Object reference not set to an instance of an object when using custom model binding

Question

I am trying to add a record into my database using custom model binder

public class PostModelBinder: DefaultModelBinder
{
    private IBlogRepository repository;

    public PostModelBinder(IBlogRepository repo)
    {
        repository = repo;
    }

    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        var post = (Post)base.BindModel(controllerContext, bindingContext);

        if (post.Category != null)
            post.Category = repository.Category(post.Category.CategoryID);

        var tags = bindingContext.ValueProvider.GetValue("Tags").AttemptedValue.Split(',');

        if (tags.Length > 0)
        {
            post.Tags = new List<Tag>();

            foreach (var tag in tags)
            {
                post.Tags.Add(repository.Tag(int.Parse(tag.Trim())));
            }
        }

        return post;
    }
}

when I try to submit my record, I get Object reference not set to an instance of an object error on this line

post.Category = repository.Category(post.Category.CategoryID);

I am not sure why its causing this error.

Here is how I set up model binder in Global.asax.cs

//model binder
var repository = DependencyResolver.Current.GetService<IBlogRepository>();
ModelBinders.Binders.Add(typeof(Post), new PostModelBinder(repository));

My repository

public Category Category(int id)
{
    return context.Categories.FirstOrDefault(c => c.CategoryID == id);
}

and my controller action

[HttpPost]
public ContentResult AddPost(Post post)
{
        string json;

        ModelState.Clear();

        if (TryValidateModel(post))
        {
            var id = repository.AddPost(post);

            json = JsonConvert.SerializeObject(new
            {
                id = id,
                success = true,
                message = "Post added successfully."
            });
        }
        else
        {
            json = JsonConvert.SerializeObject(new
            {
                id = 0,
                success = false,
                message = "Failed to add the post."
            });
        }
        return Content(json, "application/json");
}

and this is the factory i am using:

public class NinjectControllerFactory: DefaultControllerFactory
{
    private IKernel ninjectKernel;

    public NinjectControllerFactory()
    {
        ninjectKernel = new StandardKernel();
        AddBindings();
    }

    protected override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
    {
        return controllerType == null
            ? null
            : (IController)ninjectKernel.Get(controllerType);
    }

    private void AddBindings()
    {
        ninjectKernel.Bind<IBlogRepository>().To<EFBlogRepository>();
        ninjectKernel.Bind<IAuthProvider>().To<FormsAuthProvider>();
    }
}

possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — nvoigt, Aug 11 '14 at 06:14
@nvoigt Thanks, I get that post.Category is giving me null value, but what I don't understand is why it is giving me null — Mindless, Aug 11 '14 at 06:39
post your view too, also I'm not sure how you submit the data, using submit button? ajax? etc.., perhaps you need to create hidden field in the view `@Html.HiddenFieldFor(m => m.Category.CategoryID)`, I'm just guessing here with limited information — Yuliam Chandra, Aug 11 '14 at 06:59
@Yuliam my view is a jqgrid, so there isn't any custom code, just a url linking to my controller — Mindless, Aug 11 '14 at 08:16

score 2 · Accepted Answer · edited May 23 '17 at 12:11

I fixed the issue.

First of all, I switched ninject controller factory with ninject dependency resolver.

After using the dependency resolver, I get another error saying "An entity object cannot be referenced by multiple instances of IEntityChangeTracker" See Here

So I tried the suggested answer in that question and still no hope(but this also helped me in the end). I then asked another question in the comment section of this post part-5-creating-a-post, and realized there is a ninject mvc extension for dependency injection, I installed the extension, added my bindings in there and removed my own dependency resolver, with the help of previous answer, I added "InRequestScope()" (You have to have Ninject.Web.Common for this) and it worked.

So inside NinjectWebCommon.cs under App_Start folder:

kernel.Bind<IBlogRepository>().To<EFBlogRepository>().InRequestScope();

score 1 · Answer 2 · answered Aug 12 '14 at 09:23

1

This is a model binding issue. The Category object is not properly binded with the POSTed values. Use Firebug or some tool and see whether the CategoryID is POSTed from jqGrid.

answered Aug 12 '14 at 09:23

VJAI

32,167
23
102
164

The grid is indeed posting the CategoryID, I've checked my POSTed value with a WORKING solution, both are exactly the same. The only difference in coding is i am using entity framework instead of NHibernate, and my custom model binder is different, I think the problem is in my custom model binder, but I don't know what it is – Mindless Aug 12 '14 at 10:10

Object reference not set to an instance of an object when using custom model binding

2 Answers2