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Call a Web API From a .NET Client (C#)

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This content is for a previous version of .NET. New development should use ASP.NET Core. For more information on using ASP.NET Core Web API, see:

Download Completed Project.

This tutorial shows how to call a web API from a .NET application, using System.Net.Http.HttpClient.

In this tutorial, a client app is written that consumes the following web API:

Action HTTP method Relative URI
Get a product by ID GET /api/products/id
Create a new product POST /api/products
Update a product PUT /api/products/id
Delete a product DELETE /api/products/id

To learn how to implement this API with ASP.NET Web API, see Creating a Web API that Supports CRUD Operations.

For simplicity, the client application in this tutorial is a Windows console application. HttpClient is also supported for Windows Phone and Windows Store apps. For more information, see Writing Web API Client Code for Multiple Platforms Using Portable Libraries

NOTE: If you pass base URLs and relative URIs as hard-coded values, be mindful of the rules for utilizing the HttpClient API. The HttpClient.BaseAddress property should be set to an address with a trailing forward slash (/). For example, when passing hard-coded resource URIs to the HttpClient.GetAsync method, don't include a leading forward slash. To get a Product by ID:

  1. Set client.BaseAddress = new Uri("https://localhost:5001/");
  2. Request a Product. For example, client.GetAsync<Product>("api/products/4");.

Create the Console Application

In Visual Studio, create a new Windows console app named HttpClientSample and paste in the following code:

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

namespace HttpClientSample
{
    public class Product
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public decimal Price { get; set; }
        public string Category { get; set; }
    }

    class Program
    {
        static HttpClient client = new HttpClient();

        static void ShowProduct(Product product)
        {
            Console.WriteLine($"Name: {product.Name}\tPrice: " +
                $"{product.Price}\tCategory: {product.Category}");
        }

        static async Task<Uri> CreateProductAsync(Product product)
        {
            HttpResponseMessage response = await client.PostAsJsonAsync(
                "api/products", product);
            response.EnsureSuccessStatusCode();

            // return URI of the created resource.
            return response.Headers.Location;
        }

        static async Task<Product> GetProductAsync(string path)
        {
            Product product = null;
            HttpResponseMessage response = await client.GetAsync(path);
            if (response.IsSuccessStatusCode)
            {
                product = await response.Content.ReadAsAsync<Product>();
            }
            return product;
        }

        static async Task<Product> UpdateProductAsync(Product product)
        {
            HttpResponseMessage response = await client.PutAsJsonAsync(
                $"api/products/{product.Id}", product);
            response.EnsureSuccessStatusCode();

            // Deserialize the updated product from the response body.
            product = await response.Content.ReadAsAsync<Product>();
            return product;
        }

        static async Task<HttpStatusCode> DeleteProductAsync(string id)
        {
            HttpResponseMessage response = await client.DeleteAsync(
                $"api/products/{id}");
            return response.StatusCode;
        }

        static void Main()
        {
            RunAsync().GetAwaiter().GetResult();
        }

        static async Task RunAsync()
        {
            // Update port # in the following line.
            client.BaseAddress = new Uri("http://localhost:64195/");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
                new MediaTypeWithQualityHeaderValue("application/json"));

            try
            {
                // Create a new product
                Product product = new Product
                {
                    Name = "Gizmo",
                    Price = 100,
                    Category = "Widgets"
                };

                var url = await CreateProductAsync(product);
                Console.WriteLine($"Created at {url}");

                // Get the product
                product = await GetProductAsync(url.PathAndQuery);
                ShowProduct(product);

                // Update the product
                Console.WriteLine("Updating price...");
                product.Price = 80;
                await UpdateProductAsync(product);

                // Get the updated product
                product = await GetProductAsync(url.PathAndQuery);
                ShowProduct(product);

                // Delete the product
                var statusCode = await DeleteProductAsync(product.Id);
                Console.WriteLine($"Deleted (HTTP Status = {(int)statusCode})");

            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }

            Console.ReadLine();
        }
    }
}

The preceding code is the complete client app.

RunAsync runs and blocks until it completes. Most HttpClient methods are async, because they perform network I/O. All of the async tasks are done inside RunAsync. Normally an app doesn't block the main thread, but this app doesn't allow any interaction.

static async Task RunAsync()
{
    // Update port # in the following line.
    client.BaseAddress = new Uri("http://localhost:64195/");
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(
        new MediaTypeWithQualityHeaderValue("application/json"));

    try
    {
        // Create a new product
        Product product = new Product
        {
            Name = "Gizmo",
            Price = 100,
            Category = "Widgets"
        };

        var url = await CreateProductAsync(product);
        Console.WriteLine($"Created at {url}");

        // Get the product
        product = await GetProductAsync(url.PathAndQuery);
        ShowProduct(product);

        // Update the product
        Console.WriteLine("Updating price...");
        product.Price = 80;
        await UpdateProductAsync(product);

        // Get the updated product
        product = await GetProductAsync(url.PathAndQuery);
        ShowProduct(product);

        // Delete the product
        var statusCode = await DeleteProductAsync(product.Id);
        Console.WriteLine($"Deleted (HTTP Status = {(int)statusCode})");

    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
    }

    Console.ReadLine();
}

Install the Web API Client Libraries

Use NuGet Package Manager to install the Web API Client Libraries package.

