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Attribute routing in ASP.NET Web API 2

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In this video we will discuss Attribute Routing introduced in ASP.NET Web API 2

Let us understand attribute routing with an example

Step 1 : Create a new ASP.NET Web API project. Name it WebAPI

Step 2 : Right click on the "Models" folder and add a class file. Name it Student.cs. Copy and paste the following code.

namespace WebAPI.Models
    public class Student
        public int Id { get; set; }
        public string Name { get; set; }

Step 3 : Let's now add Students Controller. Right click on the Controllers folder and add a new Web API 2 Empty controller. Name it StudentsController.cs. Copy and paste the following code.

using System.Collections.Generic;
using System.Linq;
using System.Web.Http;
using WebAPI.Models;

namespace WebAPI.Controllers
    public class StudentsController : ApiController
        static List<Student> students = new List<Student>()
            new Student() { Id = 1, Name = "Tom" },
            new Student() { Id = 2, Name = "Sam" },
            new Student() { Id = 3, Name = "John" }

        public IEnumerable<Student> Get()
            return students;

        public Student Get(int id)
            return students.FirstOrDefault(s => s.Id == id);

        public IEnumerable<string> GetStudentCourses(int id)
            if (id == 1)
                return new List<string>() { "C#", "ASP.NET", "SQL Server" };
            else if (id == 2)
                return new List<string>() { "ASP.NET Web API", "C#", "SQL Server" };
                return new List<string>() { "Bootstrap", "jQuery", "AngularJs" };

In Web API 1, we had convention-based routing that define routes using route templates. When we create a new Web API project using Visual Studio, a default route is created in WebApiConfig.cs file. The default route is shown below

    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }

So with the above default route and StudentsController in place 
/api/students is mapped to Get() method in StudentsController

When we navigate to /api/students/1 we get the following exception message
Multiple actions were found that match the request: 
   Get on type WebAPI.Controllers.StudentsController 
   GetStudentCourses on type WebAPI.Controllers.StudentsController

This is because the framework does not know which of the 2 following action methods to map to the URI /api/students/1
Get(int id)
GetStudentCourses(int id)

This can be very easily resolved by using Attribute Routing. Here is what we want the framework to do
1. URI /api/students/1 should be mapped to Get(int id). This method returns the student by id.
2. URI /api/students/1/courses should be mapped to GetStudentCourses(int id). This method returns the student courses by student id.

To achieve the above, simply decorate GetStudentCourses() action method with the below [Route] attribute

At this point build the solution and navigate to /api/students/1. Notice you now get student details whose id=1. When you navigate to /api/students/1/courses you get all the courses into which student with id=1 is enrolled.

What is Attribute Routing
Using the [Route] attribute to define routes is called Attribute Routing

What are the advantages of using Attribute Routing
Attribute routing gives us more control over the URIs than convention-based routing. Creating URI patterns like hierarchies of resources (For example, students have courses, Departments have employees) is very difficult with convention-based routing. With attribute routing all you have to do is use the [Route] attribute as shown below.


How to enable Attribute Routing
In ASP.NET Web API 2, Attribute Routing is enabled by default. The following line of code in WebApiConfig.cs file enables Attribute Routing.


Can we use both Attribute Routing and Convention-based routing in a single Web API project
Yes, both the routing mechanisms can be combined in a single Web API project. The controller action methods that have the [Route] attribute uses Attribute Routing, and the others without [Route] attribute uses Convention-based routing.

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