HttpClient

Object –> HttpMessageInvoker –> HttpClient

Sending HTTP requests and receiving HTTP responses via URI.

• HttpClient is intended to be instantiated once per application.
• HttpClient works only on async methods for its long-running APIs.
• Every HttpClient instance uses its own connection pool.

Thread Safety

Warning: HttpClient is not thread safe. However, all HTTP verb methods, except Send(), are thread safe.

Flaw

When the HttpClient object gets disposed of, the underlying socket is not immediately released, which can lead to a socket exhaustion problem.

Mitigation: create HttpClient as a static object.

Construction

HttpClient() — Uses HttpClientHandler that is disposed when instance is destroyed.

• This concrete instance of HttpClientHandler has the sockets exhaustion and DNS problems mentioned in Flaw. HttpClient(HttpMessageHandler) — Uses the specified handler.
  HttpClient(HttpMessageHandler, Bool) — Uses the specified handler. Specifies whether to destroy handler when instance is destroyed.

Requests

HttpClient supports DELETE, GET, PATCH, POST, and PUT HTTP verbs.

Use the Send() methods for fine-grained control.
These methods use the HttpRequestMessage type which include headers, the verb, content, and options.

Methods

• CancelPendingRequests()
• Dispose() — Release unmanaged resources; dispose of managed resources.
• GetAsync(URI) — Send a GET request to URI.
• GetByteArrayAsync(URI) — Send a GET request to URI; return response body as byte array.
• GetStreamAsync(URI) — Send a GET request to URI; return response body as stream.
• GetStringAsync(URI) — Send a GET request to URI; return response body as string.
• PostAsync(URI, content) — Send a POST request to URI. content is HttpContent.

Properties

• BaseAddress — Get/set the URI.
• DefaultRequestHeaders — Get/set headers which should be sent with each request as HttpRequestHeaders object.
• DefaultRequestVersion — Get/set default HTTP version to use on each request.
• Timeout — Get/set request time out.

Examples

GET request example:

static readonly HttpClient client = new HttpClient();

static async Task Main() 
{
    try
    {
        // Asynchronously, send a GET request to uri and return the response body as a string.
        string responseBody = await client.GetStringAsync(uri);

        Console.WriteLine(responseBody);
    }
    catch(HttpRequestException e) 
    {
    }
}

POST request example:

static readonly HttpClient client = new HttpClient();

static async Task<string> PostApi(HttpClient client, string url, HttpContent content) 
{
    var response = await client.PostAsync(url, content);
    string result = response.Content.ReadAsStringAsync().Result;
}

HTTP/3

Availability: .NET 6 (preview), .NET 7 (production) HTTP/3 uses a different underlying transport (QUIC) than HTTP/2 and HTTP/1.1 (TCP). Advantages:

• Faster response time for the first request (negotiates connections in fewer round trips)
• Lost packets only affect the requests where data has been lost (QUIC provides native multiplexing; HTTP/2 multiplexes multiple requests via a single TCP connection, so packet loss on the connection affects all requests)
• Support for transitioning between networks (WiFi and cellular, for example)
  • Note: HttpClient and Kestrel do not support network transitions as of .NET 7.

HttpClient Settings for HTTP/3

There are two approaches:

1. Configure HTTP/3 by setting HttpRequestMessage.Version to 3.0.
   • The disadvantage of this approach is that not all network infrastructure supports HTTP/3.
2. Set HttpRequestMessage.Version to 1.1, and HttpRequestMessage.VersionPolicy to HttpVersionPolicy.RequestVersionOrHigher.
   • The advantage of this approach is that it supports HTTP/3 for network infrastructure that supports it.

Platform Dependencies

.NET’s implementation of HTTP/3 uses MsQuic. The underlying platform must meet these requirements, otherwise HTTP/3 is disabled:

• Windows
  • Windows 11, Windows Server 2022
• Linux
  • OpenSSL 1.1
  • libmsquic (sudo apt install libmsquic)
• macOS
  • HTTP/3 is not supported on macOS.

Using HttpClient with HTTP/3

// See https://aka.ms/new-console-template for more information
using System.Net;

using var client = new HttpClient
{
    DefaultRequestVersion =  HttpVersion.Version30,
    DefaultVersionPolicy = HttpVersionPolicy.RequestVersionExact
};

Console.WriteLine("--- localhost:5001 ---");

HttpResponseMessage resp = await client.GetAsync("https://localhost:5001/");
string body = await resp.Content.ReadAsStringAsync();

Console.WriteLine(
    $"status: {resp.StatusCode}, version: {resp.Version}, " +
    $"body: {body.Substring(0, Math.Min(100, body.Length))}");

Testing HTTP/3 Clients

Use Cloudflare’s test site: https://cloudflare-quic.com/