绝大多数人认为在ASP.NET中上传大文件有以下这些解决方案：
◆不要这样做。你最好是在页面中嵌入一个Silverlight或Flash进程上传文件。
◆不要这样做。因为HTTP本身设计就不是为了上传大文件，重新思考你要的功能。
◆不要这样做。ASP.NET本身设计最大也就能处理2GB大小的文件。
◆购买商业产品，如SlickUpload，它使用了一个HttpModule实现了文件流分块。
◆使用开源产品，如NeatUpload，它使用了一个HttpModule实现了文件流分块。
最近我接到一个任务，需构建一个上传工具实现以下功能：
◆必须工作在HTTP协议
◆必须允许非常大的文件上传（会大于2GB）
◆必须允许断点续传
◆必须允许并行上传
因此前三个解决方案都不适应我的需求，其它解决方案对于我而言又太笨重了，因此我开始着手解决在ASP.NET MVC中的这个问题，如果有这方面的开发背景，你一定了解大部分问题最终都归结于对ASP.NET输入流和连锁请求过程的控制，网上的资料一般都是这样描述的，只要你的代码访问了HttpRequest的InputStream属性，在你访问流之前，ASP.NET就会缓存整个上传的文件，这就意味着当我向云服务上传文件时，我必须等待整个大文件抵达服务器，然后才能将其传输到预定目的地，这意味着需要两倍的时间。
首先，我们推荐你阅读一下Scott Hanselman的有关ASP.NET MVC文件上传文章，地址http://www.hanselman.com/blog/CommentView.aspx?guid=bc137b6b-d8d0-47d1-9795-f8814f7d1903，先对文件上传有一个大致的了解，但Scott Hanselman的方法是不能上传大文件的，根据Scott Hanselman的方法，你只需要修改一下web.config文件，确保ASP.NET允许最大支持2GB大小的文件上传，不要担心，这样设置并不会吃掉你的内存，因为凡是大于256KB的数据都被缓存到磁盘上去了。
﹤system.web﹥ ﹤httpruntime requestlengthdiskthreshold="256" maxrequestlength="2097151"﹥ ﹤/httpruntime﹥﹤/system.web﹥ 这是一个简单的适合大多数应用的解决办法，但我的任务中不能借用这种方法，即使会将数据缓存到磁盘中，但这种类似于另存为的方法也会使用大量的内存。
图 1 ：通过缓存整个文件，然后另存为的方式会使内存消耗突然上升
那么在ASP.NET MVC中通过直接访问流，不触发任何缓存机制，上传大文件该如何实现呢？解决办法就是尽量远离ASP.NET，我们先来看一看UploadController，它有三个行为方法，一个是索引我们上传的文件，一个是前面讨论的缓存逻辑，另一个是基于实时流的方法。
public class UploadController : Controller
{
[AcceptVerbs(HttpVerbs.Get)]
[Authorize]
public ActionResult Index()
{
return View();
}
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult BufferToDisk()
{
var path = Server.MapPath("~/Uploads");
foreach (string file in Request.Files)
{
var fileBase = Request.Files[file];
try {
if (fileBase.ContentLength > 0)
{
fileBase.SaveAs(Path.Combine(path, fileBase.FileName));
}
}
catch (IOException)
{
}
}
return RedirectToAction("Index", "Upload");
}
//[AcceptVerbs(HttpVerbs.Post)]
//[Authorize]
public void LiveStream()
{
var path = Server.MapPath("~/Uploads");
var context = ControllerContext.HttpContext;
var provider = (IServiceProvider)context;
var workerRequest = (HttpWorkerRequest)provider.GetService(typeof(HttpWorkerRequest));
//[AcceptVerbs(HttpVerbs.Post)]
var verb = workerRequest.GetHttpVerbName();
if(!verb.Equals("POST"))
{
Response.StatusCode = (int)HttpStatusCode.NotFound;
Response.SuppressContent = true;
return;
}
//[Authorize]
if(!context.User.Identity.IsAuthenticated)
{
Response.StatusCode = (int)HttpStatusCode.Unauthorized;
Response.SuppressContent = true;
return;
}
var encoding = context.Request.ContentEncoding;
var processor = new UploadProcessor(workerRequest);
processor.StreamToDisk(context, encoding, path);
//return RedirectToAction("Index", "Upload");
Response.Redirect(Url.Action("Index", "Upload"));
}
}
虽然这里明显缺少一两个类，但基本的方法还是讲清楚了，看起来和缓存逻辑并没有太大的不同之处，我们仍然将流缓存到了磁盘，但具体处理方式却有些不同了，首先，没有与方法关联的属性，谓词和授权限制都被移除了，使用手动等值取代了，使用手工响应操作而不用ActionFilterAttribute声明的原因是这些属性涉及到了一些重要的ASP.NET管道代码，实际上在我的代码中，我还特意拦截了原生态的HttpWorkerRequest，因为它不能同时做两件事情。
HttpWorkerRequest有VIP访问传入的请求，通常它是由ASP.NET本身支持工作的，但我们绑架了请求，然后欺骗剩下的请求，让它们误以为前面的请求已经全部得到处理，为了做到这一点，我们需要上面例子中未出现的UploadProcessor类，这个类的职责是物理读取来自浏览器的每个数据块，然后将其保存到磁盘上，因为上传的内容被分解成多个部分，UploadProcessor类需要找出内容头，然后拼接成带状数据输出，这一可以在一个上传中同时上传多个文件。
internal class UploadProcessor
{
private byte[] _buffer;
private byte[] _boundaryBytes;
private byte[] _endHeaderBytes;
private byte[] _endFileBytes;
private byte[] _lineBreakBytes;
private const string _lineBreak = "\r\n";
private readonly Regex _filename = new Regex(@"Content-Disposition:\s*form-data\s*;\s*name\s*=\s*""file""\s*;\s*filename\s*=\s*""(.*)""",
RegexOptions.IgnoreCase | RegexOptions.Compiled);
private readonly HttpWorkerRequest _workerRequest;
