HttpClient对Rest服务抛出错误

我试图从我正在创建的Web API中调用多个rest服务,当调用一个Sharepoint rest服务时,出现以下错误

此实例已启动一个或多个请求。只能在发送第一个请求之前修改属性。

下面是使用HttpClient调用rest服务的代码

try
{
    var credential = new NetworkCredential(userName_SP, password_SP, domain_SP);
    var myCache = new CredentialCache();
    myCache.Add(new Uri(core_URL), "NTLM", credential);

    var handler = new HttpClientHandler();
    handler.AllowAutoRedirect = true;
    handler.Credentials = myCache;

    using (var client_sharePoint = new HttpClient(handler))
    {
        var response = client_sharePoint.GetAsync(core_URL).Result;
        client_sharePoint.BaseAddress = uri;
        client_sharePoint.DefaultRequestHeaders.Accept.Clear();
        client_sharePoint.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));

        var responsedata = await response.Content.ReadAsStringAsync();
        var returnObj = JsonConvert.DeserializeObject<SharepointDTO.RootObject>(
            responsedata);

        return returnObj;
    }

    ...

我以前从未遇到过这个错误。如果我需要设置超时,谁能建议我吗