1、遇到的问题

① 远程主机强迫关闭了一个现有的连接

相信大家在使用 HttpClient 的时候遇到过 远程主机强迫关闭了一个现有的连接 的错误，一般的解决方法就是下面这种

解决办法：在请求方法中指定 ServicePoint 安全协议
System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls11 | System.Net.SecurityProtocolType.Tls12;

② POST请求某种情况下请求会失败的解决方案

工作中遇到写正常的 http post 请求会失败的问题，于是就在原有的基础上改动了写代码使用HttpRequestMessage 指定 标头、HTTP 谓词和潜在数据，详情见代码第二个 post 请求方法。

2、使用HttpClient为什么建议使用单例

HttpClient 旨在被实例化一次并在应用程序的整个生命周期中重复使用。为每个请求实例化一个 HttpClient 类将耗尽重负载下可用的套接字数量。将导致 SocketException 错误。

3、基础代码实现

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Reflection;
using System.Text;

namespace Clear.DataServices.HttpHelper
{
    public class HttpClientService
    {
        private static HttpClientService _instance;

        private static readonly object _lock = new object();
        
        public HttpClientService() { }
        
        public static HttpClientService GetInstance()
        {
            if (_instance == null)
            {
                lock (_lock)
                {
                    if (_instance == null)
                    {
                        _instance = new HttpClientService();
                    }
                }
            }
            return _instance;
        }
      
        /// <summary>
        /// HttpClient Get请求
        /// </summary>
        /// <param name="url">请求地址</param>
        /// <returns></returns>
        public string HttpGet(string url)
        {
            try
            {
                using (
                    HttpClient client = new HttpClient
                    {
                        BaseAddress = new Uri(url)
                    })
                {
                    client.DefaultRequestHeaders.Accept.Clear();
                    HttpResponseMessage response = client.GetAsync(url).Result;

                    response.EnsureSuccessStatusCode();
                    string responseBody = response.Content.ReadAsStringAsync().Result;

                    return responseBody;
                }
            }
            catch (Exception e)
            {                
                return e.Message;
            }
        }

        /// <summary>
        /// HttpClient Post请求
        /// </summary>
        /// <param name="url">请求地址</param>
        /// <param name="content">HttpContent</param>
        /// <returns></returns>
        public string HttpPost(string url, string content, string token = null)
        {
            try
            {
                using (
                    HttpClient client = new HttpClient
                    {
                        BaseAddress = new Uri(url)
                    })
                {
                    // System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls11 | System.Net.SecurityProtocolType.Tls12;

                    client.DefaultRequestHeaders.Accept.Clear();
                    client.DefaultRequestHeaders.Connection.Add("keep-alive");
                    if (!token.IsNullOrEmpty())
                    {
                        client.DefaultRequestHeaders.Add("Authorization-Token", token);
                    }
                    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
             
                    HttpResponseMessage response = client.PostAsync(url, new StringContent(content, Encoding.UTF8, "application/json")).Result;
                    response.EnsureSuccessStatusCode();
                    string responseBody = response.Content.ReadAsStringAsync().Result;
             
                    return responseBody;
                }
            }
            catch (Exception e)
            {                
                return e.Message;
            }
        }

        /// <summary>
        /// HttpClient Post请求
        /// </summary>
        /// <remarks>
        /// 用于非正常http请求
        /// </remarks>
        /// <param name="url"></param>
        /// <param name="content"></param>
        /// <returns></returns>
        public string HttpPost(string url, string content)
        {
            try
            {
                using (var httpClient = new HttpClient())
                {
                    using (var request = new HttpRequestMessage(new HttpMethod("Post"), url))
                    {
                        // System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls11 | System.Net.SecurityProtocolType.Tls12;

                        request.Headers.TryAddWithoutValidation("Content-Type", "application/json");

                        request.Content = new StringContent(content);

                        request.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json");
                        var response = httpClient.SendAsync(request).Result;
                        return response.Content.ReadAsStringAsync().Result;
                    }
                }
            }
            catch (Exception e)
            {
                return e.Message;
            }
        }

        /// <summary>
        /// Model对象转换为uri网址参数形式
        /// </summary>
        /// <param name="obj">Model对象</param>
        /// <param name="url">前部分网址</param>
        /// <returns></returns>
        public string GetUriParam(object obj, string url = "")
        {
            PropertyInfo[] propertis = obj.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public);
            StringBuilder sb = new StringBuilder();
            sb.Append(url);
            sb.Append("?");
            foreach (var p in propertis)
            {
                var v = p.GetValue(obj, null);
                if (v == null) continue;

                sb.Append(p.Name);
                sb.Append("=");
                sb.Append(Uri.EscapeDataString(v.ToString()));//将字符串转换为它的转义表示形式，HttpUtility.UrlEncode是小写
                sb.Append("&");
            }
            sb.Remove(sb.Length - 1, 1);

            return sb.ToString();
        }
    }
}