如果你想重用HttpClient，建议不要使用DefaultRequestHeaders，因为它们是用来发送每个请求的。

你可以试试这个:

var requestMessage = new HttpRequestMessage
    {
        Method = HttpMethod.Post,
        Content = new StringContent("...", Encoding.UTF8, "application/json"),
        RequestUri = new Uri("...")
    };

requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Basic", 
    Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes($"{user}:{password}")));

var response = await _httpClient.SendAsync(requestMessage);

这可以工作，如果你从服务接收到一个json或xml，我认为这可以给你一个关于标题和T类型如何工作的想法，如果你使用函数MakeXmlRequest(把结果放在xmldocumentnet)和MakeJsonRequest(把json放在类你希望有相同的结构，json响应)在下一种方式

/*-------------------------example of use-------------*/
MakeXmlRequest<XmlDocument>("your_uri",result=>your_xmlDocument_variable =     result,error=>your_exception_Var = error);

MakeJsonRequest<classwhateveryouwant>("your_uri",result=>your_classwhateveryouwant_variable=result,error=>your_exception_Var=error)
/*-------------------------------------------------------------------------------*/


public class RestService
{
    public void MakeXmlRequest<T>(string uri, Action<XmlDocument> successAction, Action<Exception> errorAction)
    {
        XmlDocument XMLResponse = new XmlDocument();
        string wufooAPIKey = ""; /*or username as well*/
        string password = "";
        StringBuilder url = new StringBuilder();
        url.Append(uri);
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url.ToString());
        string authInfo = wufooAPIKey + ":" + password;
        authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
        request.Timeout = 30000;
        request.KeepAlive = false;
        request.Headers["Authorization"] = "Basic " + authInfo;
        string documento = "";
        MakeRequest(request,response=> documento = response,
                            (error) =>
                            {
                             if (errorAction != null)
                             {
                                errorAction(error);
                             }
                            }
                   );
        XMLResponse.LoadXml(documento);
        successAction(XMLResponse);
    }



    public void MakeJsonRequest<T>(string uri, Action<T> successAction, Action<Exception> errorAction)
    {
        string wufooAPIKey = "";
        string password = "";
        StringBuilder url = new StringBuilder();
        url.Append(uri);
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url.ToString());
        string authInfo = wufooAPIKey + ":" + password;
        authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
        request.Timeout = 30000;
        request.KeepAlive = false;
        request.Headers["Authorization"] = "Basic " + authInfo;
       // request.Accept = "application/json";
      //  request.Method = "GET";
        MakeRequest(
           request,
           (response) =>
           {
               if (successAction != null)
               {
                   T toReturn;
                   try
                   {
                       toReturn = Deserialize<T>(response);
                   }
                   catch (Exception ex)
                   {
                       errorAction(ex);
                       return;
                   }
                   successAction(toReturn);
               }
           },
           (error) =>
           {
               if (errorAction != null)
               {
                   errorAction(error);
               }
           }
        );
    }
    private void MakeRequest(HttpWebRequest request, Action<string> successAction, Action<Exception> errorAction)
    {
        try{
            using (var webResponse = (HttpWebResponse)request.GetResponse())
            {
                using (var reader = new StreamReader(webResponse.GetResponseStream()))
                {
                    var objText = reader.ReadToEnd();
                    successAction(objText);
                }
            }
        }catch(HttpException ex){
            errorAction(ex);
        }
    }
    private T Deserialize<T>(string responseBody)
    {
        try
        {
            var toReturns = JsonConvert.DeserializeObject<T>(responseBody);
             return toReturns;
        }
        catch (Exception ex)
        {
            string errores;
            errores = ex.Message;
        }
        var toReturn = JsonConvert.DeserializeObject<T>(responseBody);
        return toReturn;
    }
}
}

对于性能和端口耗尽问题，重用HttpClient实例是一个很好的实践，而且因为没有一个答案给出了这个解决方案(甚至会把你引向糟糕的实践:)，所以我在这里放了一个链接，指向我对类似问题的回答:

https://stackoverflow.com/a/40707446/717372

一些关于如何正确使用HttpClient的源代码:

https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/ https://blogs.msdn.microsoft.com/alazarev/2017/12/29/disposable-finalizers-and-httpclient/

如果你想重用HttpClient，建议不要使用DefaultRequestHeaders，因为它们是用来发送每个请求的。

你可以试试这个:

var requestMessage = new HttpRequestMessage
    {
        Method = HttpMethod.Post,
        Content = new StringContent("...", Encoding.UTF8, "application/json"),
        RequestUri = new Uri("...")
    };

requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Basic", 
    Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes($"{user}:{password}")));

var response = await _httpClient.SendAsync(requestMessage);

Oauth流程是复杂的，总是有一个或另一个错误的空间。 我的建议是始终使用样板代码和一组用于OAuth身份验证流的库。这会让你的生活更轻松。

下面是库集的链接。用于。net的OAuth库

使用基本授权和Json参数。

using (HttpClient client = new HttpClient())
{
    var request_json = "your json string";

    var content = new StringContent(request_json, Encoding.UTF8, "application/json");

    var authenticationBytes = Encoding.ASCII.GetBytes("YourUsername:YourPassword");

    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic",
            Convert.ToBase64String(authenticationBytes));
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

    var result = await client.PostAsync("YourURL", content);

    var result_string = await result.Content.ReadAsStringAsync();
}