How to consume Asp.Net WebAPI endpoints from other applications using C#?

The HttpClient class provides a base class for sending and receiving HTTP requests and responses from URLs. It is a supported async feature of the .NET framework that can process multiple concurrent requests. HttpClient is available in the System.Net.Http namespace and acts as a layer over HttpWebRequest and HttpWebResponse.

This article demonstrates how to consume ASP.NET Web API endpoints from external applications using HttpClient. We'll create a Web API with student data and then consume it from a console application.

Creating the Web API

Student Model

First, let's define the Student model −

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

Student Controller

The controller provides endpoints to retrieve student data −

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

namespace DemoWebApplication.Controllers {
    public class StudentController : ApiController {
        List<Student> students = new List<Student> {
            new Student {
                Id = 1,
                Name = "Mark"
            },
            new Student {
                Id = 2,
                Name = "John"
            }
        };

        public IEnumerable<Student> Get() {
            return students;
        }

        public Student Get(int id) {
            var studentForId = students.FirstOrDefault(x => x.Id == id);
            return studentForId;
        }
    }
}

This creates two endpoints:

• GET /api/student − Returns all students
• GET /api/student/{id} − Returns a specific student by ID

Consuming the Web API with HttpClient

Basic HttpClient Usage

Here's how to consume the Web API endpoints using HttpClient −

using System;
using System.Net.Http;

namespace DemoApplication {
    public class Program {
        static void Main(string[] args) {
            using (var httpClient = new HttpClient()) {
                Console.WriteLine("Calling WebApi for get all students");
                var students = GetResponse("student");
                Console.WriteLine($"All Students: {students}");
                
                Console.WriteLine("Calling WebApi for student id 2");
                var studentForId = GetResponse("student/2");
                Console.WriteLine($"Student for Id 2: {studentForId}");
            }
        }

        private static string GetResponse(string url) {
            using (var httpClient = new HttpClient()) {
                httpClient.BaseAddress = new Uri("http://localhost:58174/api/");
                var responseTask = httpClient.GetAsync(url);
                var result = responseTask.Result;
                var readTask = result.Content.ReadAsStringAsync();
                return readTask.Result;
            }
        }
    }
}

The output of the above code is −

Calling WebApi for get all students
All Students: [{"Id":1,"Name":"Mark"},{"Id":2,"Name":"John"}]
Calling WebApi for student id 2
Student for Id 2: {"Id":2,"Name":"John"}

Using Async/Await Pattern

Improved Asynchronous Implementation

Since HttpClient is designed for asynchronous operations, here's a better implementation using async/await −

using System;
using System.Net.Http;
using System.Threading.Tasks;

namespace DemoApplication {
    public class Program {
        private static readonly HttpClient httpClient = new HttpClient();

        static async Task Main(string[] args) {
            httpClient.BaseAddress = new Uri("http://localhost:58174/api/");
            
            Console.WriteLine("Calling WebApi for all students (async)");
            var students = await GetResponseAsync("student");
            Console.WriteLine($"All Students: {students}");
            
            Console.WriteLine("Calling WebApi for student id 1 (async)");
            var studentForId = await GetResponseAsync("student/1");
            Console.WriteLine($"Student for Id 1: {studentForId}");
        }

        private static async Task<string> GetResponseAsync(string url) {
            var response = await httpClient.GetAsync(url);
            response.EnsureSuccessStatusCode();
            return await response.Content.ReadAsStringAsync();
        }
    }
}

The output of the above code is −

Calling WebApi for all students (async)
All Students: [{"Id":1,"Name":"Mark"},{"Id":2,"Name":"John"}]
Calling WebApi for student id 1 (async)
Student for Id 1: {"Id":1,"Name":"Mark"}

Key Best Practices

Conclusion

HttpClient provides a powerful and flexible way to consume ASP.NET Web API endpoints from external applications. Always prefer the asynchronous methods and reuse HttpClient instances for optimal performance. The async/await pattern ensures your application remains responsive while making HTTP calls.