<form id="form1" runat="server" action="UserManageHandler.ashx" method="post" enctype="multipart/form-data">
 <div>
  名稱： <input type="text" name="uname" class="uname" /><br/>
  郵件： <input type="text" name="email" class="email" /><p/>
  附件1： <input type="file" name="file1" class="file" /><p/>
  附件2： <input type="file" name="file2" class="file" /><p/>
  附件3： <input type="file" name="file3" class="file" /><p/>
    <input type="submit" name="submit" value="提交" />
 </div>
</form>

HttpFileCollection files = HttpContext.Current.Request.Files;
 HttpPostedFile postedFile = files["fileUpload"];
 postedFile.SaveAs(postedFile.FileName);

/// <summary>
/// HTTP請求(包含多分部數(shù)據(jù),multipart/form-data)。
/// 將多個文件以及多個參數(shù)以多分部數(shù)據(jù)表單方式上傳到指定url的服務器
/// </summary>
/// <param name="url">請求目標URL</param>
/// <param name="fileFullNames">待上傳的文件列表(包含全路徑的完全限定名)。如果某個文件不存在，則忽略不上傳</param>
/// <param name="kVDatas">請求時表單鍵值對數(shù)據(jù)。</param>
/// <param name="method">請求的方法。請使用 WebRequestMethods.Http 的枚舉值</param>
/// <param name="timeOut">獲取或設置 <see cref="M:System.Net.HttpWebRequest.GetResponse" /> 和
///      <see cref="M:System.Net.HttpWebRequest.GetRequestStream" /> 方法的超時值（以毫秒為單位）。
///      -1 表示永不超時
/// </param>
/// <returns></returns>
public HttpResult UploadFormByMultipart(string url, string[] fileFullNames, NameValueCollection kVDatas = null, string method = WebRequestMethods.Http.Post, int timeOut = -1)
{
 #region 說明
 /* 阿里云文檔：https://www.alibabacloud.com/help/zh/doc-detail/42976.htm
  C# 示例： https://github.com/aliyun/aliyun-oss-csharp-sdk/blob/master/samples/Samples/PostPolicySample.cs?spm=a2c63.p38356.879954.18.7f3f7c34W3bR9U&file=PostPolicySample.cs
     (C#示例中僅僅是把文件中的文本內容當做 FormData 中的項，與文件流是不一樣的。本方法展示的是文件流，更通用)
  */

 /* 說明：multipart/form-data 方式提交文件
  *  (1) Header 一定要有 Content-Type: multipart/form-data; boundary={boundary}。
  *  (2) Header 和bod y之間由 \r\n--{boundary} 分割。
  *  (3) 表單域格式 ：Content-Disposition: form-data; name="{key}"\r\n\r\n
  *     {value}\r\n
  *     --{boundary}
  *  (4)表單域名稱大小寫敏感，如policy、key、file、OSSAccessKeyId、OSSAccessKeyId、Content-Disposition。
  *  (5)注意:表單域 file 必須為最后一個表單域。即必須放在最后寫。
  */
 #endregion

 #region ContentType 說明
 /* 該ContentType的屬性包含請求的媒體類型。分配給ContentType屬性的值在請求發(fā)送Content-typeHTTP標頭時替換任何現(xiàn)有內容。
  
  要清除Content-typeHTTP標頭，請將ContentType屬性設置為null。
  
  * 注意：此屬性的值存儲在WebHeaderCollection中。如果設置了WebHeaderCollection，則屬性值將丟失。
  *  所以放置在Headers 屬性之后設置
  */
 #endregion

 #region Method 說明
 /* 如果 ContentLength 屬性設置為-1以外的任何值，則必須將 Method 屬性設置為上載數(shù)據(jù)的協(xié)議屬性。 */
 #endregion

 #region HttpWebRequest.CookieContainer 在 .NET3.5 與 .NET4.0 中的不同
 /* 請參考：https://www.crifan.com/baidu_emulate_login_for_dotnet_4_0_error_the_fisrt_two_args_should_be_string_type_0_1/ */
 #endregion

 HttpResult httpResult = new HttpResult();

