Sending requests with different headers using HttpClient

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
  }
}