C#发送post请求支持发送文件

 public static string HttpPost(string Url, string postDataStr)
 {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";


        byte[] contentByte = System.Text.Encoding.UTF8.GetBytes(postDataStr);
        request.ContentLength = contentByte.Length;

        Stream webstream = request.GetRequestStream();
        webstream.Write(contentByte, 0, contentByte.Length);
        webstream.Close();

        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        string encoding = response.ContentEncoding;
        if (encoding == null || encoding.Length < 1)
        {
            encoding = "UTF-8"; //默认编码
        }
        StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding(encoding));
        string retString = reader.ReadToEnd();

        CookieCollection cookcollection = response.Cookies;

        return retString;
  
 }

发送文本及文件

 public static string HttpPost(string url,Dictionary<string,string> postData,Dictionary<string,string> files)
 {
        HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
        CookieContainer cookieContainer = new CookieContainer();
        request.CookieContainer = cookieContainer;
        request.AllowAutoRedirect = true;
        request.Method = "POST";
        string boundary = DateTime.Now.Ticks.ToString("X"); // 随机分隔线
        request.ContentType = "multipart/form-data;charset=utf-8;boundary=" + boundary;

        byte[] itemBoundaryBytes = Encoding.UTF8.GetBytes("\r\n--" + boundary + "\r\n");
        byte[] endBoundaryBytes = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");
       
        Stream postStream = request.GetRequestStream();
        postStream.Write(itemBoundaryBytes, 0, itemBoundaryBytes.Length);

        //写入文本
        if (postData != null && postData.Count > 0)
        {

            var keys = postData.Keys;
            foreach (var key in keys)
            {
                string strHeader = string.Format("Content-Disposition: form-data; name=\"{0}\"\r\n\r\n", key);
                byte[] strByte = System.Text.Encoding.UTF8.GetBytes(strHeader);
                postStream.Write(strByte, 0, strByte.Length);

                byte[] value = System.Text.Encoding.UTF8.GetBytes(string.Format("{0}", postData[key]));
                postStream.Write(value, 0, value.Length);

                postStream.Write(itemBoundaryBytes, 0, itemBoundaryBytes.Length);

            } 
        }
        postStream.Write(itemBoundaryBytes, 0, itemBoundaryBytes.Length);

       //写入文件
        if (files != null && files.Count > 0)
        {
            var keys = files.Keys;

            foreach (var key in keys)
            {
                string filePath = files[key];
                int pos = filePath.LastIndexOf("\\");
                string fileName = filePath.Substring(pos + 1);
                StringBuilder sbHeader = new StringBuilder(string.Format("Content-Disposition:form-data;name=\"{0}\";filename=\"{1}\"\r\nContent-Type:application/octet-stream\r\n\r\n", key,fileName));
                byte[] postHeaderBytes = Encoding.UTF8.GetBytes(sbHeader.ToString());

                FileStream fs = new FileStream(files[key], FileMode.Open, FileAccess.Read);
                byte[] bArr = new byte[fs.Length];
                fs.Read(bArr, 0, bArr.Length);
                fs.Close();
                postStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);
                postStream.Write(bArr, 0, bArr.Length);

                postStream.Write(itemBoundaryBytes, 0, itemBoundaryBytes.Length);
            }

        }
        postStream.Write(endBoundaryBytes, 0, endBoundaryBytes.Length); //结束标志
        postStream.Close();
        //发送请求并获取相应回应数据
        HttpWebResponse response = request.GetResponse() as HttpWebResponse;
        //直到request.GetResponse()程序才开始向目标网页发送Post请求
        Stream instream = response.GetResponseStream();
        StreamReader sr = new StreamReader(instream, Encoding.UTF8);
        //返回结果网页（html）代码
        string content = sr.ReadToEnd();
        return content;
    
 }