 #region 校驗

 if (fileFullNames == null || fileFullNames.Length == 0)
 {
  httpResult.Status = HttpResult.STATUS_FAIL;

  httpResult.RefCode = (int)HttpStatusCode2.USER_FILE_NOT_EXISTS;
  httpResult.RefText = HttpStatusCode2.USER_FILE_NOT_EXISTS.GetCustomAttributeDescription();

  return httpResult;
 }

 List<string> lstFiles = new List<string>();
 foreach (string fileFullName in fileFullNames)
 {
  if (File.Exists(fileFullName))
  {
   lstFiles.Add(fileFullName);
  }
 }

 if (lstFiles.Count == 0)
 {
  httpResult.Status = HttpResult.STATUS_FAIL;

  httpResult.RefCode = (int)HttpStatusCode2.USER_FILE_NOT_EXISTS;
  httpResult.RefText = HttpStatusCode2.USER_FILE_NOT_EXISTS.GetCustomAttributeDescription();

  return httpResult;
 }

 #endregion

 string boundary = CreateFormDataBoundary();          // 邊界符
 byte[] beginBoundaryBytes = Encoding.UTF8.GetBytes("--" + boundary + "\r\n");  // 邊界符開始?！尽睢坑覀缺仨氁?\r\n 。
 byte[] endBoundaryBytes = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n"); // 邊界符結束?！尽睢績蓚缺仨氁?--\r\n 。
 byte[] newLineBytes = Encoding.UTF8.GetBytes("\r\n"); //換一行
 MemoryStream memoryStream = new MemoryStream();

 HttpWebRequest httpWebRequest = null;
 try
 {
  httpWebRequest = WebRequest.Create(url) as HttpWebRequest; // 創(chuàng)建請求
  httpWebRequest.ContentType = string.Format(HttpContentType.MULTIPART_FORM_DATA + "; boundary={0}", boundary);
  //httpWebRequest.Referer = "http://bimface.com/user-console";
  httpWebRequest.Method = method;
  httpWebRequest.KeepAlive = true;
  httpWebRequest.Timeout = timeOut;
  httpWebRequest.UserAgent = GetUserAgent();

  #region 步驟1：寫入鍵值對
  if (kVDatas != null)
  {
   string formDataTemplate = "Content-Disposition: form-data; name=\"{0}\"\r\n\r\n" +
          "{1}\r\n";

   foreach (string key in kVDatas.Keys)
   {
    string formItem = string.Format(formDataTemplate, key.Replace(StringUtils.Symbol.KEY_SUFFIX, String.Empty), kVDatas[key]);
    byte[] formItemBytes = Encoding.UTF8.GetBytes(formItem);

    memoryStream.Write(beginBoundaryBytes, 0, beginBoundaryBytes.Length); // 1.1 寫入FormData項的開始邊界符
    memoryStream.Write(formItemBytes, 0, formItemBytes.Length);   // 1.2 將鍵值對寫入FormData項中
   }
  }
  #endregion

  #region 步驟2：寫入文件(表單域 file 必須為最后一個表單域)

  const string filePartHeaderTemplate = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\n" +
            "Content-Type: application/octet-stream\r\n\r\n";

  int i = 0;
  foreach (var fileFullName in lstFiles)
  {
   FileInfo fileInfo = new FileInfo(fileFullName);
   string fileName = fileInfo.Name;

   string fileHeaderItem = string.Format(filePartHeaderTemplate, "file", fileName);
   byte[] fileHeaderItemBytes = Encoding.UTF8.GetBytes(fileHeaderItem);

   if (i > 0)
   {
    // 第一筆及第一筆之后的數(shù)據(jù)項之間要增加一個換行 
    memoryStream.Write(newLineBytes, 0, newLineBytes.Length);
   }
   memoryStream.Write(beginBoundaryBytes, 0, beginBoundaryBytes.Length);  // 2.1 寫入FormData項的開始邊界符
   memoryStream.Write(fileHeaderItemBytes, 0, fileHeaderItemBytes.Length); // 2.2 將文件頭寫入FormData項中

   int bytesRead;
   byte[] buffer = new byte[1024];

