnnotations. but if I use FluentValidation or similar, It won't work. So seems something is missing here. — SajithK, Dec 05 '16 at 08:24

score 3 · Accepted Answer · answered Dec 05 '16 at 11:55

I found an answer. The default validation process can be invoked in custom model binder as follows,

public abstract class PaginationModelBinder : IModelBinder
{
        public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
        {
              var model = (PagingParamsVM)bindingContext.Model ?? new PagingParamsVM();
              //model population logic
              .....

              bindingContext.Model = model;

              //following lines invoke default validation on model
              bindingContext.ValidationNode.ValidateAllProperties = true;
              bindingContext.ValidationNode.Validate(actionContext);

              return true;
        }
}

Thank you guys for your support.

@Sajith I have tried the same, but the validation not triggered for the sub entity used in the model. Any option to invoke validations for the entities used in the main model — Magendran V, Mar 27 '18 at 12:01
@MagendranV i'm quite curious if you've managed to find a solution since I've hit the same dead end. The DataAnnotations validation works brilliantly for the 1st level properties, but anything any deeper and it doesn't work — Andrei U, Apr 30 '18 at 15:50

Bob Dust · Answer 2 · 2016-12-05T08:42:50.100

DefaultModelBinder.CreateModel should help you keep model state:

public class PaginationModelBinder : DefaultModelBinder
{
    protected override object CreateModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Type modelType)
    {
        if(modelType == typeof(PagingParamsVM))
        {
            var page = default(int?);
            var model = bindingContext.Model;
            var valueProvider = bindingContext.ValueProvider;
            var pageValue = valueProvider.GetValue("Page");
            var tmp = default(int);
            if(pageValue != null && int.TryParse(pageValue.AttemptedValue, out tmp))
            {
                page = tmp;
            }

            var pageSize = default(int?);
            var sizeValue = valueProvider.GetValue("PageSize");
            if(sizeValue != null && int.TryParse(sizeValue.AttemptedValue, out tmp))
            {
                pageSize = tmp;
            }
            return new PagingParamsVM { Page = page, PageSize = pageSize };
        }
        return base.CreateModel(controllerContext, bindingContext, modelType);
    }
}

A web api controller that uses the binder can be:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;

public class NewsItemController : ApiController
{
    public IEnumerable<NewsItemVM> Get([ModelBinder(typeof(PaginationModelBinder))]PagingParamsVM pegination)
    {
        //Validate(pegination); //if I call this explicitly ModelState.IsValid is set correctly.
        var valid = ModelState.IsValid; //this is always true
        return Enumerable.Empty<NewsItemVM>();
    }
}

Web API does not have DefaultModelBinder. DefaultModelBinder comes with MVC. — SajithK, Dec 05 '16 at 08:21
@sajith, when you develop Web API in a MVC .NET application, `DefaultModelBinder` will come :) — Bob Dust, Dec 05 '16 at 08:27
Yah I know that. But the thing is that you can not use your PaginationModelBinder : DefaultModelBinder in a controller extended ApiController. — SajithK, Dec 05 '16 at 08:36
Have you tried your code? I tried this earlier. it gives the following error, Could not create a 'IModelBinder' from 'PaginationModelBinder'. Please ensure it derives from 'IModelBinder' and has a public parameterless constructor. I think the reason is that the model binder must be derived from System.Web.Http.ModelBinding.IModelBinder. But DefaultModelBinder is in System.Web.Mvc namespace. — SajithK, Dec 05 '16 at 09:01

Web API Validation not trigger with custom model binder

2 Answers2