引用：

using System.Threading.Tasks;
using System.Net.Http;
using Newtonsoft.Json;
using System.Net.Http.Headers;

帮助类：

 public static class HttpHelper
    {
        public static async Task<T> Get<T>(string url)
        {
            try
            {
                using (var client = new HttpClient())
                {
                    var responseMsg = await client.GetAsync(url);
                    if (responseMsg.IsSuccessStatusCode)
                    {
                        string strJson = await responseMsg.Content.ReadAsStringAsync();
                        return JsonConvert.DeserializeObject<T>(strJson);
                    }
                    else
                    {
                        return default(T);
                    }
                }
            }
            catch
            {
                return default(T);
            }
        }

        public static async Task<T> Post<T>(string url, dynamic para)
        {
            try
            {
                if (para != null)
                {
                    var requestJson = JsonConvert.SerializeObject(para);
                    HttpContent httpContent = new StringContent(requestJson);
                    httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
                    using (var client = new HttpClient())
                    {
                        var responseJson = await client.PostAsync(url, httpContent).Result.Content.ReadAsStringAsync();
                        return JsonConvert.DeserializeObject<T>(responseJson);
                    }
                }
                return default(T);
            }
            catch
            {
                return default(T);
            }
        }
    }