内容类型是内容的标头，而不是请求的标头，这就是失败的原因。
AddWithoutValidation如Robert
Levy所建议的那样可能有效，但是您也可以在创建请求内容本身时设置内容类型（请注意，代码段在两个位置为“ Accept”和“ Content-
Type”标头添加了“ application / json”）：
HttpClient client = new HttpClient();client.baseAddress = new Uri("http://example.com/");client.DefaultRequestHeaders .Accept .Add(new MediaTypeWithQualityHeaderValue("application/json"));//ACCEPT headerHttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, "relativeAddress");request.Content = new StringContent("{"name":"John Doe","age":33}", Encoding.UTF8, "application/json");//CONTENT-TYPE headerclient.SendAsync(request) .ContinueWith(responseTask => { Console.WriteLine("Response: {0}", responseTask.Result); });
