Fixed HttpClient Error: This instance has already started one or more requests. Properties can only be modified before sending the first request

In our project, we encountered the need to efficiently handle parallel HTTP requests while using HttpClient in our .NET Core 5.0 application. To address this, we explored the solution , which involves creating an HttpClientFactory.

First, we define an interface, IHttpClientFactory, with a method CreateClient() to create HttpClient instances. Then, we implement interface in the HttpClientFactory class.

In the HttpClientFactory, we set a base address and configure default settings for the HttpClient instances created. These defaults include setting a timeout and specifying the base address. We can customize these settings based on our requirements.

public interface ICustomHttpClientFactory
{
    HttpClient CreateCustomClient();
}

public class CustomHttpClientFactory : ICustomHttpClientFactory
{
    static string _customBaseAddress = "http://samplerestapi.com/api/petslover";

    public HttpClient CreateCustomClient()
    {
        var customClient = new HttpClient();
        SetupCustomClientDefaults(customClient);
        return customClient;
    }

    protected virtual void SetupClientDefaults(HttpClient client)
    {
        client.Timeout = TimeSpan.FromSeconds(50); //set your own timeout.
        client.BaseAddress = new Uri(_customBaseAddress);
    }
}

public ProductController(ICustomHttpClientFactory customHttpClientFactory)
{
    _customHttpClientFactory = customHttpClientFactory;
}

readonly ICustomHttpClientFactory _customHttpClientFactory;

public IActionResult Index()
{
    var customClient = _customHttpClientFactory.CreateCustomClient();
   
    return View();
}

In our controller, we inject IHttpClientFactory and utilize it to create HttpClient instances whenever needed. However, it's important to note that with this approach, a new instance of HttpClient is created every time CreateClient() is called, and there's no reuse of HttpClient objects.

The HttpClientFactory pattern offers some benefits in terms of organization and configurability, it falls short in efficiently managing HttpClient instances for parallel requests. Alternative approaches such as HttpClient pooling may be more suitable for scenarios requiring optimal resource utilization and performance in handling concurrent HTTP requests.

In our ASP.NET Core application, we encountered the error message "HttpClient: This instance has already started one or more requests. Properties can only be modified before sending the first request" to resolve this issue, we need to ensure that we configure the HttpClient instance correctly and avoid modifying its properties after it has been used for sending requests. One common mistake that leads to this error is the reuse of the same HttpClient instance for multiple requests without proper configuration.

The best approach to use HttpClient in ASP.NET Core is to register it as a transient service, allowing the framework to manage its lifecycle efficiently, this ensures that a new instance of HttpClient is created for each request, avoiding issues related to concurrent requests and property modification.

Use HttpClient in an ASP.NET Core controller:

using System.Net.Http;
using Microsoft.AspNetCore.Mvc;

public class PetsController : ControllerBase
{
    private readonly HttpClient _httpClient;

    public PetsController(HttpClient httpClient)
    {
        _httpClient = httpClient;
        ConfigureHttpClient();
    }

    private void ConfigureHttpClient()
    {
        // Configure HttpClient properties here
        _httpClient.Timeout = TimeSpan.FromSeconds(30);
        _httpClient.BaseAddress = new Uri("http://samplerestapi.com");
    }

    public IActionResult Index()
    {
        // Use the configured HttpClient instance for requests
        HttpResponseMessage response = _httpClient.GetAsync("api/employee").Result;
        
        return Ok();
    }
}
    

In above code we ensure that each request in our ASP.NET Core application gets its own instance of HttpClient, preventing conflicts and errors related to concurrent requests and property modification.

In our ASP.NET Core application, we know that HttpClient.DefaultRequestHeaders and BaseAddress should only be set once, before making any requests. HttpClient is only safe to use as a singleton if we don't modify it once it's in use.

We should avoid setting DefaultRequestHeaders and BaseAddress and instead set the headers on each HttpRequestMessage we send to ensures that the HttpClient instance remains thread-safe and can be safely reused as a singleton throughout the application.

how we can set headers on each HttpRequestMessage:

var httpRequest = new HttpRequestMessage(HttpMethod.Post, url);
httpRequest.Headers.Accept.Clear();
httpRequest.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
httpRequest.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
httpRequest.Content = new StringContent("{}", Encoding.UTF8, "application/json");
var httpResponse = await _httpClient.SendAsync(request, CancellationToken.None);
    

By following this approach, we ensure that HttpClient remains thread-safe and can be safely reused as a singleton. Each HttpRequestMessage is configured with the necessary headers before being sent, allowing us to maintain consistent behavior and prevent conflicts in our ASP.NET Core application.