Webclient 实现异步调用

        private void CallTaxonomyNodeAttributeApi(TaxonomyNodeAttribute taxonomyNodeAttribute)
        {
            string apiUri = string.Format(nodeAttributeURL, taxonomyNodeAttribute.NodeID);
            taxonomyNodeAttribute.RetryCount += 1;
            WebClient webClient = new WebClient();
            webClient.Headers.Add("Authorization", authorizationStr);
            webClient.Proxy = HttpAPIClient.GetProxy();
            webClient.DownloadDataCompleted +=
                new DownloadDataCompletedEventHandler(ProcessTaxonomyAttributeResponseData);
            webClient.DownloadDataAsync(new Uri(apiUri), taxonomyNodeAttribute);
        }

        private void ProcessTaxonomyAttributeResponseData(object sender, DownloadDataCompletedEventArgs e)
        {

            TaxonomyNodeAttribute taxonomyNodeAttribute = e.UserState as TaxonomyNodeAttribute;
            if (e.Error == null)
            {
                taxonomyNodeAttribute.ResponseData = e.Result;
               //...
            }
            else
            {
                //...
            }
        }