接上面的章节，我们这篇要讲解的是Pipeline是执行的各种事件，我们知道，在自定义的HttpModule的Init方法里，我们可以添加自己的事件，比如如下代码：
public class Test : IHttpModule
{
public void Init(HttpApplication context)
{
context.BeginRequest += new EventHandler(context_BeginRequest);
context.AuthenticateRequest += new EventHandler(context_AuthenticateRequest);
}
void context_AuthenticateRequest(object sender, EventArgs e)
{
throw new NotImplementedException();
}
void context_BeginRequest(object sender, EventArgs e)
{
throw new NotImplementedException();
}
}
然后添加的代码，在Pipeline里执行的时候就会把这些事件给执行了，那么如何执行并且按照什么顺序执行的呢？ 在了解这些之前，我们先看看这些事件是如何在HttpApplication里暴露出来了，添加事件存放在何处的呢？阅读HttpApplication 的源码，我们可以看到，所有的事件都是按照如下的形式暴露的，选择其中两个看一下：
/// <devdoc><para>[To be supplied.]</para></devdoc>
public event EventHandler BeginRequest {
add { AddSyncEventHookup(EventBeginRequest, value, RequestNotification.BeginRequest); }
remove { RemoveSyncEventHookup(EventBeginRequest, value, RequestNotification.BeginRequest); }
}
/// <devdoc><para>[To be supplied.]</para></devdoc>
public event EventHandler AuthenticateRequest {
add { AddSyncEventHookup(EventAuthenticateRequest, value, RequestNotification.AuthenticateRequest); }
remove { RemoveSyncEventHookup(EventAuthenticateRequest, value, RequestNotification.AuthenticateRequest); }
}
可以发现，所有的事件都是调用AddSyncEventHookup方法添加进去的，其中第一个参数是以Event+事件名称的值，这个值是如何得来的，我们找到声明的代码：
private static readonly object EventDisposed = new object();
private static readonly object EventErrorRecorded = new object();
private static readonly object EventPreSendRequestHeaders = new object();
private static readonly object EventPreSendRequestContent = new object();
private static readonly object EventBeginRequest = new object();
private static readonly object EventAuthenticateRequest = new object();
private static readonly object EventDefaultAuthentication = new object();
private static readonly object EventPostAuthenticateRequest = new object();
private static readonly object EventAuthorizeRequest = new object();
private static readonly object EventPostAuthorizeRequest = new object();
private static readonly object EventResolveRequestCache = new object();
private static readonly object EventPostResolveRequestCache = new object();
private static readonly object EventMapRequestHandler = new object();
private static readonly object EventPostMapRequestHandler = new object();
private static readonly object EventAcquireRequestState = new object();
private static readonly object EventPostAcquireRequestState = new object();
private static readonly object EventPreRequestHandlerExecute = new object();
private static readonly object EventPostRequestHandlerExecute = new object();
private static readonly object EventReleaseRequestState = new object();
private static readonly object EventPostReleaseRequestState = new object();
private static readonly object EventUpdateRequestCache = new object();
private static readonly object EventPostUpdateRequestCache = new object();
private static readonly object EventLogRequest = new object();
private static readonly object EventPostLogRequest = new object();
private static readonly object EventEndRequest = new object();
再结合add和remove方法，可以大胆猜想，这些值应该是作为key值用的，我们先看完第2个参数，再来验证我们的猜想，第2个参数是枚举类型 RequestNotification，这里我们再猜想一下，所有的事件都应该放在统一的地方，然后用这个枚举来区分。让我们先看看这个枚举类的代码：
[Flags]
public enum RequestNotification
{
BeginRequest = 1,
AuthenticateRequest = 2,
AuthorizeRequest = 4,
ResolveRequestCache = 8,
MapRequestHandler = 16,
AcquireRequestState = 32,
PreExecuteRequestHandler = 64,
ExecuteRequestHandler = 128,
ReleaseRequestState = 256,
UpdateRequestCache = 512,
LogRequest = 1024,
EndRequest = 2048,
SendResponse = 536870912,
}
发现什么了没有？虽然使用了Flags标记来记录以便进行异或查询，但是这里的枚举类型好像少了一些吧，仔细对照代码发现所有以Post开头的事件都没出现在这个枚举类里，为什么呢？那这些事件是如何声明的？回到HttpApplication类来继续查看代码，
/// <devdoc><para>[To be supplied.]</para></devdoc>
public event EventHandler PostAuthenticateRequest {
add { AddSyncEventHookup(EventPostAuthenticateRequest, value, RequestNotification.AuthenticateRequest, true); }
remove { RemoveSyncEventHookup(EventPostAuthenticateRequest, value, RequestNotification.AuthenticateRequest, true); }
}