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" };

            else

                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

config.Routes.MapHttpRoute(

    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

[Route("api/students/{id}/courses")]

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.

[Route("api/students/{id}/courses")]

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.

config.MapHttpAttributeRoutes();

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.