使用 HttpWebRequest 使用 multipart/form-data 发布数据/上传图像

Question

我正在尝试使用 ImageShack API 上传图片。要使用它，我应该使用multipart/form-data 来POST 图像。我是这样做的......

var postData = "";
var req = HttpWebRequest.Create("http://www.imageshack.us/upload_api.php");
req.Method = "POST";
req.ContentType = "multipart/form-data";
postData += "key=my_key_here&";
postData += "type=base64&";

// get base64 data from image
byte[] bytes = File.ReadAllBytes(@"D:\tmp\WpfApplication1\WpfApplication1\Images\Icon128.gif");
string encoded = Convert.ToBase64String(bytes);
postData += "fileupload=" + encoded;

byte[] reqData = Encoding.UTF8.GetBytes(postData);
using (Stream dataStream = req.GetRequestStream())
{
    dataStream.Write(reqData, 0, reqData.Length);
}

var res = (HttpWebResponse)req.GetResponse();
var resStream = res.GetResponseStream();
var reader = new StreamReader(resStream);
string resString = reader.ReadToEnd();
txt1.Text = resString;

但 ImageShack 抱怨说

<links>
    <error id="parameter_missing">Sorry, but we've detected that unexpected data is received. Required parameter 'fileupload' is missing or your post is not multipart/form-data</error>
</links>

FileUpload 存在，我正在使用 multipart/form-data 有什么问题？

更新：

新代码http://pastebin.com/TN6e0CD8

发布数据http://pastebin.com/fYE9fsxs

更新 2

我看了另一个问题Multipart forms from C# client。用边界修改了我的代码，删除了 expect 100 标题，但我仍然无法让它工作......

ServicePointManager.Expect100Continue = false;
var boundary = "-----------------------------28520690214962";
var newLine = Environment.NewLine;
var propFormat = boundary + newLine +
                    "Content-Disposition: form-data; name=\"{0}\"" + newLine + newLine + 
                    "{1}" + newLine + newLine;
var fileHeaderFormat = boundary + newLine +
                        "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"" + newLine;

var req = (HttpWebRequest)HttpWebRequest.Create("http://jm/php/upload.php");
req.Method = WebRequestMethods.Http.Post;
req.ContentType = "multipart/form-data; boundary=" + boundary;

using (var reqStream = req.GetRequestStream()) {
    var reqWriter = new StreamWriter(reqStream);
    var tmp = string.Format(propFormat, "str1", "hello world");
    reqWriter.Write(tmp);
    tmp = string.Format(propFormat, "str2", "hello world 2");
    reqWriter.Write(tmp);
    reqWriter.Write(boundary + "--");
    reqWriter.Flush();
}
var res = req.GetResponse();
using (var resStream = res.GetResponseStream()) {
    var reader = new StreamReader(resStream);
    txt1.Text = reader.ReadToEnd();
}

Answer 1

我终于用下面的代码搞定了……

var boundary = "------------------------" + DateTime.Now.Ticks;
var newLine = Environment.NewLine;
var propFormat = "--" + boundary + newLine +
                    "Content-Disposition: form-data; name=\"{0}\"" + newLine + newLine + 
                    "{1}" + newLine;
var fileHeaderFormat = "--" + boundary + newLine +
                        "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"" + newLine;

var req = (HttpWebRequest)HttpWebRequest.Create("http://jm/php/upload.php");
req.Method = WebRequestMethods.Http.Post;
req.ContentType = "multipart/form-data; boundary=" + boundary;

using (var reqStream = req.GetRequestStream()) {
    var reqWriter = new StreamWriter(reqStream);
    var tmp = string.Format(propFormat, "str1", "hello world");
    reqWriter.Write(tmp);
    tmp = string.Format(propFormat, "str2", "hello world 2");
    reqWriter.Write(tmp);
    reqWriter.Write("--" + boundary + "--");
    reqWriter.Flush();
}
var res = req.GetResponse();
using (var resStream = res.GetResponseStream()) {
    var reader = new StreamReader(resStream);
    txt1.Text = reader.ReadToEnd();
}

注意边界必须以 -- {boundary declaration in ContentType} 开头，结束边界必须以 -- 开头和结尾。就我而言，我最初使用的是

var propFormat = boundary + newLine +
                    "Content-Disposition: form-data; name=\"{0}\"" + newLine + newLine + 
                    "{1}" + newLine;

替换为

var propFormat = "--" + boundary + newLine +
                    "Content-Disposition: form-data; name=\"{0}\"" + newLine + newLine + 
                    "{1}" + newLine;

一切正常

Answer 2

我相信您没有正确构建请求正文。 首先，您需要在内容类型标题中包含部分边界（随机文本）。例如，

内容类型：multipart/form-data； 边界=----WebKitFormBoundarySkAQdHysJKel8YBM

现在请求正文的格式将类似于

------WebKitFormBoundarySkAQdHysJKel8YBM 
Content-Disposition: form-data;name="key"

KeyValueGoesHere
------WebKitFormBoundarySkAQdHysJKel8YBM 
Content-Disposition: form-data;name="param2"

ValueHere
------WebKitFormBoundarySkAQdHysJKel8YBM 
Content-Disposition: form-data;name="fileUpload"; filename="y1.jpg"
Content-Type: image/jpeg 

[image data goes here]

我会建议你使用诸如 Fiddler 之类的工具来了解这些请求是如何构建的。

Answer 3

这与 multipart/form-data 完全不同

1. 字段之间没有界限（即使只有一个字段也需要）。
2. 为什么要使用 base-64 编码？
3. 没有指明图片的内容类型。

查看 RFC 2388 以了解实际的格式规范。查看从网页上传的文件的 Fiddler 抓取也很有用。