参考链接

• postman 请求直接查看Code
• https://www.jb51.cc/csharp/95741.html 发送 Multipart / form-data
• https://www.cnblogs.com/xuliangxing/p/8746277.html 通过NuGet引用RestSharp

普通http请求

            //通过base64加密：
	    //string str = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(strJson));
            String strURL = "Post请求地址";
            System.Net.HttpWebRequest request = (HttpWebRequest)WebRequest.Create(strURL);
            request.Method = "POST";
            HttpWebRequest.DefaultWebProxy = null;
            request.ContentType = "application/json;charset=utf-8";

            string data = "{\"key\":\"value\"}";

            byte[] param = System.Text.Encoding.UTF8.GetBytes(data);
            request.ContentLength = param.Length;
            System.IO.Stream writer = request.GetRequestStream();
            writer.Write(param, 0, param.Length);
            writer.Close();
            System.Net.HttpWebResponse response;
            try
            {
                response = (System.Net.HttpWebResponse)request.GetResponse();
            }
            catch (WebException ex)
            {
                response = (System.Net.HttpWebResponse)ex.Response;
            }

            System.IO.StreamReader myreader = new System.IO.StreamReader(response.GetResponseStream(), Encoding.UTF8);
            //返回的结果
            string strResult = myreader.ReadToEnd();
            myreader.Close();

使用RestSharp

           string filePath = @"E:\12.png";//上传文件
            var client = new RestClient("https://baidu.com");//url接口地址
            client.Timeout = -1;
            var request = new RestRequest(Method.POST);//提交方法
            request.AddHeader("Content-Type", "multipart/form-data");//请求头信息
           // request.AddHeader("Content-Type", "application/json");
            request.AddFile("media", filePath);//添加文件
            request.AddParameter("access_token", "asd");//添加参数
            request.AddParameter("type", "image");
         
            IRestResponse response = client.Execute(request);
            Console.WriteLine(response.Content);//返回值

C# Post请求