我可以从 HTTPModule 访问会话状态吗？

HttpContext.Current.Session should Just Work, assuming your HTTP Module isn't handling any pipeline events that occur prior to the session state being initialized...

EDIT, after clarification in comments: when handling the BeginRequest event, the Session object will indeed still be null/Nothing, as it hasn't been initialized by the ASP.NET runtime yet. To work around this, move your handling code to an event that occurs after PostAcquireRequestState -- I like PreRequestHandlerExecute for that myself, as all low-level work is pretty much done at this stage, but you still pre-empt any normal processing.

Found this over on the ASP.NET forums:

using System;
using System.Web;
using System.Web.Security;
using System.Web.SessionState;
using System.Diagnostics;


// This code demonstrates how to make session state available in HttpModule,
// regardless of requested resource.
// author: Tomasz Jastrzebski


public class MyHttpModule : IHttpModule
{
public void Init(HttpApplication application)
{
application.PostAcquireRequestState += new EventHandler(Application_PostAcquireRequestState);
application.PostMapRequestHandler += new EventHandler(Application_PostMapRequestHandler);
}


void Application_PostMapRequestHandler(object source, EventArgs e)
{
HttpApplication app = (HttpApplication)source;


if (app.Context.Handler is IReadOnlySessionState || app.Context.Handler is IRequiresSessionState) {
// no need to replace the current handler
return;
}


// swap the current handler
app.Context.Handler = new MyHttpHandler(app.Context.Handler);
}


void Application_PostAcquireRequestState(object source, EventArgs e)
{
HttpApplication app = (HttpApplication)source;


MyHttpHandler resourceHttpHandler = HttpContext.Current.Handler as MyHttpHandler;


if (resourceHttpHandler != null) {
// set the original handler back
HttpContext.Current.Handler = resourceHttpHandler.OriginalHandler;
}


// -> at this point session state should be available


Debug.Assert(app.Session != null, "it did not work :(");
}


public void Dispose()
{


}


// a temp handler used to force the SessionStateModule to load session state
public class MyHttpHandler : IHttpHandler, IRequiresSessionState
{
internal readonly IHttpHandler OriginalHandler;


public MyHttpHandler(IHttpHandler originalHandler)
{
OriginalHandler = originalHandler;
}


public void ProcessRequest(HttpContext context)
{
// do not worry, ProcessRequest() will not be called, but let's be safe
throw new InvalidOperationException("MyHttpHandler cannot process requests.");
}


public bool IsReusable
{
// IsReusable must be set to false since class has a member!
get { return false; }
}
}
}

If you're writing a normal, basic HttpModule in a managed application that you want to apply to asp.net requests through pages or handlers, you just have to make sure you're using an event in the lifecycle after session creation. PreRequestHandlerExecute instead of Begin_Request is usually where I go. mdb has it right in his edit.

The longer code snippet originally listed as answering the question works, but is complicated and broader than the initial question. It will handle the case when the content is coming from something that doesn't have an ASP.net handler available where you can implement the IRequiresSessionState interface, thus triggering the session mechanism to make it available. (Like a static gif file on disk). It's basically setting a dummy handler that then just implements that interface to make the session available.

If you just want the session for your code, just pick the right event to handle in your module.

Accessing the HttpContext.Current.Session in a IHttpModule can be done in the PreRequestHandlerExecute handler.

PreRequestHandlerExecute: "Occurs just before ASP.NET starts executing an event handler (for example, a page or an XML Web service)." This means that before an 'aspx' page is served this event gets executed. The 'session state' is available so you can knock yourself out.

Example:

public class SessionModule : IHttpModule
{
public void Init(HttpApplication context)
{
context.BeginRequest += BeginTransaction;
context.EndRequest += CommitAndCloseSession;
context.PreRequestHandlerExecute += PreRequestHandlerExecute;
}






public void Dispose() { }


public void PreRequestHandlerExecute(object sender, EventArgs e)
{
var context = ((HttpApplication)sender).Context;
context.Session["some_sesion"] = new SomeObject();
}
...
}

Try it: in class MyHttpModule declare:

private HttpApplication contextapp;

Then:

public void Init(HttpApplication application)
{
//Must be after AcquireRequestState - the session exist after RequestState
application.PostAcquireRequestState += new EventHandler(MyNewEvent);
this.contextapp=application;
}

And so, in another method (the event) in the same class:

public void MyNewEvent(object sender, EventArgs e)
{
//A example...
if(contextoapp.Context.Session != null)
{
this.contextapp.Context.Session.Timeout=30;
System.Diagnostics.Debug.WriteLine("Timeout changed");
}
}

Since .NET 4.0 there is no need for this hack with IHttpHandler to load Session state (like one in most upvoted answer). There is a method HttpContext.SetSessionStateBehavior to define needed session behaviour. If Session is needed on all requests set runAllManagedModulesForAllRequests to true in web.config HttpModule declaration, but be aware that there is a significant performance cost running all modules for all requests, so be sure to use preCondition="managedHandler" if you don't need Session for all requests. For future readers here is a complete example:

web.config declaration - invoking HttpModule for all requests:

<system.webServer>
<modules runAllManagedModulesForAllRequests="true">
<add name="ModuleWithSessionAccess" type="HttpModuleWithSessionAccess.ModuleWithSessionAccess, HttpModuleWithSessionAccess"/>
</modules>
</system.webServer>

web.config declaration - invoking HttpModule only for managed requests:

<system.webServer>
<modules>
<add name="ModuleWithSessionAccess" type="HttpModuleWithSessionAccess.ModuleWithSessionAccess, HttpModuleWithSessionAccess" preCondition="managedHandler"/>
</modules>
</system.webServer>

IHttpModule implementation:

namespace HttpModuleWithSessionAccess
{
public class ModuleWithSessionAccess : IHttpModule
{
public void Init(HttpApplication context)
{
context.BeginRequest += Context_BeginRequest;
context.PreRequestHandlerExecute += Context_PreRequestHandlerExecute;
}


private void Context_BeginRequest(object sender, EventArgs e)
{
var app = (HttpApplication)sender;
app.Context.SetSessionStateBehavior(System.Web.SessionState.SessionStateBehavior.Required);
}
    

private void Context_PreRequestHandlerExecute(object sender, EventArgs e)
{
var app = (HttpApplication)sender;
if (app.Context.Session != null)
{
app.Context.Session["Random"] = $"Random value: {new Random().Next()}";
}
}


public void Dispose()
{
}
}
}