大文件上传之异常处理

等Application_Error事件处理捕获到“Maximum request length exceeded.”异常为时已晚，页面已无法正常Redirect至Custom Error Page，因此应该设法使系统不抛出“Maximum request length exceeded.”异常。而要使系统不抛出该异常，就应在HttpHandler处理Request之前，使HttpHandler得到的HttpContext的内容小于maxRequestLength。看来，可以在Global.asax中的Application_BeginRequest事件处理过程中作一番文章。下面的代码就给出了解决方案：
protected void Application_BeginRequest(Object sender, EventArgs e)
{
double maxRequestLength = 4096000 ;

HttpRequest request = HttpContext.Current.Request;
if (request.ContentLength > maxRequestLength)//4096000 is maxRequestLength
{
HttpApplication app = sender as HttpApplication;
HttpContext context = app.Context;

HttpWorkerRequest wr =
(HttpWorkerRequest)(context.GetType().GetProperty
("WorkerRequest", BindingFlags.Instance |
BindingFlags.NonPublic).GetValue(context, null));
byte[] buffer;
if (wr.HasEntityBody())
{
int contentlen = Convert.ToInt32(wr.GetKnownRequestHeader(

HttpWorkerRequest.HeaderContentLength));
buffer = wr.GetPreloadedEntityBody();
int received = buffer.Length;
int totalrecv = received;
if (!wr.IsEntireEntityBodyIsPreloaded())
{
buffer = new byte[65535];
while ( contentlen - totalrecv >= received )
{
received =
wr.ReadEntityBody(buffer,
buffer.Length);
totalrecv += received;
}
received =
wr.ReadEntityBody(buffer, contentlen - totalrecv);
}
}
context.Response.Redirect("./errTimeout.aspx");//Redirect to custom error page.
}
}

从代码可以看出，关键的思路就是：如果上传的文件超过maxRequestLength，则上传文件已可以看作无效文件，于是就可以放心大胆的为其瘦身，使Request的HttpContext的内容小于maxRequestLength，这样“Maximum request length exceeded.”异常就不会再产生，页面也就可以正常跳转到Custom Error Page了。代码中的循环部分就是将上传的文件内容读入buffer中，因为上传文件已属于无效文件，所以文件内容也就不重要，这里buffer的用途仅仅是用来接收上传文件内容，buffer中的数据也无需再被读取。