用 C# 做一個 file upload proxy server

實現功能

讓 js 用 ajax 的方式傳檔案到跨網域的 server 上，而不會遇到跨網域問題。

核心程式(C#)

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Web;

namespace Tools
{
    public class HttpCaller
    {
        public static string MultipartForm(string url, HttpRequestBase request)
        {
            var ignoreHeader = new string[] { "Content-Length", "Content-Type", "Host", "Referer" };
            string result = string.Empty;
            var mediaType = "";
            try
            {
                using (HttpClient client = new HttpClient())
                {
                    client.DefaultRequestHeaders.Clear();
                    foreach (var key in request.Headers.AllKeys)
                    {
                        if (key.Equals("Content-Type"))
                        {
                            mediaType = request.Headers[key];
                        }
                        if (ignoreHeader.Contains(key))
                        {
                            continue;
                        }
                        client.DefaultRequestHeaders.Add(key, request.Headers[key]);
                    }
                    HttpRequestMessage message = new HttpRequestMessage(HttpMethod.Post, url);
                    message.Content = new ByteArrayContent(StreamToBytes(request.InputStream));
                    message.Content.Headers.Remove("Content-Type");
                    message.Content.Headers.Add("Content-Type", mediaType);
                    result = client.SendAsync(message).Result.Content.ReadAsStringAsync().Result;
                }
            }
            catch (Exception ex)
            {
                result = ex.Message;
            }
            return result;
        }

        private static byte[] StreamToBytes(Stream stream)
        {
            byte[] bytes = new byte[stream.Length];
            stream.Read(bytes, 0, bytes.Length);
            stream.Seek(0, SeekOrigin.Begin);
            return bytes;
        }
    }
}

使用方法(C# ASP.NET MVC 專案)

public ActionResult upload()
{
   var targetUrl = Request.Form["targetUrl"];
   var json = HttpCaller.MultipartForm(targetUrl, Request);
   return Content(json);
}

使用方法(JS)

function uploadRequest(targetUrl, requestObject, files, callback) {
    var formData = new FormData();
    for (var key in requestObject) {
        formData.append(key, requestObject[key]);
    }
    for (var i = 0 ; i < files.length ; i++){
        formData.append("file" + (i+1), files[i]);
    }
    formData.append("targetUrl", targetUrl);

    var request = new XMLHttpRequest();
    request.open("POST", "upload");
    request.onload = function (e) {
        var response = e.target.response;
        callback(response);
    };
    request.send(formData);
}