How Default Model Binder works under the hood

If you have a requirement to write a custom model binding by implementing interface IModelBinder, then it's better to have a peek on DefaultModelBinder which is implemented in System.Web.Mvc. There is a lot which happens in this class and for most of the cases I prefer to create my custom model binder by deriving from DefaultModelBinder. In this way I can override whatever I need to and rest I can get for free.

There is a very less documentation around DefaultModelBinder so to find out what happens under the hood I decompiled System.Web.Mvc.DefaultModelBinder. Here are some of the important methods

• BindModel - Only Member of IModelBinder interface. This binds simple as well as complex model.
• BindComplexModel - This Creates Model by calling CreateModel and then call BindComplexElementalModel which calls BindProperties.
• CreateModel - This create an instance of Model Type which you specify as a parameter type in the controller. So in the particular case CreateModel will return an object of type Employee.
  

  [HttpPost]
      public ActionResult EmployeeList(Employee employee)
     

  one case in which I had to override this was when I had a requirement to support for Interface type parameter. Refer to my posting on custom model binding
  
• BindProperties - Binds all the properties of the model class by using controller context and binding context. This is the place where lot of magic happens. In the above case this will loop through all the properties of Employee and then bind from the controller context. This is one of the best part of default model binder which saves lot of development time. So for example it will bind the value of the property from HttpContext.Request.Form, which is collection of form variables. By default there are five ValueProviders:ChildActionValueProvider, FormValueProvider, RouteDataValueProvider, QueryStringValueProvider, HttpFileCollectionValueProvider.
• BindProperty Calls GetPropertyValue which call BindModel which calls BindSimpleModel which gets the value.

In case of filters, DefaultModelBinder creates the Model object and in the filter you can access this model object using filterContext.ActionParameters[key] and populates the model object. In this case no binding happens in the DefaultModelBinder. Put a break point at the following location CreateModel, BindModel in the DefaultModelBinder and in your filter actionexecuting. You will notice that BindModel is called first and then CreateModel which creates the model object and then the filter action will be called. In filter action you can access the model object which CreateModel created using filterContext.ActionParameters[key]


The ModelBindingContext object is created and populated by whoever calls into the BindModel() method. If the model is coming in as an argument to your action method, this is done by ControllerActionInvoker.GetParameterValue(), and the ModelName property will be set to the name of the parameter (unless overridden by [Bind(Prefix = ... )]).

ModelBindingContext bindingContext = new ModelBindingContext() {
  FallbackToEmptyPrefix = (parameterDescriptor.BindingInfo.Prefix == null), // only fall back if prefix not specified 
  ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, parameterType),
  ModelName = parameterName, 
  ModelState = controllerContext.Controller.ViewData.ModelState, 
  PropertyFilter = propertyFilter,
  ValueProvider = controllerContext.Controller.ValueProvider 
 };
 

If the model is being updated via UpdateModel(), the ModelBindingContext object is created by the UpdateModel() method itself. The ModelName parameter will be the prefix argument passed to UpdateModel().