ModelState is valid with null model

1. Declare your model as a struct
2. Change type of all your required properties to be nullable

I had the same problem before and the answer is already available in a few forums and even here at SO: ModelState.IsValid even when it should not be?

You can also add a custom filter to validate (invalidate) missing fields and/or null values http://www.asp.net/web-api/overview/formats-and-model-binding/model-validation-in-aspnet-web-api

http://www.strathweb.com/2012/10/clean-up-your-web-api-controllers-with-model-validation-and-null-check-filters/

I have found the hint for this problem here. So i will give my solution here.

How do you use my solution? You can register it globally:

config.Filters.Add(new ValidateModelStateAttribute());

Or use it on demand for a class

[ValidateModelState]
public class UsersController : ApiController
{...

or for a methode

[ValidateModelState]
public IHttpActionResult Create([Required] UserModel data)
{...

As you can see, a [System.ComponentModel.DataAnnotations.Required] atribute has been placed in the method parameter. This indicates that the model is required and can not be null.

You can also use with a custom message:

[ValidateModelState]
public IHttpActionResult Create([Required(ErrorMessage = "Custom message")] UserModel data)
{...

Here is my code:

using System;
using System.Collections.Concurrent;
using System.ComponentModel.DataAnnotations;
using System.Net;
using System.Net.Http;
using System.Reflection;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;

namespace your_base_namespace.Web.Http.Filters
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true)]
    public class ValidateModelStateAttribute : ActionFilterAttribute
    {
        private delegate void ValidateHandler(HttpActionContext actionContext);

        private static readonly ConcurrentDictionary<HttpActionBinding, ValidateHandler> _validateActionByActionBinding;

        static ValidateModelStateAttribute()
        {
            _validateActionByActionBinding = new ConcurrentDictionary<HttpActionBinding, ValidateHandler>();
        }

        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            GetValidateHandler(actionContext.ActionDescriptor.ActionBinding)(actionContext);

            if (actionContext.ModelState.IsValid)
                return;

            actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
        }

        private ValidateHandler GetValidateHandler(HttpActionBinding actionBinding)
        {
            ValidateHandler validateAction;

            if (!_validateActionByActionBinding.TryGetValue(actionBinding, out validateAction))
                _validateActionByActionBinding.TryAdd(actionBinding, validateAction = CreateValidateHandler(actionBinding));

            return validateAction;
        }

        private ValidateHandler CreateValidateHandler(HttpActionBinding actionBinding)
        {
            ValidateHandler handler = new ValidateHandler(c => { });

            var parameters = actionBinding.ParameterBindings;

            for (int i = 0; i < parameters.Length; i++)
            {
                var parameterDescriptor = (ReflectedHttpParameterDescriptor)parameters[i].Descriptor;
                var attribute = parameterDescriptor.ParameterInfo.GetCustomAttribute<RequiredAttribute>(true);

                if (attribute != null)
                    handler += CreateValidateHandler(attribute, parameterDescriptor.ParameterName);
            }

            return handler;
        }

        private static ValidateHandler CreateValidateHandler(ValidationAttribute attribute, string name)
        {            
            return CreateValidateHandler(attribute, new ValidationContext(new object()) { MemberName = name });
        }

        private static ValidateHandler CreateValidateHandler(ValidationAttribute attribute, ValidationContext context)
        {
            return new ValidateHandler(actionContext =>
            {
                object value;
                actionContext.ActionArguments.TryGetValue(context.MemberName, out value);

                var validationResult = attribute.GetValidationResult(value, context);
                if (validationResult != null)
                    actionContext.ModelState.AddModelError(context.MemberName, validationResult.ErrorMessage);
            });
        }
    }
}

Here is an action filter to check for null models or invalid models.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;

namespace Studio.Lms.TrackingServices.Filters
{
    public class ValidateViewModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.ActionArguments.Any(kv => kv.Value == null)) {
                actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Arguments cannot be null");
            }

            if (actionContext.ModelState.IsValid == false) {
                actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            }
        }
    }
}

You can register it globally:

config.Filters.Add(new ValidateViewModelAttribute());

Or use it on demand on classes/actions

 [ValidateViewModel]
 public class UsersController : ApiController
 { ...