public UploadProcessor(HttpWorkerRequest workerRequest)
{
_workerRequest = workerRequest;
}
public void StreamToDisk(IServiceProvider provider, Encoding encoding, string rootPath)
{
var buffer = new byte[8192];
if (!_workerRequest.HasEntityBody())
{
return;
}
var total = _workerRequest.GetTotalEntityBodyLength();
var preloaded = _workerRequest.GetPreloadedEntityBodyLength();
var loaded = preloaded;
SetByteMarkers(_workerRequest, encoding);
var body = _workerRequest.GetPreloadedEntityBody();
if (body == null) // IE normally does not preload
{
body = new byte[8192];
preloaded = _workerRequest.ReadEntityBody(body, body.Length);
loaded = preloaded;
}
var text = encoding.GetString(body);
var fileName = _filename.Matches(text)[0].Groups[1].Value;
fileName = Path.GetFileName(fileName); // IE captures full user path; chop it
var path = Path.Combine(rootPath, fileName);
var files = new List {fileName};
var stream = new FileStream(path, FileMode.Create);
if (preloaded > 0)
{
stream = ProcessHeaders(body, stream, encoding, preloaded, files, rootPath);
}
// Used to force further processing (i.e. redirects) to avoid buffering the files again
var workerRequest = new StaticWorkerRequest(_workerRequest, body);
var field = HttpContext.Current.Request.GetType().GetField("_wr", BindingFlags.NonPublic | BindingFlags.Instance); field.SetValue(HttpContext.Current.Request, workerRequest);
if (!_workerRequest.IsEntireEntityBodyIsPreloaded())
{
var received = preloaded;
while (total - received >= loaded && _workerRequest.IsClientConnected())
{
loaded = _workerRequest.ReadEntityBody(buffer, buffer.Length);
stream = ProcessHeaders(buffer, stream, encoding, loaded, files, rootPath);
received += loaded;
}
var remaining = total - received;
buffer = new byte[remaining];
loaded = _workerRequest.ReadEntityBody(buffer, remaining);
stream = ProcessHeaders(buffer, stream, encoding, loaded, files, rootPath);
}
stream.Flush();
stream.Close();
stream.Dispose();
}
private void SetByteMarkers(HttpWorkerRequest workerRequest, Encoding encoding)
{
var contentType = workerRequest.GetKnownRequestHeader(HttpWorkerRequest.HeaderContentType);
var bufferIndex = contentType.IndexOf("boundary=") + "boundary=".Length;
var boundary = String.Concat("--", contentType.Substring(bufferIndex));
_boundaryBytes = encoding.GetBytes(string.Concat(boundary, _lineBreak));
_endHeaderBytes = encoding.GetBytes(string.Concat(_lineBreak, _lineBreak));
_endFileBytes = encoding.GetBytes(string.Concat(_lineBreak, boundary, "--", _lineBreak));
_lineBreakBytes = encoding.GetBytes(string.Concat(_lineBreak + boundary + _lineBreak));
}
private FileStream ProcessHeaders(byte[] buffer, FileStream stream, Encoding encoding, int count, ICollection files, string rootPath) {
buffer = AppendBuffer(buffer, count);
var startIndex = IndexOf(buffer, _boundaryBytes, 0);
if (startIndex != -1)
{
var endFileIndex = IndexOf(buffer, _endFileBytes, 0);
if (endFileIndex != -1)
{
var precedingBreakIndex = IndexOf(buffer, _lineBreakBytes, 0);
if (precedingBreakIndex > -1)
{
startIndex = precedingBreakIndex;
}
endFileIndex += _endFileBytes.Length;
var modified = SkipInput(buffer, startIndex, endFileIndex, ref count);
stream.Write(modified, 0, count);
}
else
{
var endHeaderIndex = IndexOf(buffer, _endHeaderBytes, 0);
if (endHeaderIndex != -1)
{
endHeaderIndex += _endHeaderBytes.Length;
var text = encoding.GetString(buffer);
var match = _filename.Match(text);
var fileName = match != null ? match.Groups[1].Value : null;
fileName = Path.GetFileName(fileName); // IE captures full user path; chop it
