ASP.NET MVC Model Binding

Model Binding: 

Model binding is the process of creating .NET objects using the data sent by the browser in an HTTP request. MVC Framework has to map the details of the request in such a way that it can pass appropriate values or objects as parameters to action method using default model binder (DefaultModelBinder).

Default Model Binder:  Action invoker used default model binder when it can’t find a custom binder to bind the type for action method. Default model binder search in following location and order for named parameters data.

Source	Description
Request.Form	Contents HTML form elements data.
RouteData.Values	Application routes values
Request.QueryString	Data in the query string of the request URL
Request.Files	Files that have been uploaded as part of the request.

Example: How to find id parameter

• Request.Form["id"]
• RouteData.Values["id"]
• Request.QueryString["id"]
• Request.Files["id"]

System.ComponentModel.TypeDescriptor class use to convert request data into parameter type. If value cannot be convertible then default model binder won’t be able to bind to the model.

Default model binder use reflation to bind for complex data type like class.

Bind attribute: We can use the Bind attribute to include or exclude model properties from the binding process.

Example: To include first and last name in person object. Person FirstName and LastName property value will be consider other will ignored.

public ActionResult Register([Bind(Include = "FirstName, LastName")] Person person)

Example: To exclude sex property in person object. Person sex property value will be ignored.

public ActionResult Register([Bind(Exclude = "Sex")] Person person)

Example: Bind attribute at Model level. Policy applied to the model class cannot be overridden by applying a less restrictive policy to the action method parameter.

[Bind(Exclude = "PersonId")]

We can also restrict binding to a specific data source like form collection or file.

Example:

public ActionResult Register(FormCollection form)

public ActionResult Register(HttpPostedFileBase file)

Custom Model Binder: We can create a custom model binder to override default binder for specific types. We can create custom model binder to create a custom class that implements IModelBinder interface or can create a class that inherits from DefaultModelBinder class.

The IModelBinder interface contains a single method BindModel() that allows you to bind model with the form data.

Example:

public class PersonBinder : IModelBinder

    {

        public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)

        {

            Person person = new Person();

            person.FirstName = controllerContext.HttpContext.Request.Form["FirstName"];

            person.LastName = controllerContext.HttpContext.Request.Form["LastName"];

            person.FullName = string.Format("{0} {1}", person.FirstName, person.LastName);

            person.PersonId = int.Parse(controllerContext.HttpContext.Request.Form["PersonId"]);

            return person;

        }

    }

Use Custom Model Binder:  We can use custom model binder by applying attribute on model or by registering globally using “Global.asax” file.

ModelBinder Attribute : Way of registering a custom model binder is to apply the ModelBinder attribute to the model.

Example:

[ModelBinder(typeof(PersonBinder))]

    public class Person

    {

        public int PersonId { get; set; }

        public string FirstName { get; set; }

        public string LastName { get; set; }

        public string Sex { get; set; }

        [HiddenInput(DisplayValue = false)]

        public string FullName { get; set; }

    }

Assigning model binders in Global.asax: We register our binder through the ModelBinders.Binders.Add method, passing in the type that our binder supports and an instance of the binder class. We can do that in the Application_Start event inside the Global.asax.

ModelBinders.Binders.Add(typeof(Person), new PersonBinder());