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ASP.NET Core Model Binding

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In this video we will discuss Model Binding in ASP.NET Core with examples.


What is Model Binding
  • Model binding maps data in an HTTP request to controller action method parameters
  • The action parameters may be simple types such as integers, strings, etc or complex types like Customer, Employee, Order etc.
  • Model binding is great, because without it we have to write lot of custom code to map request data to action method parameters which is not only tedious but also error prone.

ASP.NET Core Model Binding Example

When an HTTP request arrives at our MVC application it is the Controller action method that handles the incoming request. Let's say we want to view employee details whose ID is 2. For this we issue a GET request to the following URL

http://localhost:48118/home/details/2

Our application default route template ({controller=Home}/{action=Index}/{id?}) routes this request to Details(int? id) action method of the HomeController.

public ViewResult Details(int? id)
{
    HomeDetailsViewModel homeDetailsViewModel = new HomeDetailsViewModel()
    {
        Employee = _employeeRepository.GetEmployee(id ?? 1),
        PageTitle = "Employee Details"
    };

    return View(homeDetailsViewModel);
}

The id value 2 in the request URL is mapped to the id parameter of the details(int? id) action method. MVC will bind the data in the request to the action parameters by name. Notice, in the above example, the parameter name in the default route template is "id" and the parameter name on the Details(int? id) action method is also id. So the value 2 in the URL (http://localhost:48118/home/details/2) is mapped to the id parameter on the Details(int? id) action method.

Another Example

Request URL
http://localhost:48118/home/details/2?name=pragim

The following Details() action method handles the above request URL and maps the value 2 to the id parameter and the value pragim to the name parameter

public string Details(int? id, string name)
{
    return "id = " + id.Value.ToString() + " and name = " + name;
}

HTTP Request Data

To bind the request data to the controller action method parameters, model binding looks for data in the HTTP request in the following places in the order specified below.
  • Form values
  • Route values
  • Query strings
Model Binding Complex Types

Model Binding also works with complex types such as Customer, Order, Employee etc. Consider the following "Create Employee Form"

asp.net core model binding complex object

When the above form is posted to the server, the values in the form are mapped to the Employee object parameter of the following Create() action method.

public RedirectToActionResult Create(Employee employee)
{
    Employee newEmployee = _employeeRepository.Add(employee);
    return RedirectToAction("details", new { id = newEmployee.Id });
}
  • Model binder in asp.net core binds the posted form values to the properties of the Employee object that is passed as a parameter to the Create() action method.
  • The value in the input element that has the name attribute set to "Name" is mapped to the Name property of the Employee object
  • Similarly value in the input element with name "Email" is mapped to Email property of the Employee object
  • The same is true for Department
Add() method - IEmployeeRepository interface

public interface IEmployeeRepository
{
    Employee GetEmployee(int Id);
    IEnumerable<Employee> GetAllEmployees();
    Employee Add(Employee employee);
}

Add() method - MockEmployeeRepository class 

public class MockEmployeeRepository : IEmployeeRepository
{
    private List<Employee> _employeeList;

    public MockEmployeeRepository()
    {
        _employeeList = new List<Employee>()
        {
            new Employee() { Id = 1, Name = "Mary", Department = Dept.HR, Email = "mary@pragimtech.com" },
            new Employee() { Id = 2, Name = "John", Department = Dept.IT, Email = "john@pragimtech.com" },
            new Employee() { Id = 3, Name = "Sam", Department = Dept.IT, Email = "sam@pragimtech.com" },
        };
    }

    public Employee Add(Employee employee)
    {
        employee.Id = _employeeList.Max(e => e.Id) + 1;
        _employeeList.Add(employee);
        return employee;
    }

    public IEnumerable<Employee> GetAllEmployees()
    {
        return _employeeList;
    }

    public Employee GetEmployee(int Id)
    {
        return this._employeeList.FirstOrDefault(e => e.Id == Id);
    }
}

HttpGet vs HttpPost

At the moment in the HomeController we have the following 2 Create() action methods. 

public ViewResult Create()
{
    return View();
}

public RedirectToActionResult Create(Employee employee)
{
    Employee newEmployee = _employeeRepository.Add(employee);
    return RedirectToAction("details", new { id = newEmployee.Id });
}

If we now navigate to the URL (http://localhost:1234/home/create) we get the following error
AmbiguousActionException: Multiple actions matched.

This is because asp.net core does not know which action method to execute. We want the first Create() action method to respond to GET request and the second Create() action method to respond to the POST request. To tell this to asp.net core decorate the Create() action methods with HttpGet and HttpPost attributes as shown below.

[HttpGet]
public ViewResult Create()
{
    return View();
}

[HttpPost]
public RedirectToActionResult Create(Employee employee)
{
    Employee newEmployee = _employeeRepository.Add(employee);
    return RedirectToAction("details", new { id = newEmployee.Id });
}

The Create() action method that responds to the POST action adds the new employee to the EmployeeRepository and redirects the user to the Details() action method passing it the ID of the newly created employee. 

Upon redirection if you get a NullReferenceException, make sure to use AddSingleton() method instead of AddTransient() method to register IEmployeeRepository service in ConfigureServices() method of Startup.cs file.

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
        services.AddSingleton<IEmployeeRepository, MockEmployeeRepository>();
    }

    // Other code here....
}

We will discuss what's causing this error and the difference between AddSingleton(), AddTransient() and AddScoped() methods in our upcoming videos.

At the moment Create Employee Form does not have any validation in place. If we submit the form without filling any of the form fields, we will end up creating a new employee whose name and email fields are null. We will discuss form validation in our next video.

asp.net core tutorial for beginners

1 comment:

  1. Great work!. Simple explanation, Excellent presentation, apt slides, it gives us better concept wise understanding, more over, allows text downloads.

    Salute you sir. abu_ashfak@yahoo.com

    ReplyDelete

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