错误'请求消息已发送。无法多次发送相同的请求消息'

Question

这就是我正在尝试的：

public static async Task<HttpResponseMessage> post(string specificUrl, string token, StringContent inputData) // specificUrl like "user/" at the end "/"
{
    var policy = Policy
        .Handle<Exception>()
        .OrResult<HttpResponseMessage>(res => !res.IsSuccessStatusCode)
        .RetryAsync(3);

    string fullUrl = baseUrl + specificUrl;
    using (var requestMessage = new HttpRequestMessage(HttpMethod.Post, fullUrl))
    {
        requestMessage.Content = inputData;
        requestMessage.Headers.Add("access_token", token);
        //HttpResponseMessage res = await client.SendAsync(requestMessage).ConfigureAwait(false);
        HttpResponseMessage res = await policy.ExecuteAsync(() => client.SendAsync(requestMessage)).ConfigureAwait(false);
        return res;
    }
}

只要代码达到 waitAndRetryPolicy 并等待所需的时间，我就会收到以下错误：

System.InvalidOperationException: '请求消息已经发送。不能多次发送相同的请求消息。

Answer 1

简而言之，您不能重用HttpRequestMessage，它会处理在SendAsync 级别无法缓解的一大堆问题（例如流式内容等）。您需要在ExecuteAsync lambda 中创建/重新创建您的HttpRequestMessage，或者更好的是，使用IHttpClientFactory，然后在HttpClientBuilder 级别使用Polly。

重新创建HttpRequestMessage的示例

return await policy.ExecuteAsync(() =>
{
   // note the using here is a superfluous, but good practice
   using var requestMessage = new HttpRequestMessage(HttpMethod.Post, fullUrl) 
   {
       Content = new StringContent(aString) 
   };

   requestMessage.Headers.Add("access_token", token);
   return client.SendAsync(requestMessage);

}).ConfigureAwait(false);

但理想情况下，您应该将 HttpClientFactory 与类型化或命名的客户端一起使用

给定一个类型化的客户端

public class MyTypedClient
{
   private readonly HttpClient _client;

   public MyTypedClient(HttpClient client) => _client = client;
   public async Task<HttpResponseMessage> MyCall(string inputData)
   {
      using var requestMessage = new HttpRequestMessage(HttpMethod.Post, "/someRoute") { Content = new StringContent(inputData) };
      var result = await _client.SendAsync(requestMessage);
      result.EnsureSuccessStatusCode();
      return result;
   }
}

Di 用法示例

private static readonly AsyncRetryPolicy<HttpResponseMessage> retryPolicy
   = HttpPolicyExtensions
      .HandleTransientHttpError() // what ever ??
      .RetryAsync(3);

public static void Main()
{

   var serviceCollection = new ServiceCollection();

   serviceCollection
      .AddHttpClient<MyTypedClient>(ClientConfiguration)
      .AddPolicyHandler(retryPolicy);

   var provider = serviceCollection.BuildServiceProvider();
   // what ever
}

private static void ClientConfiguration(HttpClient client)
{
   client.BaseAddress = new Uri("http://bob.com");
   /// blah blah blah
}

_{注意：这并不是关于 Di、Polly 或其他任何东西的教程，例如世界最佳实践，如何在 AddHttpClient 级别使用身份验证等等，这只是一个使用策略如何组合在一起的示例用例}

Nuget

• Microsoft.Extensions.DependencyInjection
• Microsoft.Extensions.Http
• Microsoft.Extensions.Http.Polly
• 波莉

---

其他资源

• Use IHttpClientFactory to implement resilient HTTP requests

• Implement HTTP call retries with exponential backoff with IHttpClientFactory and Polly policies