Sending requests with different headers using HttpClient

Question

There is a similar SO question here which talks about the performance of HttpClient objects and recommends to use one HttpClient instance per application. My project requires me to send multiple webservice requests to the same URI but each with a different set of headers. Should I create a new HttpClient instance for every request, knowing the fact that DefaultRequestHeaders will be same for all requests if I use a static instance.

Answer 1

You can:

• Set default headers on your global instance
• Create multiple global (logical) instances with different default configurations
• Set (additional) headers per request
• Create new HttpClient using IHttpClientFactory

Docs: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-2.1

Global Default Headers

Here you create one client instance and add headers that will be applied to all requests.

 var client = new HttpClient();
 client.DefaultRequestHeaders.Add("Content-Type", contentTypeValue);

Multiple Preconfigured Instances

In this dotnet core 2.1 example, we register a preconfigured named instance:

services.AddHttpClient("github", c =>
{
    c.BaseAddress = new Uri("https://api.github.com/");
    // Github API versioning
    c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
    // Github requires a user-agent
    c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample");
});

Later we'll inject IHttpClientFactory, and get this pre configured client like so:

var client = _clientFactory.CreateClient("github");

An alternative to named client are "typed clients": https://learn.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-5.0#typed-clients. Both approaches will keep the basic configuration at one place.

Headers per Request

If your headers belong to a single request only, simple set them per request.

var client = new HttpClient();

var request = new HttpRequestMessage();
request.Headers.Add("Content-Type", "text/plain");

var response = await client.SendAsync(request);

Using this approach you can use a shared HttpClient instance.

IHttpClientFactory

If you want a new "clean" HttpClient instance, the recommended approach for asp.net core is to inject IHttpClientFactory and use _clientFactory.CreateClient().

public class MyService {
  public MyService (IHttpClientFactory clientFactory)
  {
    _clientFactory = clientFactory;
  }
  public async Task DoSomething()
  {
    var client = _clientFactory.CreateClient();
    // do request
  }
}