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Question

Hello community,

I'm working on a .NET application that makes frequent calls to external REST APIs. I'm trying to ensure my application remains responsive, especially under load, by using asynchronous programming. I've started using async and await with HttpClient, but I'm not entirely confident I'm doing it correctly, particularly regarding potential deadlocks or resource leaks.

Here's a simplified example of what I have:


public async Task<string> GetDataFromApiAsync(string url)
{
    using (HttpClient client = new HttpClient())
    {
        try
        {
            string result = await client.GetStringAsync(url);
            return result;
        }
        catch (HttpRequestException e)
        {
            Console.WriteLine($"Request error: {e.Message}");
            return null;
        }
    }
}

Key areas of concern:

Should HttpClient be instantiated for each call, or is it better to have a single, long-lived instance?
Are there any common pitfalls with ConfigureAwait(false) in this context?
What are the best practices for handling exceptions and timeouts with HttpClient?

Any guidance, best practices, or code examples would be greatly appreciated!

Thanks in advance!

Answer 1

Great question, Dave! You're on the right track with async/await. Let's address your concerns:

1. HttpClient Lifetime: You should absolutely reuse HttpClient instances. Creating a new one for each request is inefficient and can lead to socket exhaustion. The recommended approach is to use a single, static instance (or a managed one via dependency injection) that lives for the lifetime of your application.


// Recommended way using a static instance
public static class ApiClient
{
    private static readonly HttpClient client = new HttpClient();

    public static async Task<string> GetDataFromApiAsync(string url)
    {
        try
        {
            string result = await client.GetStringAsync(url);
            return result;
        }
        catch (HttpRequestException e)
        {
            Console.WriteLine($"Request error: {e.Message}");
            return null;
        }
    }
}

2. ConfigureAwait(false): For library code or when you don't need to resume on the original synchronization context, ConfigureAwait(false) is highly recommended. It prevents potential deadlocks, especially in UI or ASP.NET Classic applications. In modern ASP.NET Core, it's often less critical as there's no captured context by default, but it's still a good habit.


public async Task<string> GetDataFromApiAsync(string url)
{
    // Assuming 'client' is a shared, static HttpClient instance
    string result = await client.GetStringAsync(url).ConfigureAwait(false);
    return result;
}

3. Exception Handling and Timeouts: Use CancellationToken for timeouts and wrap calls in try-catch blocks for HttpRequestException and potentially other network-related exceptions.


public async Task<string> GetDataWithTimeoutAsync(string url, TimeSpan timeout)
{
    using (var cts = new CancellationTokenSource(timeout))
    {
        try
        {
            string result = await client.GetStringAsync(url, cts.Token).ConfigureAwait(false);
            return result;
        }
        catch (HttpRequestException e)
        {
            Console.WriteLine($"Request error: {e.Message}");
            return null;
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Request timed out.");
            return null;
        }
    }
}

Hope this helps clarify things!

Answer 2

Echoing Sarah's points, especially about reusing HttpClient. It's a common mistake beginners make. Also, consider adding default headers or base addresses to your shared HttpClient instance if you're calling the same API frequently.

For timeouts, using CancellationTokenSource with a specified delay is indeed the way to go. Remember that GetStringAsync itself accepts a CancellationToken.

Another pattern is to use Polly for more sophisticated retry policies and circuit breakers, which can significantly improve resilience.

Answer 3

To add to the excellent advice, when instantiating a shared HttpClient, consider managing its lifetime through Dependency Injection. In ASP.NET Core, the IHttpClientFactory interface is specifically designed for this, providing robust management of HttpClient instances, including advanced configuration, health checks, and retry policies.

Here's a glimpse of how you'd set it up:


// In Startup.cs or Program.cs
services.AddHttpClient(); // Basic setup
// Or for typed clients:
services.AddHttpClient<MyApiClient>();

// Then inject IHttpClientFactory into your service
public class MyService
{
    private readonly HttpClient _httpClient;

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

    public async Task<string> GetDataAsync(string url)
    {
        return await _httpClient.GetStringAsync(url).ConfigureAwait(false);
    }
}

Answers