我在网上找了一个比较贴切的描述：
Session.Clear()就是把Session对象中的所有项目都删除了，Session对象里面啥都没有。但是Session对象还保留。Session.Abandon()就是把当前Session对象删除了，下一次就是新的Session了。(下一次新会话开始貌似并不会产生新的SESSIONID)主要的不同之处在于当使用Session.Abandon时，会调用Session_End方法（InProc模式下）。当下一个请求到来时将激发Session_Start方法。而Session.Clear只是清除Session中的所有数据并不会中止该Session，因此也不会调用那些方法。其中标红也不是很正确。
首先我们来看一下code：
Dictionary<string, HttpSessionState> dict = new Dictionary<string, HttpSessionState>();
// Abandon
dict.Remove(key);
//Clear()
dict[key].Clear();
假设session存储一个字典结构，key就是我们客户端的sessionID，value 就是我们的SessionSate（它也是一个集合），调用Abandon后就把key删除了，但是clear是value里面的内容清空了。这2个方法的实现大家可以参考HttpSessionStateContainer的实现，Clear 清空集合，Abandon设置变量使其属性IsAbandoned为true。
真正的核心code在SessionStateModule类的OnReleaseState方法里面，在有Session的Http请求中，有一个地方加载我们的session，也有一个地方保存我们的session。
/// <devdoc>
/// <para>[To be supplied.]</para>
/// </devdoc>
void OnReleaseState(Object source, EventArgs eventArgs) {
HttpApplication app;
HttpContext context;
bool setItemCalled = false; Debug.Trace("SessionStateOnReleaseState", "Beginning SessionStateModule::OnReleaseState"); Debug.Assert(!(_rqAddedCookie && !_rqIsNewSession),
"If session id was added to the cookie, it must be a new session."); // !!!
// Please note that due to InProc session id optimization, this function should not
// use _rqId directly because it can still be null. Instead, use DelayedGetSessionId(). _releaseCalled = true; app = (HttpApplication)source;
context = app.Context; ChangeImpersonation(context, false); try {
if (_rqSessionState != null) {
bool delayedSessionState = (_rqSessionState == s_delayedSessionState); Debug.Trace("SessionStateOnReleaseState", "Remove session state from context");
SessionStateUtility.RemoveHttpSessionStateFromContext(_rqContext, delayedSessionState); /*
* Don't store untouched new sessions.
*/ if (
// The store doesn't have the session state.
// ( Please note we aren't checking _rqIsNewSession because _rqIsNewSession
// is lalso true if the item is converted from temp to perm in a GetItemXXX() call.)
_rqSessionStateNotFound // OnStart is not defined
&& _sessionStartEventHandler == null // Nothing has been stored in session state
&& (delayedSessionState || !_rqSessionItems.Dirty)
&& (delayedSessionState || _rqStaticObjects == null || _rqStaticObjects.NeverAccessed)
) { Debug.Trace("SessionStateOnReleaseState", "Not storing unused new session.");
}
else if (_rqSessionState.IsAbandoned) {
Debug.Trace("SessionStateOnReleaseState", "Removing session due to abandonment, SessionId=" + _rqId); if (_rqSessionStateNotFound) {
// The store provider doesn't have it, and so we don't need to remove it from the store. // However, if the store provider supports session expiry, and we have a Session_End in global.asax,
// we need to explicitly call Session_End.
if (_supportSessionExpiry) {
if (delayedSessionState) {
Debug.Assert(s_allowDelayedStateStoreItemCreation, "s_allowDelayedStateStoreItemCreation");
Debug.Assert(_rqItem == null, "_rqItem == null"); InitStateStoreItem(false /*addToContext*/);
} _onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);
}
}
else {
Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session"); // Remove it from the store because the session is abandoned.
_store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem);
}
}
else if (!_rqReadonly ||
(_rqReadonly &&
_rqIsNewSession &&
_sessionStartEventHandler != null &&
!SessionIDManagerUseCookieless)) {
// We need to save it since it isn't read-only
// See Dev10 588711: Issuing a redirect from inside of Session_Start event
// triggers an infinite loop when using pages with read-only session state // We save it only if there is no error, and if something has changed (unless it's a new session)
if ( context.Error == null // no error
&& ( _rqSessionStateNotFound
|| _rqSessionItems.Dirty // SessionItems has changed.
