How do I set up HttpClient (C#) to use system proxy settings automatically?

In C#, the HttpClient class is used to send HTTP requests and receive HTTP responses. By default, HttpClient uses the system proxy settings that are configured in the operating system, unless you override these settings.

To ensure that HttpClient uses the system proxy settings, you can set up an HttpClientHandler and pass it to the HttpClient constructor. Here's a step-by-step guide:

1. Create an instance of HttpClientHandler.
2. Set the UseProxy property to true to enable proxy use.
3. Optionally, set the UseDefaultCredentials property to true if your proxy requires authentication and you want to use the default network credentials.
4. Create an instance of HttpClient with the configured HttpClientHandler.

Here is an example in C#:

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

class Program
{
    static async Task Main(string[] args)
    {
        // Create an instance of HttpClientHandler
        var httpClientHandler = new HttpClientHandler
        {
            // Use system proxy settings
            UseProxy = true,
            // Use default credentials for proxy authentication if required
            UseDefaultCredentials = true
        };

        // Create an instance of HttpClient with the handler
        using (var httpClient = new HttpClient(httpClientHandler))
        {
            try
            {
                // Send a GET request
                string response = await httpClient.GetStringAsync("http://example.com");
                Console.WriteLine(response);
            }
            catch (HttpRequestException e)
            {
                Console.WriteLine("Error while making request: " + e.Message);
            }
        }
    }
}

In this example, the HttpClientHandler is configured to use the system's proxy settings. The UseProxy property is by default set to true, but it's explicitly set in this example for clarity. The UseDefaultCredentials is also set to true to ensure that if the proxy server requires authentication, the system's default credentials are used. If you don't require default credentials for proxy authentication, you can omit that line or set it to false.

When the HttpClient instance is created using this handler, it will automatically adhere to the proxy settings as configured on the user's system.

Remember that the HttpClient instance should be instantiated once and reused throughout the life of an application, rather than per request. This is to reduce the number of sockets used and to improve performance.

Also, note that some applications might run in environments where the default proxy settings are not appropriate or need to be bypassed for specific requests. In such cases, you can set the proxy manually in the HttpClientHandler:

httpClientHandler.Proxy = new WebProxy("http://yourproxyaddress:port", true);

Replace "http://yourproxyaddress:port" with your proxy server's URI and port number. The second parameter to WebProxy constructor is bypassOnLocal, which you can set to true if you want to bypass the proxy for local addresses.

Always make sure to handle exceptions that may occur due to network errors or invalid proxy settings by using appropriate try-catch blocks around your HTTP calls.