What are the best practices for using HttpClient (C#) instances?

Using HttpClient properly is essential to ensure efficient resource utilization and avoid common pitfalls such as socket exhaustion. Here are several best practices to consider when using HttpClient instances in C#:

1. Use a Singleton or Static Instance

Creating a new instance of HttpClient for every request can lead to resource exhaustion, as each instance may not release the underlying socket immediately upon disposal. Instead, it is recommended to reuse a single instance of HttpClient for the lifetime of the application.

public static class HttpClientFactory
{
    private static readonly HttpClient instance = new HttpClient();

    static HttpClientFactory()
    {
        // Configure the HttpClient instance (e.g., default headers, timeout)
    }

    public static HttpClient Instance => instance;
}

2. Dispose of HttpResponseMessage

While it's recommended to reuse the HttpClient instance, you should always dispose of the HttpResponseMessage instances once you're done with them to free up resources.

var response = await HttpClientFactory.Instance.GetAsync("http://example.com");
if (response.IsSuccessStatusCode)
{
    var content = await response.Content.ReadAsStringAsync();
    // Use the content
}
response.Dispose(); // Make sure to dispose of the response

3. Configure HttpClient Properly

Before you start making requests, configure the HttpClient instance as needed. Common configurations include setting timeouts, default request headers, and handlers.

var httpClient = new HttpClient()
{
    Timeout = TimeSpan.FromSeconds(30)
};
httpClient.DefaultRequestHeaders.Accept.Clear();
httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

4. Handle Exceptions

Network operations are inherently unreliable, so you should handle exceptions that may occur when sending requests.

try
{
    var response = await HttpClientFactory.Instance.GetAsync("http://example.com");
    response.EnsureSuccessStatusCode(); // Throws an exception if the HTTP response status is an error code
    var content = await response.Content.ReadAsStringAsync();
}
catch (HttpRequestException e)
{
    // Handle the exception (e.g., retry, log the error)
}

5. Use IHttpClientFactory in ASP.NET Core

If you're developing an ASP.NET Core application, consider using the built-in IHttpClientFactory to handle the creation and management of HttpClient instances.

public class MyService
{
    private readonly HttpClient _httpClient;

    public MyService(IHttpClientFactory httpClientFactory)
    {
        _httpClient = httpClientFactory.CreateClient();
    }

    public async Task<string> GetWebContentAsync(string url)
    {
        var response = await _httpClient.GetAsync(url);
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadAsStringAsync();
    }
}

Register the IHttpClientFactory in the Startup class:

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient();
}

6. Use HttpMessageHandler for Testing/Mocking

When unit testing, you might want to mock the behavior of HttpClient. You can do this by passing a custom HttpMessageHandler to the HttpClient constructor.

var handlerMock = new Mock<HttpMessageHandler>();
handlerMock
    .Protected()
    .Setup<Task<HttpResponseMessage>>(
        "SendAsync",
        ItExpr.IsAny<HttpRequestMessage>(),
        ItExpr.IsAny<CancellationToken>()
    )
    .ReturnsAsync(new HttpResponseMessage
    {
        StatusCode = HttpStatusCode.OK,
        Content = new StringContent("Fake content"),
    });

var httpClient = new HttpClient(handlerMock.Object);

7. Consider Cancellation

Use CancellationToken to allow for cooperative cancellation of async operations, such as when a user navigates away from a page or when a timeout is reached.

var cancellationTokenSource = new CancellationTokenSource();
cancellationTokenSource.CancelAfter(TimeSpan.FromSeconds(10)); // Set a timeout

try
{
    var response = await HttpClientFactory.Instance.GetAsync("http://example.com", cancellationTokenSource.Token);
    var content = await response.Content.ReadAsStringAsync();
}
catch (OperationCanceledException)
{
    // Handle cancellation
}

By following these best practices, you'll be able to use HttpClient in a way that's both efficient and reliable.