   FileStream fileStream = new FileStream(fileFullName, FileMode.Open, FileAccess.Read);
   while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
   {
    memoryStream.Write(buffer, 0, bytesRead);        // 2.3 將文件流寫入FormData項中
   }

   i++;
  }

  memoryStream.Write(endBoundaryBytes, 0, endBoundaryBytes.Length);    // 2.4 寫入FormData的結束邊界符

  #endregion

  #region 步驟3：將表單域(內存流)寫入 httpWebRequest 的請求流中，并發(fā)起請求
  httpWebRequest.ContentLength = memoryStream.Length;

  Stream requestStream = httpWebRequest.GetRequestStream();

  memoryStream.Position = 0;
  byte[] tempBuffer = new byte[memoryStream.Length];
  memoryStream.Read(tempBuffer, 0, tempBuffer.Length);
  memoryStream.Close();

  requestStream.Write(tempBuffer, 0, tempBuffer.Length);  // 將內存流中的字節(jié)寫入 httpWebRequest 的請求流中
  requestStream.Close();
  #endregion

  HttpWebResponse httpWebResponse = httpWebRequest.GetResponse() as HttpWebResponse; // 獲取響應
  if (httpWebResponse != null)
  {
   //GetHeaders(ref httpResult, httpWebResponse);
   GetResponse(ref httpResult, httpWebResponse);
   httpWebResponse.Close();
  }
 }
 catch (WebException webException)
 {
  GetWebExceptionResponse(ref httpResult, webException);
 }
 catch (Exception ex)
 {
  GetExceptionResponse(ref httpResult, ex, method, HttpContentType.MULTIPART_FORM_DATA);
 }
 finally
 {
  if (httpWebRequest != null)
  {
   httpWebRequest.Abort();
  }
 }

 return httpResult;
}

/// <summary>
/// HTTP請求(包含多分部數(shù)據(jù),multipart/form-data)。
/// 將文件以及多個參數(shù)以多分部數(shù)據(jù)表單方式上傳到指定url的服務器
/// </summary>
/// <param name="url">請求目標URL</param>
/// <param name="fileFullName">待上傳的文件(包含全路徑的完全限定名)</param>
/// <param name="kVDatas">請求時表單鍵值對數(shù)據(jù)。</param>
/// <param name="method">請求的方法。請使用 WebRequestMethods.Http 的枚舉值</param>
/// <param name="timeOut">獲取或設置 <see cref="M:System.Net.HttpWebRequest.GetResponse" /> 和
///      <see cref="M:System.Net.HttpWebRequest.GetRequestStream" /> 方法的超時值（以毫秒為單位）。
///      -1 表示永不超時
/// </param>
/// <returns></returns>
public HttpResult UploadFormByMultipart(string url, string fileFullName, NameValueCollection kVDatas = null, string method = WebRequestMethods.Http.Post, int timeOut = -1)
{
 string[] fileFullNames = { fileFullName };

 return UploadFormByMultipart(url, fileFullNames, kVDatas, method, timeOut);
}

/// <summary>
/// HTTP請求(包含多分部數(shù)據(jù),multipart/form-data)。
/// 將文件以多分部數(shù)據(jù)表單方式上傳到指定url的服務器
/// </summary>
/// <param name="url">請求目標URL</param>
/// <param name="fileFullName">待上傳的文件(包含全路徑的完全限定名)</param>
/// <param name="kVDatas">請求時表單鍵值對數(shù)據(jù)。</param>
/// <param name="method">請求的方法。請使用 WebRequestMethods.Http 的枚舉值</param>
/// <param name="timeOut">獲取或設置 <see cref="M:System.Net.HttpWebRequest.GetResponse" /> 和
///      <see cref="M:System.Net.HttpWebRequest.GetRequestStream" /> 方法的超時值（以毫秒為單位）。
///      -1 表示永不超時
/// </param>
/// <returns></returns>
public HttpResult UploadFormByMultipart(string url, string fileFullName, Dictionary<string, string> kVDatas = null, string method = WebRequestMethods.Http.Post, int timeOut = -1)
{
 var nvc = kVDatas.ToNameValueCollection();
 return UploadFormByMultipart(url, fileFullName, nvc, method, timeOut);
}