Exception with custom model binding in ASP.NET 5 MVC 6 RC1 API

Question

When trying to parse using a custom model binder in ASP.NET 5 RC1 I get a NullReferenceException thrown within Microsoft.AspNet.Mvc.ModelBinding.CompositeModelBinder when calling the action.

Model binding in Startup:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
    {
        options.ModelBinders.Insert(0, new MyCustomModelBinder());
}

Custom model binder:

public class MyCustomModelBinder : IModelBinder
{
    public Task<ModelBindingResult> BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType == typeof(MyCustomClass) && bindingContext.ValueProvider.GetValue(bindingContext.ModelName) != null)
        {
            MyCustomClass model;
            var val = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).FirstValue as string;

            if (MyCustomClass.TryParse(val, out model))
            {
                return Task.FromResult(ModelBindingResult.Success(bindingContext.ModelName, model));
            }
        }

        return null;
    }
}

Controller action:

[HttpGet]
public IActionResult GetSomething([ModelBinder(BinderType = typeof(MyCustomModelBinder))]MyCustomClass key)
{
    return Json("Success!");
}

Exception:

System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.AspNet.Mvc.ModelBinding.CompositeModelBinder.d__5.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Controllers.DefaultControllerActionArgumentBinder.d__6.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Controllers.DefaultControllerActionArgumentBinder.d__9.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Controllers.DefaultControllerActionArgumentBinder.d__5.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.d__52.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.d__51.MoveNext() --- End of stack trace from previous location where exception was thrown --- at Microsoft.AspNet.Mvc.Controllers.FilterActionInvoker.d__44.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Infrastructure.MvcRouteHandler.d__6.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Mvc.Routing.InnerAttributeRoute.d__10.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Routing.RouteCollection.d__9.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Builder.RouterMiddleware.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Hosting.Internal.RequestServicesContainerMiddleware.d__3.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Hosting.Internal.HostingEngine.<>c__DisplayClass32_0.<b__0>d.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNet.Server.Kestrel.Http.Frame.d__79.MoveNext()

Mike Mike · Accepted Answer · 2015-12-12T13:14:00

Please change return null to return Task.FromResult(ModelBindingResult.NoResult); in BindModelAsync method call.

In CompositeModelBinder.cs on 64 line var result = await binder.BindModelAsync(bindingContext); error is occuring when framework awaits Task that is null

Exception with custom model binding in ASP.NET 5 MVC 6 RC1 API

1 Answers