|| (_rqStaticObjects != null && !_rqStaticObjects.NeverAccessed) // Static objects have been accessed
|| _rqItem.Timeout != _rqSessionState.Timeout // Timeout value has changed
)
) { if (delayedSessionState) {
Debug.Assert(_rqIsNewSession, "Saving a session and delayedSessionState is true: _rqIsNewSession must be true");
Debug.Assert(s_allowDelayedStateStoreItemCreation, "Saving a session and delayedSessionState is true: s_allowDelayedStateStoreItemCreation");
Debug.Assert(_rqItem == null, "Saving a session and delayedSessionState is true: _rqItem == null"); InitStateStoreItem(false /*addToContext*/);
} #if DBG
if (_rqSessionItems.Dirty) {
Debug.Trace("SessionStateOnReleaseState", "Setting new session due to dirty SessionItems, SessionId=" + _rqId);
}
else if (_rqStaticObjects != null && !_rqStaticObjects.NeverAccessed) {
Debug.Trace("SessionStateOnReleaseState", "Setting new session due to accessed Static Objects, SessionId=" + _rqId);
}
else if (_rqSessionStateNotFound) {
Debug.Trace("SessionStateOnReleaseState", "Setting new session because it's not found, SessionId=" + _rqId);
}
else {
Debug.Trace("SessionStateOnReleaseState", "Setting new session due to options change, SessionId=" + _rqId +
"\n\t_rq.timeout=" + _rqItem.Timeout.ToString(CultureInfo.InvariantCulture) +
", _rqSessionState.timeout=" + _rqSessionState.Timeout.ToString(CultureInfo.InvariantCulture));
}
#endif
if (_rqItem.Timeout != _rqSessionState.Timeout) {
_rqItem.Timeout = _rqSessionState.Timeout;
} s_sessionEverSet = true;
setItemCalled = true;
_store.SetAndReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqItem, _rqLockId, _rqSessionStateNotFound);
}
else {
// Can't save it because of various reason. Just release our exclusive lock on it.
Debug.Trace("SessionStateOnReleaseState", "Release exclusive lock on session, SessionId=" + _rqId); if (!_rqSessionStateNotFound) {
Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session");
_store.ReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqLockId);
}
}
}
#if DBG
else {
Debug.Trace("SessionStateOnReleaseState", "Session is read-only, ignoring SessionId=" + _rqId);
}
#endif Debug.Trace("SessionStateOnReleaseState", "Returning from SessionStateModule::OnReleaseState");
} if (_rqAddedCookie && !setItemCalled && context.Response.IsBuffered()) {
_idManager.RemoveSessionID(_rqContext);
} }
finally {
RestoreImpersonation();
} // WOS 1679798: PERF: Session State Module should disable EndRequest on successful cleanup
bool implementsIRequiresSessionState = context.RequiresSessionState;
if (HttpRuntime.UseIntegratedPipeline
&& (context.NotificationContext.CurrentNotification == RequestNotification.ReleaseRequestState)
&& (s_canSkipEndRequestCall || !implementsIRequiresSessionState)) {
context.DisableNotifications(RequestNotification.EndRequest, /*postNotifications*/);
_acquireCalled = false;
_releaseCalled = false;
ResetPerRequestFields();
}
}
其中主要的code是：
else if (_rqSessionState.IsAbandoned) {
Debug.Trace("SessionStateOnReleaseState", "Removing session due to abandonment, SessionId=" + _rqId);
if (_rqSessionStateNotFound) {
// The store provider doesn't have it, and so we don't need to remove it from the store.
// However, if the store provider supports session expiry, and we have a Session_End in global.asax,
// we need to explicitly call Session_End.
if (_supportSessionExpiry) {
if (delayedSessionState) {
Debug.Assert(s_allowDelayedStateStoreItemCreation, "s_allowDelayedStateStoreItemCreation");
Debug.Assert(_rqItem == null, "_rqItem == null");
InitStateStoreItem(false /*addToContext*/);
}
_onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);
}
}
else {
Debug.Assert(_rqItem != null, "_rqItem cannot null if it's not a new session");
// Remove it from the store because the session is abandoned.
_store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem);
}
}if (_rqAddedCookie && !setItemCalled && context.Response.IsBuffered()) {
_idManager.RemoveSessionID(_rqContext);
}
_rqAddedCookie 一般情况下为false，所以SessionID 默认也就没有被移除。也就是调用Abandon方法默认Session ID是不会改变的。 _onEndTarget.RaiseSessionOnEnd(ReleaseStateGetSessionID(), _rqItem);就是调用我们的Session_End方法。看到这里大家应该知道什么情况调用Session_End，什么有调用 _store.RemoveItem(_rqContext, ReleaseStateGetSessionID(), _rqLockId, _rqItem); 还有后面 _store.SetAndReleaseItemExclusive(_rqContext, ReleaseStateGetSessionID(), _rqItem, _rqLockId, _rqSessionStateNotFound);这个方法也很重要哦， 就是保存我们的Session大家可以参考 asp.net mvc Session RedisSessionStateProvider锁的实现