处理来自类型化 WebAPI HttpClient 调用的错误

Question

我对返回任务的 webapi 客户端进行了多次调用 像这样的

public async Task<TResp> GetMyThingAsync(TReq req)
{
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri(BaseURI);
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
        client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/xml"));

        await HttpRuntime.Cache.GetToken().ContinueWith((t) =>
        {
            client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("XXX", t.Result);
        });

        var httpResponseMessage = await client.PostAsXmlAsync<TReq>("This/That/", req);
        httpResponseMessage.EnsureSuccessStatusCode();
        var resp = await httpResponseMessage.Content.ReadAsAsync<TResp>();
        return resp;
    }
}

对 api 的调用当然可以返回 500 或其他一些问题。 EnsureSuccessStatusCode() 如果发生类似的事情，显然会抛出，但是对于响应中的任何信息做任何事情都为时已晚。 有没有一个很好的方法来处理这个？ 我了解您可以向客户端添加 messageHandler，例如 HttpClient client = HttpClientFactory(new ErrorMessageHandler()) ..

var customHandler = new ErrorMessageHandler()
        { InnerHandler = new HttpClientHandler()};
HttpClient client = new HttpClient(customHandler);

这是要走的路吗？ ErroMessageHandler 会是什么样子并会做什么来向调用控制器返回有用的东西...？

非常感谢

自然

Answer 1

创建自定义处理程序可能是记录异常或验证响应的优雅解决方案。如果遇到异常，我不确定被调用的控制器是否正在等待来自客户端的有意义的响应。我认为真正重要的部分是确保您（客户端）优雅地处理 Web api 错误。

这可以通过两种方式完成：

1. 您可以在调用方法中本地处理异常。您可以使用HttpResponseMessage.IsSuccessStatusCode 属性来指示是否返回了错误的响应，而不是调用引发异常的httpResponseMessage.EnsureSuccessStatusCode()，并返回自定义的ErrorResponse（或做任何您决定的事情）：

   var client = new HttpClient() // No need to dispose HttpClient
   
   client.BaseAddress = new Uri(BaseURI);
   client.DefaultRequestHeaders.Accept.Clear();
   
   client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
   client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/xml"));
   
   var token = await HttpRuntime.Cache.GetToken();
   client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("XXX", token);
   
   var httpResponseMessage = await client.PostAsXmlAsync<TReq>("This/That/", req);
   if (!httpResponseMessage.IsSuccessStatusCode)
   {
      return Task.FromResult(new ErrorResponse()) // Create some kind of error response to indicate failure
   }
   
   var resp = await httpResponseMessage.Content.ReadAsAsync<TResp>();
   return resp;
   

2. 创建一个可以记录从 web api 接收到的异常（或执行其他操作）的 ErrorLoggingHandler：

   public class ErrorLoggingHandler : DelegatingHandler
   {
      private readonly StreamWriter _writer; // As a sample, log to a StreamWriter
   
      public ErrorLoggingHandler(Stream stream)
      {
         _writer = new StreamWriter(stream);
      }
   
      protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
      {
         var response = await base.SendAsync(request, cancellationToken);
   
         if (!response.IsSuccessStatusCode)
         {
             // This would probably be replaced with real error 
             // handling logic (return some kind of special response etc..)
             _writer.WriteLine("{0}\t{1}\t{2}", request.RequestUri,
               (int) response.StatusCode, response.Headers.Date);
         }
   
         return response;
      }
   
      protected override void Dispose(bool disposing)
      {
         if (disposing)
         {
            _writer.Dispose();
         }
   
         base.Dispose(disposing);
      }
   }
   

然后，您可以使用HttpClientFactory 创建您的HttpClient：

var httpclient = HttpClientFactory.Create(new ErrorLoggingHandler(new FileStream(@"Location", FileMode.OpenOrCreate)));