/// 
        /// 使用HttpClient上传文件及传递参数        /// 
        ///         ///         ///         /// 
             public static async Task
     
       HttpPostAsync(string url, NameValueCollection postParameters, List
      
        lstFilePath = null)        {            //var handler = new HttpClientHandler() { AutomaticDecompression = DecompressionMethods.GZip };                        using (var httpClient = new HttpClient())            {                using (MultipartFormDataContent formData = new MultipartFormDataContent())                 {                                        if (lstFilePath != null)                    {                        for (int i = 0; i < lstFilePath.Count; i++)                        {                            int start = lstFilePath[i].LastIndexOf('\\');                            string name = lstFilePath[i].Substring(start + 1);                            var fileContent = new ByteArrayContent(File.ReadAllBytes(lstFilePath[i]));                            fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")                            {                                FileName = name                            };                            formData.Add(fileContent);                        }                                           }                    foreach (string key in postParameters.Keys)                    {                        formData.Add(new StringContent(postParameters[key]), key);                    }                                        Task
       
         response = httpClient.PostAsync(url, formData);                    string tempResult = await response.Result.Content.ReadAsStringAsync();                    return tempResult;                }                           }        }
       
      
     

后端接受 

NameValueCollection request = context.Request.Params;

HttpPostedFile file = context.Request.Files[0];