I also have a javascript service (in Angular) that makes an AJAX request to one of my controllers.

...

The problem with this is that I can't use my custom binder as the else block is never invoked.

• I am going to assume that you have properly registered your binder either globally OR on the action itself on one or more of the parameters.
• I will also assume that your binder is being called when you expect it to.

Its null because it can't find the data based on your model name that it is trying to bind to. Whether the value can be found by this name depends on the model name and the data sent in the client request which have to align/match. But before anyone can tell you why its not matching data (including your model with the array) can be sent from the client in one of three ways:

1. If using the URL you will reuse the same property name in the query string. Example: ?myArray=1&myArray=2&myArray=3. This means that in your model binder you will have to consider this.
2. If using the data (body) in a POST then it might be an actual array object. Example in json: {"myArray":[1,2,3,4]}
3. You might also be serializing the entire form and sending it over with Angular (which would allow you to better use the binding functionality in MVC)

So to better answer your question you will need to provide

1. The format of the data is being sent from the browser and how its being sent (query string or in the data payload) (this is probably in your Angular factory,service, or controller)
2. The model definition you are trying to bind to

So to recap: bindingContext.ModelName is the expected name and must match the data that your model binder is trying to find. If you are sending {"myArray":[1,2,3,4]} but your model property is named ProductIds then it will always be null.

I recently updated from 4.5 to 5.2.something

No, not that I am aware of.

Final thought. You could also let the default model binder execute and then do something with the returned value if the types match. If the binding happens now without a problem but you want to do some post-processing this would be a better choice. Example:

public override object BindModel(Controller context, ModelBindingContext bindingContext)
{
    var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);

    object returnVal = base.BindModel(controllerContext, bindingContext);

    /* check returnVal and then additional custom logic here */.
    
    return returnVal;
}

Please make sure you have done something like this:-

1. Custom Model Binding:-

   public class HomeCustomDataBinder : DefaultModelBinder
   {
       public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
       {
           if (bindingContext.ModelType == typeof(HomePageModels))
           {
               HttpRequestBase request = controllerContext.HttpContext.Request;
   
               string title = request.Form.Get("Title");
               string day = request.Form.Get("Day");
               string month = request.Form.Get("Month");
               string year = request.Form.Get("Year");
   
               return new HomePageModels
               {
                   Title = title,
                   Date = day + "/" + month + "/" + year
               };
   
               //// call the default model binder this new binding context
               //return base.BindModel(controllerContext, newBindingContext);
           }
           else
           {
               return base.BindModel(controllerContext, bindingContext);
           }
       }
   
   } 
   

2. Once we have completed coding our custom class we will need to register the class which I do in the Global.asax under Application_Start().

   protected void Application_Start()
   {
       AreaRegistration.RegisterAllAreas();
       WebApiConfig.Register(GlobalConfiguration.Configuration);
       FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
       RouteConfig.RegisterRoutes(RouteTable.Routes);
       BundleConfig.RegisterBundles(BundleTable.Bundles);
       AuthConfig.RegisterAuth();
       ModelBinders.Binders.Add(typeof(HomePageModels), new HomeCustomBinder());
   }
   

   3) Finally we need to inform the controller as to the binding we want it to use. This we can specify using attributes [ModelBinder(typeof(HomeCustomBinder))] as below:

   [HttpPost]
   public ActionResult Index([ModelBinder(typeof(HomeCustomBinder))] HomePageModels home)
   {
       if (ModelState.IsValid)
       {
           ViewBag.Title = home.Title;
           ViewBag.Date = home.Date;
       }
       return View();
   }