c# HttpClient超時重試

當使用c# HttpClient 發送請求時，由於網路等原因可能會出現超時的情況。為了提高請求的成功率，我們可以使用超時重試的機制。

超時重試的實現方式可以使用迴圈結構，在請求發起後等待一定時間，若超時未收到響應，則再次發起請求。迴圈次數可以根據實際情況進行設置，一般建議不超過三次。

百度搜索的關於c#HttpClient 的比較少，簡單整理了下，代碼如下

　　　　　　　　　　　　//調用方式 3秒後超時 重試2次        .net framework 4.5               
 　　　　　　　　　  　HttpMessageHandler handler = new TimeoutHandler(2,3000);
                    using (var client = new HttpClient(handler))
                    {
                        using (var content = new StringContent(""), null, "application/json"))
                        {
                            var response = client.PostAsync("url", content).Result;
                            string result = response.Content.ReadAsStringAsync().Result;
                            JObject jobj = JObject.Parse(result);
                            if (jobj.Value<int>("code") == 1)
                            {
                            }
                        }
                    }

public class TimeoutHandler : DelegatingHandler
{
    private int _timeout;
    private int _max_count;
    ///
    /// 超時重試
    /// 
    ///重試次數
    ///超時時間
    public TimeoutHandler( int max_count = 3,  int timeout = 5000)
    {
        base .InnerHandler =  new HttpClientHandler();
        _timeout = timeout;
        _max_count = max_count;
    }

    protected async  override Task SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        HttpResponseMessage response =  null ;
        for ( int i = 1; i <= _max_count + 1; i++)
        {
            var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            cts.CancelAfter(_timeout);
            try
            {
                 response = await  base .SendAsync(request, cts.Token);

                if (response.IsSuccessStatusCode)
                {
                    return response;
                }
            }
            catch (Exception ex)
            {
                //請求超時
                if (ex  is TaskCanceledException)
                {
                    MyLogService.Print( "介面請求超時：" + ex.ToString());
                    if (i > _max_count)
                    {
                        return new HttpResponseMessage(HttpStatusCode.OK)
                        {
                            Content =  new StringContent( "{\"code\":-1,\"data\":\"\",\"msg\":\"介面請求超時\"}" , Encoding.UTF8,  "text/json" )
                        };
                    }
                    MyLogService.Print($ "介面第{i}次重新請求" );
                }
                else
                {
                    MyLogService.Error( "介面請求出錯：" + ex.ToString());
                    return new HttpResponseMessage(HttpStatusCode.OK)
                    {
                        Content =  new StringContent( "{\"code\":-1,\"data\":\"\",\"msg\":\"介面請求出錯\"}" , Encoding.UTF8,  "text/json" )
                    };
                }
            }
        }
        return response;
    }
}