public class FormItem
    {
        public string Name { get; set; }
        public ParamType ParamType { get; set; }
        public string Value { get; set; }
    }

    public enum ParamType
    { 
        ///
        /// 文本类型
        ///
        Text,
        ///
        /// 文件路径，需要全路径（例：C:\A.JPG)
        ///
        File
    }

    public static class Funcs
    {
        public static string PostFormData(List list,string uri)
        {
            string boundary = "---------------------------" + DateTime.Now.Ticks.ToString("x");
            //请求 
            WebRequest req = WebRequest.Create(uri);
            req.Method = "POST";
            req.ContentType = "multipart/form-data; boundary=" + boundary;
            //组织表单数据 
            StringBuilder sb = new StringBuilder();
            foreach (FormItem item in list)
            {
                switch (item.ParamType)
                {
                    case ParamType.Text:
                        sb.Append("--" + boundary);
                        sb.Append("\r\n");
                        sb.Append("Content-Disposition: form-data; name="" + item.Name + """);
                        sb.Append("\r\n\r\n");
                        sb.Append(item.Value);
                        sb.Append("\r\n");
                        break;
                    case ParamType.File:
                        sb.Append("--" + boundary);
                        sb.Append("\r\n");
                        sb.Append("Content-Disposition: form-data; name="media"; filename=""+item.Value+""");
                        sb.Append("\r\n");
                        sb.Append("Content-Type: application/octet-stream");
                        sb.Append("\r\n\r\n");
                        break;
                }
            }
            string head = sb.ToString();
            //post字节总长度
            long length = 0;
            byte[] form_data = Encoding.UTF8.GetBytes(head);
            //结尾 
            byte[] foot_data = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");
            List fileList = list.Where(f => f.ParamType == ParamType.File).ToList();
            length = form_data.Length + foot_data.Length;
            foreach (FormItem fi in fileList)
            {
                FileStream fileStream = new FileStream(fi.Value, FileMode.Open, FileAccess.Read);
                length += fileStream.Length;
                fileStream.Close();
            }
            req.ContentLength = length;           

            Stream requestStream = req.GetRequestStream();
            //发送表单参数 
            requestStream.Write(form_data, 0, form_data.Length);
            foreach (FormItem fd in fileList)
            {
                FileStream fileStream = new FileStream(fd.Value, FileMode.Open, FileAccess.Read);
                //文件内容 
                byte[] buffer = new Byte[checked((uint)Math.Min(4096, (int)fileStream.Length))];
                int bytesRead = 0;
                while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
                    requestStream.Write(buffer, 0, bytesRead);
                //结尾 
                requestStream.Write(foot_data, 0, foot_data.Length);
            }
            requestStream.Close();

            //响应 
            WebResponse pos = req.GetResponse();
            StreamReader sr = new StreamReader(pos.GetResponseStream(), Encoding.UTF8);
            string html = sr.ReadToEnd().Trim();
            sr.Close();
            if (pos != null)
            {
                pos.Close();
                pos = null;
            }
            if (req != null)
            {
                req = null;
            }
            return html;
        }
        ///
        /// 从URL地址下载文件到本地磁盘
        ///
        /// 本地磁盘地址
        /// URL网址
        ///
        public static string SaveFileFromUrl(string FileName, string Url)
        {
            WebResponse response = null;
            Stream stream = null;
            try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
                response = request.GetResponse();
                stream = response.GetResponseStream();

                if (!response.ContentType.ToLower().StartsWith("text/"))
                {
                    SaveBinaryFile(response, FileName);
                }
                else
                {
                    StreamReader sr = new StreamReader(stream, System.Text.Encoding.UTF8);
                    return sr.ReadToEnd();
                }

            }
            catch (Exception err)
            {
                return err.ToString();
            }
            return "complete";
        }
        ///
        /// 将二进制文件保存到磁盘
        ///
        /// 将二进制文件保存到磁盘
        // 将二进制文件保存到磁盘
        private static bool SaveBinaryFile(WebResponse response, string FileName)
        {
            bool Value = true;
            byte[] buffer = new byte[1024];

            try
            {
                if (File.Exists(FileName))
                    File.Delete(FileName);
                Stream outStream = System.IO.File.Create(FileName);
                Stream inStream = response.GetResponseStream();

                int l;
                do
                {
                    l = inStream.Read(buffer, 0, buffer.Length);
                    if (l > 0)
                        outStream.Write(buffer, 0, l);
                }
                while (l > 0);

                outStream.Close();
                inStream.Close();
            }
            catch
            {
                Value = false;
            }
            return Value;
        }
}

//新建一个form表单项，就是需要提交哪些字段和数据的列表
 List list = new List();
//添加微信接口上的access_token参数，注意，access_token是有过期时间的，代码中的access_token肯定过期了，获取access_token的地址请点击我，需要有自己的服务号才会有的
 list.Add(new FormItem() { Name = "access_token", ParamType = ParamType.Text, Value = "MlfTORyg_dRTuiQThmKUxVVkK7q_SMEd0y9GwBmj6NJw3E0J2jVnC3RxgdO1Yjog2QD4DDxhdqEkZaklR7czq8sSbW4mnhM7n9-5lIIymVGkrBAv2nnnktUyYcuYTMs2SYtp-pn6IWEtTpsFVlUFZQ" });
//添加FORM表单中这条数据的类型，目前只做了两种，一种是文本，一种是文件
 list.Add(new FormItem() { Name = "type", Value = "image", ParamType = ParamType.Text });
//添加Form表单中文件的路径，路径必须是基于硬盘的绝对路径
 list.Add(new FormItem() { Name = "media", Value = @"d:\1.jpg", ParamType = ParamType.File });
//通过Funcs静态类中的PostFormData方法，将表单数据发送至http://file.api.weixin.qq.com/cgi-bin/media/upload腾讯上传下载文件接口
 string result = Funcs.PostFormData(list, "http://file.api.weixin.qq.com/cgi-bin/media/upload");
//获取返回值，并取出的结果中的media_id，注意，有可能返回的是腾讯的错误代码，请自行判断
 System.Web.Script.Serialization.JavaScriptSerializer jss = new System.Web.Script.Serialization.JavaScriptSerializer();
 var mydata=jss.Deserialize(result);
//通过Funcs静态类中的SaveFileFromUrl方法，将指定微信media_id的文件下载到本机
 var saveResult=Funcs.SaveFileFromUrl(@"d:\lee.jpg", "http://file.api.weixin.qq.com/cgi-bin/media/get?access_token=MlfTORyg_dRTuiQThmKUxVVkK7q_SMEd0y9GwBmj6NJw3E0J2jVnC3RxgdO1Yjog2QD4DDxhdqEkZaklR7czq8sSbW4mnhM7n9-5lIIymVGkrBAv2nnnktUyYcuYTMs2SYtp-pn6IWEtTpsFVlUFZQ&media_id=" + mydata["media_id"].ToString());