Looks like the Model's don't participate in the Data Validation system, when we custom Model Bind.

I added some code so that Data Model Validation would work:

   public class JsonModelBinder : IModelBinder
    {
        private readonly MvcJsonOptions _options;

        public JsonModelBinder(IOptions<MvcJsonOptions> options) =>
            _options = options.Value;

        public Task BindModelAsync(ModelBindingContext bindingContext)
        {
            if (bindingContext == null)
            {
                throw new ArgumentNullException(nameof(bindingContext));
            }

            // Test if a value is received
            var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
            if (valueProviderResult != ValueProviderResult.None)
            {
                bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);

                // Deserialize from string
                string serialized = valueProviderResult.FirstValue;

                // Use custom json options defined in startup if available
                object deserialized = _options?.SerializerSettings == null ?
                    JsonConvert.DeserializeObject(serialized, bindingContext.ModelType) :
                    JsonConvert.DeserializeObject(serialized, bindingContext.ModelType, _options.SerializerSettings);

                // DataAnnotation Validation. Validate Properties and Fields.
                var validationResultProps = from prop in TypeDescriptor.GetProperties(deserialized).Cast<PropertyDescriptor>()
                                       from attribute in prop.Attributes.OfType<ValidationAttribute>()
                                       where !attribute.IsValid(prop.GetValue(deserialized))
                                       select new { Propertie = prop.Name, ErrorMessage = attribute.FormatErrorMessage(string.Empty) };

                var validationResultFields = from field in TypeDescriptor.GetReflectionType(deserialized).GetFields().Cast<FieldInfo>()
                                       from attribute in field.GetCustomAttributes<ValidationAttribute>()
                                       where !attribute.IsValid(field.GetValue(deserialized))
                                       select new { Propertie = field.Name, ErrorMessage = attribute.FormatErrorMessage(string.Empty) };

                var errors = validationResultFields.Concat(validationResultFields);

                // Add the ValidationResult's to the ModelState
                foreach (var validationResultItem in errors)
                    bindingContext.ModelState.AddModelError(validationResultItem.Propertie, validationResultItem.ErrorMessage);

                // Set successful binding result
                bindingContext.Result = ModelBindingResult.Success(deserialized);
#if NET451
                return Task.FromResult(0);
#else
                return Task.CompletedTask;
#endif
            }
#if NET451
            return Task.FromResult(0);
#else
            return Task.CompletedTask;
#endif
        }
    }

I hacked this together from various internet posts. I wonder if there isn't an easier way to just ask the deserialzied object to validate itself?

For now, I iterate the Properties and Fields, and call the ValidationAttribute.IsValid against the data.

Im trying with your solution but I have a problem. Model is always null. Only image file was read correctly. This is post request which is send from my video camera after license plate is recognized:

POST /alpv/receive.php HTTP/1.1\r\n Host: 10.13.24.13\r\n Content-Length: 7160\r\n Content-Type: multipart/form-data; boundary=------------------------3df4e8eee91da308\r\n --------------------------3df4e8eee91da308 Content-Disposition: form-data; name="event"; filename="20190204103017_10536event_7000175.json" Content-Type: application/octet-stream { "packetCounter":"7000175", "datetime":"20190204 103017000", "plateText":"\u004c\u0042\u0045\u0033\u0039\u0037", "plateCountry":"SWE", "plateConfidence":"0.716159", "carState":"new", "geotag":{"lat": 50.418114,"lon": 30.476213}, "imageType": "plate", "plateImageType": "png", "plateImageSize": "0", "carMoveDirection":"in", "timeProcessing":"0", "plateCoordinates":[1614, 880, 172, 28], "carID":"18", "GEOtarget":"Camera", "sensorProviderID":"defaultID" } --------------------------3df4e8eee91da308 Content-Disposition: form-data; name="image"; filename="20190204103016_999776lp_LBE397_7000175.png" Content-Type: application/octet-stream <IMAGE DATA> --------------------------3df4e8eee91da308--

There are two files: "image" and "event".

In controller I write this:

public IActionResult NewEvent([ModelBinder(BinderType = typeof(JsonModelBinder))] DataFromCameraModel @event, IFormFile image) { return Ok(); }

and DataFromCameraModel

public class DataFromCameraModel { public int packetCounter {get;set;} public string Datetime {get;set;} public string plateText {get;set;} }

Just wondering anyone who using this package met the error as the title? Reference as below:

.NET Core 3.0 introduces a new platform native Json deserializer. This is used by default by ASP.NET Core 3.0. This package, however, does imply you use Newtonsoft.Json and we blindly inject IOptions<MvcNewtonsoftJsonOptions>.

This is suboptimal as you cannot use this package with the new default Json deserializer and needs to be addressed.

First of all, thank you very much for this code.
I just wanted to let you know there is a bug in your code.

This line:
var errors = validationResultFields.Concat(validationResultFields);
should be
var errors = validationResultFields.Concat(validationResultProps);

Again, thank you!

Please, make a model validation optional.
As it is a slow operation via Reflection + LINQ etc.
It would be great to have bool IsValidateModel property in the binding constructor.

Hi, I think declaring this library as Microsoft.AspNetCore.Mvc.ModelBinding is not "fair".

This makes programmers understand something wrong.

For example: 1 parameter acquired but value is empty, it turn unhanded exception and Controller not invoked.

If add in JsonModelBinder.cs: 43 fix below, Controller will be invoked and parameter == null as is.

// Add null check for parameter
if (string.IsNullOrEmpty(serialized))
{
bindingContext.Result = ModelBindingResult.Success(null);
return Task.CompletedTask;
}

Before fix:

System.ArgumentNullException: Value cannot be null. (Parameter 'instance') at System.ComponentModel.TypeDescriptor.GetReflectionType(Object instance) at BrunoZell.ModelBinding.JsonModelBinder.BindModelAsync(ModelBindingContext bindingContext) in C:\Users\b.yanish\source\repos\JsonModelBinder\src\JsonModelB inder\JsonModelBinder.cs:line 59 at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.BinderTypeModelBinder.BindMo delAsync(ModelBindingContext bindingContext) at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.BindModelAsync(Actio nContext actionContext, IModelBinder modelBinder, IValueProvider valueProvider, ParameterDescriptor parameter, ModelMetadata metadata, Object value) at Microsoft.AspNetCore.Mvc.Controllers.ControllerBinderDelegateProvider.<>c_ _DisplayClass0_0.<<CreateBinderDelegate>g__Bind|0>d.MoveNext() --- End of stack trace from previous location where exception was thrown --- at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInn erFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, St ate next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourc eFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scop e scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceEx ecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipel ineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Sco pe scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Aw aited|17_0(ResourceInvoker invoker, Task task, IDisposable scope) at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTas k|6_0(Endpoint endpoint, Task requestTask, ILogger logger) at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpCont ext context) at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.Proces sRequests[TContext](IHttpApplication1 application)
`

After fix:

Hi,

I saw the reference to this library at a StackOverflow question.

Does this library work with dot net core 2, as I always get null value for the parameter decorated with
the [ModelBinder(BinderType = typeof(JsonModelBinder))] controller action method.

I had created an empty project and then installed the library via Nuget.