if (!string.IsNullOrEmpty(fileName) && !files.Contains(fileName))
{
files.Add(fileName);
var filePath = Path.Combine(rootPath, fileName);
stream = ProcessNextFile(stream, buffer, count, startIndex, endHeaderIndex, filePath);
}
else
{
var modified = SkipInput(buffer, startIndex, endHeaderIndex, ref count);
stream.Write(modified, 0, count);
}
}
else
{
_buffer = buffer;
}
}
}
else
{
stream.Write(buffer, 0, count);
}
return stream;
}
private static FileStream ProcessNextFile(FileStream stream, byte[] buffer, int count, int startIndex, int endIndex, string filePath) {
var fullCount = count;
var endOfFile = SkipInput(buffer, startIndex, count, ref count);
stream.Write(endOfFile, 0, count);
stream.Flush();
stream.Close();
stream.Dispose();
stream = new FileStream(filePath, FileMode.Create);
var startOfFile = SkipInput(buffer, 0, endIndex, ref fullCount);
stream.Write(startOfFile, 0, fullCount);
return stream;
}
private static int IndexOf(byte[] array, IList value, int startIndex)
{
var index = 0;
var start = Array.IndexOf(array, value[0], startIndex);
if (start == -1)
{
return -1;
}
while ((start + index) < array.Length)
{
if (array[start + index] == value[index])
{
index++;
if (index == value.Count)
{
return start;
}
}
else
{
start = Array.IndexOf(array, value[0], start + index);
if (start != -1)
{
index = 0;
}
else
{
return -1;
}
}
}
return -1;
}
private static byte[] SkipInput(byte[] input, int startIndex, int endIndex, ref int count)
{
var range = endIndex - startIndex;
var size = count - range;
var modified = new byte[size];
var modifiedCount = 0;
for (var i = 0; i < input.Length; i++)
{
if (i >= startIndex && i < endIndex)
{
continue;
}
if (modifiedCount >= size)
{
break;
}
modified[modifiedCount] = input[i];
modifiedCount++;
}
input = modified;
count = modified.Length;
return input;
}
private byte[] AppendBuffer(byte[] buffer, int count)
{
var input = new byte[_buffer == null ? buffer.Length : _buffer.Length + count];
if (_buffer != null)
{
Buffer.BlockCopy(_buffer, 0, input, 0, _buffer.Length);
}
Buffer.BlockCopy(buffer, 0, input, _buffer == null ? 0 : _buffer.Length, count);
_buffer = null;
return input;
}
}
在处理代码的中间位置，你应该注意到了另一个类StaticWorkerRequest，这个类负责欺骗ASP.NET，在点击提交按钮时，它欺骗ASP.NET，让他认为没有文件上传，这是必需的，因为当上传完毕时，如果我们要重定向到所需的页面时，ASP.NET将会检查到在HTTP实体主体中仍然有数据，然后会尝试缓存整个上传，于是我们兜了一圈又回到了原点，为了避免这种情况，我们必须欺骗HttpWorkerRequest，将它注入到HttpContext中，获得请求开始部分的StaticWorkerRequest，它是唯一有用的数据。
internal class StaticWorkerRequest : HttpWorkerRequest
{
readonly HttpWorkerRequest _request;
private readonly byte[] _buffer;
public StaticWorkerRequest(HttpWorkerRequest request, byte[] buffer)
{
_request = request;
_buffer = buffer;
}
public override int ReadEntityBody(byte[] buffer, int size)
{
return 0;
}
public override int ReadEntityBody(byte[] buffer, int offset, int size)
{
return 0;
}
public override byte[] GetPreloadedEntityBody()
{
return _buffer;
}
public override int GetPreloadedEntityBody(byte[] buffer, int offset)
{
Buffer.BlockCopy(_buffer, 0, buffer, offset, _buffer.Length);
return _buffer.Length;
}
public override int GetPreloadedEntityBodyLength()
{
return _buffer.Length;
}
public override int GetTotalEntityBodyLength()
{
return _buffer.Length;
}
public override string GetKnownRequestHeader(int index)
{
return index == HeaderContentLength ? "0" : _request.GetKnownRequestHeader(index);
}
// All other methods elided, they're just passthrough } 使用StaticWorkerRequest建立虚假的声明，现在你可以在ASP.NET MVC中通过直接访问数据流上传大文件，使用这个代码作为开始，你可以很容易地保存过程数据，并使用Ajax调用另一个控制器行为展示其进度，将大文件缓存到一个临时区域，可以实现断点续传，不用再等待ASP.NET进程将整个文件缓存到磁盘上，同样，保存文件时也不用消耗另存为方法那么多的内存了。
图 2： 内存消耗更平稳