private void button1_Click(object sender, EventArgs e)

{

    string postURL = "http://localhost:40694/test1/Handler1.ashx";

    string[] files = { "F:\\1.png", "F:\\2.png" };

    string str = HttpUploadFile(postURL, files);

    MessageBox.Show(str);

}

public string HttpUploadFile(string url, string[] files)

{

    //HttpContext context

    //参考http://www.cnblogs.com/greenerycn/archive/2010/05/15/csharp_http_post.html

    var memStream = new MemoryStream();

    // 边界符

    var boundary = "---------------" + DateTime.Now.Ticks.ToString("x");

    var beginBoundary = Encoding.ASCII.GetBytes("--" + boundary + "\r\n");

    // 文件参数头

    foreach (var item in files)

    {

        string filePath = item;

        string fileName = Path.GetFileName(item);

        var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read);

        var fileHeaderBytes = Encoding.UTF8.GetBytes($"Content-Disposition: form-data; name=\"{"file"}\"; filename=\"{fileName}\"\r\nContent-Type: application/octet-stream\r\n\r\n");

        // 开始拼数据

        memStream.Write(beginBoundary, , beginBoundary.Length);

        // 文件数据

        memStream.Write(fileHeaderBytes, , fileHeaderBytes.Length);

        var buffer = new byte[];

        int bytesRead;

        while ((bytesRead = fileStream.Read(buffer, , buffer.Length)) != )

        {

            memStream.Write(buffer, , bytesRead);

        }

        fileStream.Close();

        //必须得写入一个换行

        byte[] bytes = Encoding.UTF8.GetBytes($"\r\n");

        memStream.Write(bytes, , bytes.Length);

    }

    //// Key-Value数据

    //Dictionary<string, string> stringDict = new Dictionary<string, string>();

    //stringDict.Add("len", "500");

    //stringDict.Add("wid", "300");

    //stringDict.Add("test1", "1");

    //foreach (var item in stringDict)

    //{

    //    string name = item.Key;

    //    string value = item.Value;

    //    byte[] bytes = Encoding.UTF8.GetBytes($"\r\n--{boundary}\r\nContent-Disposition: form-data; name=\"{name}\"\r\n\r\n{value}\r\n");

    //    memStream.Write(bytes, 0, bytes.Length);

    //}

    // 写入最后的结束边界符

    var endBoundary = Encoding.ASCII.GetBytes("--" + boundary + "--\r\n");// 最后的结束符

    memStream.Write(endBoundary, , endBoundary.Length);

    //写入到tempBuffer

    memStream.Position = ;

    var tempBuffer = new byte[memStream.Length];

    memStream.Read(tempBuffer, , tempBuffer.Length);

    memStream.Close();

    // 创建webRequest并设置属性

    var webRequest = (HttpWebRequest)WebRequest.Create(url);

    webRequest.Method = "POST";

    webRequest.Timeout = ;

    webRequest.ContentType = "multipart/form-data; boundary=" + boundary;

    webRequest.ContentLength = tempBuffer.Length;

    var requestStream = webRequest.GetRequestStream();

    requestStream.Write(tempBuffer, , tempBuffer.Length);

    requestStream.Close();

    var httpWebResponse = (HttpWebResponse)webRequest.GetResponse();

    string responseContent;

    using (var httpStreamReader = new StreamReader(httpWebResponse.GetResponseStream(), Encoding.GetEncoding("utf-8")))

    {

        responseContent = httpStreamReader.ReadToEnd();

    }

    httpWebResponse.Close();

    webRequest.Abort();

    return responseContent;

}

 public class Handler1 : IHttpHandler

    {

        public void ProcessRequest(HttpContext context)

        {

            context.Response.ContentType = "text/plain";

            string strFileNames = "";

            if (context.Request.Files.Count > )

            {

                for (int i = ; i < context.Request.Files.Count; i++)

                {

                    HttpPostedFile _HttpPostedFile = context.Request.Files[i];

                    strFileNames += _HttpPostedFile.FileName + ",";

                    _HttpPostedFile.SaveAs(context.Server.MapPath($"./{_HttpPostedFile.FileName}"));

                }

                //context.Response.Write($"files:{context.Request.Files.Count}  filenames:{strFileNames}");

            }

            //context.Response.Write(context.Request.Form["len"]);

            //context.Response.Write(JsonConvert.SerializeObject(context.Request.Form.AllKeys.Length.ToString()));

            context.Response.Write(strFileNames);

        }

        public bool IsReusable

        {

            get

            {

                return false;

            }

        }

    }