From the Tools menu, select NuGet Package Manager > Package Manager Console. In the Package Manager Console (PMC), type the following command:

Install-Package Microsoft.AspNet.WebApi.Client

The preceding command adds the following NuGet packages to the project:

  • Microsoft.AspNet.WebApi.Client
  • Newtonsoft.Json

Newtonsoft.Json (also known as Json.NET) is a popular high-performance JSON framework for .NET.

Add a Model Class

Examine the Product class:

public class Product
{
    public string Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Category { get; set; }
}

This class matches the data model used by the web API. An app can use HttpClient to read a Product instance from an HTTP response. The app doesn't have to write any deserialization code.

Create and Initialize HttpClient

Examine the static HttpClient property:

static HttpClient client = new HttpClient();

HttpClient is intended to be instantiated once and reused throughout the life of an application. The following conditions can result in SocketException errors:

  • Creating a new HttpClient instance per request.
  • Server under heavy load.

Creating a new HttpClient instance per request can exhaust the available sockets.

The following code initializes the HttpClient instance:

static async Task RunAsync()
{
    // Update port # in the following line.
    client.BaseAddress = new Uri("http://localhost:64195/");
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(
        new MediaTypeWithQualityHeaderValue("application/json"));

The preceding code:

  • Sets the base URI for HTTP requests. Change the port number to the port used in the server app. The app won't work unless port for the server app is used.
  • Sets the Accept header to "application/json". Setting this header tells the server to send data in JSON format.

Send a GET request to retrieve a resource

The following code sends a GET request for a product:

static async Task<Product> GetProductAsync(string path)
{
    Product product = null;
    HttpResponseMessage response = await client.GetAsync(path);
    if (response.IsSuccessStatusCode)
    {
        product = await response.Content.ReadAsAsync<Product>();
    }
    return product;
}

The GetAsync method sends the HTTP GET request. When the method completes, it returns an HttpResponseMessage that contains the HTTP response. If the status code in the response is a success code, the response body contains the JSON representation of a product. Call ReadAsAsync to deserialize the JSON payload to a Product instance. The ReadAsAsync method is asynchronous because the response body can be arbitrarily large.

HttpClient does not throw an exception when the HTTP response contains an error code. Instead, the IsSuccessStatusCode property is false if the status is an error code. If you prefer to treat HTTP error codes as exceptions, call HttpResponseMessage.EnsureSuccessStatusCode on the response object. EnsureSuccessStatusCode throws an exception if the status code falls outside the range 200–299. Note that HttpClient can throw exceptions for other reasons — for example, if the request times out.

Media-Type Formatters to Deserialize

When ReadAsAsync is called with no parameters, it uses a default set of media formatters to read the response body. The default formatters support JSON, XML, and Form-url-encoded data.

Instead of using the default formatters, you can provide a list of formatters to the ReadAsAsync method. Using a list of formatters is useful if you have a custom media-type formatter:

var formatters = new List<MediaTypeFormatter>() {
    new MyCustomFormatter(),
    new JsonMediaTypeFormatter(),
    new XmlMediaTypeFormatter()
};
resp.Content.ReadAsAsync<IEnumerable<Product>>(formatters);

For more information, see Media Formatters in ASP.NET Web API 2

Sending a POST Request to Create a Resource

The following code sends a POST request that contains a Product instance in JSON format:

static async Task<Uri> CreateProductAsync(Product product)
{
    HttpResponseMessage response = await client.PostAsJsonAsync(
        "api/products", product);
    response.EnsureSuccessStatusCode();

    // return URI of the created resource.
    return response.Headers.Location;
}

The PostAsJsonAsync method:

  • Serializes an object to JSON.
  • Sends the JSON payload in a POST request.

If the request succeeds:

  • It should return a 201 (Created) response.
  • The response should include the URL of the created resources in the Location header.

Sending a PUT Request to Update a Resource

The following code sends a PUT request to update a product:

static async Task<Product> UpdateProductAsync(Product product)
{
    HttpResponseMessage response = await client.PutAsJsonAsync(
        $"api/products/{product.Id}", product);
    response.EnsureSuccessStatusCode();

    // Deserialize the updated product from the response body.
    product = await response.Content.ReadAsAsync<Product>();
    return product;
}

The PutAsJsonAsync method works like PostAsJsonAsync, except that it sends a PUT request instead of POST.

Sending a DELETE Request to Delete a Resource

The following code sends a DELETE request to delete a product:

static async Task<HttpStatusCode> DeleteProductAsync(string id)
{
    HttpResponseMessage response = await client.DeleteAsync(
        $"api/products/{id}");
    return response.StatusCode;
}

Like GET, a DELETE request does not have a request body. You don't need to specify JSON or XML format with DELETE.

Test the sample

To test the client app:

  1. Download and run the server app. Verify the server app is working. For example, http://localhost:64195/api/products should return a list of products.

  2. Set the base URI for HTTP requests. Change the port number to the port used in the server app.

    static async Task RunAsync()
{
    // Update port # in the following line.
    client.BaseAddress = new Uri("http://localhost:64195/");
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Accept.Add(
        new MediaTypeWithQualityHeaderValue("application/json"));

  3. Run the client app. The following output is produced:

    Created at http://localhost:64195/api/products/4
Name: Gizmo     Price: 100.0    Category: Widgets
Updating price...
Name: Gizmo     Price: 80.0     Category: Widgets
Deleted (HTTP Status = 204)