将WebProxy与HttpClient.SendAsync()和HttpRequestMessage一起使用

我可以通过实现类IWebProxy并执行以下操作来使用代理：HttpClientHandler aHandler=新的HttpClientHandler（）；

aHandler.UseCookies = true;
aHandler.AllowAutoRedirect = true;
IWebProxy proxy = new AWProxy(new Uri("http://xx.xx.xxx.xxx:xxxx"));
proxy.Credentials = new NetworkCredential("xxxx", "xxxx");
aHandler.Proxy = proxy;
HttpClient client = new HttpClient(aHandler);

通过呼叫客户端。GetAsync（"http://google.com"）我能够得到一个成功的响应消息。然而，我希望能够使用HttpRequestMessage来控制标题和发布/放置内容。

HttpRequestMessage requestMessage = new HttpRequestMessage(HttpMethod.Get, uri); 
//add other headers
requestMessage.Headers.UserAgent.ParseAdd("Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)");
HttpResponseMessage response = await client.SendAsync(requestMessage);

但是如果我使用HttpRequestMessage，我会得到这个异常：

System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The underlying connection was closed: Unable to connect to the remote server. ---> System.NullReferenceException: Object reference not set to an instance of an object.

感谢您的帮助！

client.DefaultRequestHeaders.Add("user-agent", "Mozilla/5.0 (compatible; MSIE 10.0;     Windows NT 6.2; WOW64; Trident/6.0)");