HttpClient在SendAsync（C＃）上抛出错误

Question

我的代码（C＃）：

        HttpClientHandler httpClientHandler = new HttpClientHandler();
        httpClientHandler.PreAuthenticate = true;
        //httpClientHandler.UseDefaultCredentials = true;
        httpClientHandler.Credentials = CredentialCache.DefaultNetworkCredentials;
        //httpClientHandler.Credentials = new NetworkCredential("usr", "pw", "domain");
        httpClientHandler.CookieContainer = new CookieContainer();

        var httpcontent = new StringContent(content);

        HttpClient httpClient = new HttpClient(httpClientHandler);
        httpClient.DefaultRequestHeaders.ExpectContinue = false;
        httpClient.BaseAddress = ews_url;

        // var request = httpClient.PostAsync(ews_url, httpcontent);
        var requestMessage = new HttpRequestMessage();
        requestMessage.RequestUri = ews_url;
        requestMessage.Content = httpcontent;
        requestMessage.Method = HttpMethod.Post;
        var request = httpClient.SendAsync(requestMessage); 
        HttpResponseMessage response = await request; // --> Error

HttpClient（httpClient）的实例总是在标有 - ＆gt;的行中遇到异常。错误评论。我找不到任何错误的代码行。例外情况说：“底层连接已关闭”，“发送时出错”，但连接未按您所见关闭。我尝试了几个选项，如“UseDefaultCredentials true / false”，“new NetworkCredentials”。

我的方案是一个应用程序发送肥皂消息来交换Web服务（ews）。

你有什么想法解决我的问题吗？