关于释放session

If you are using ASP 3.0 (the version of ASP that comes with Windows 2000 / IIS 5) then you can use the following syntax:

Session.Contents.Remove "name"

where Name is the name of the Session variable you wish to remove. Removing Session variables in this way has its advantages over using the following method:

Session("Name") = Null

Namely, the above method (setting a Session variable to Null) only removes the memory associated with the Session variable itself... the Session object still maintains a reference to it, though. Each Session variables is stored in the Session object with a key/item pair, similar to the Scripting.Dictionary object. Therefore, using the Null method above, you are not removing the key from the Session Contents collection that contains the reference to the variable...

With the Remove method that we looked at first, you are removing both the key and item associated with a Session variable. There is also a RemoveAll method that can be used to scrap all of the Session variables completely:

Session.Contents.RemoveAll

Again, the Remove and RemoveAll methods are new to ASP 3.0. If you have ASP 2.0, you will need to use the Null method


Deleting a Subset of Session Variables

When using Sessions to store variables in, I use a naming convention - for example, for all Customer related info I prefix the session variable with the substring Customer. (so for the CustomerID it would be Customer.ID, the customer username would be Customer.Name, etc.) This is very useful when viewing the session objects as you can see the related objects straight off.

The problem I had the other day was that I wanted to remove only those items from the Session which were prefixed SW.. So first off I used the following code:

'---------------------------------------
Session("SW.1")="Test 1"
Session("SW.2")="Test 2"
Session("Other")="Other"

For Each SessionItem in Session.Contents
If Left(SessionItem,3) = "SW." then
Session.Contents.Remove(SessionItem)
end if
Next
'---------------------------------------

This seems fine, but when it's run, what happens is that SW.1 is removed, but SW.2 is NOT removed. Why? I'm not exactly sure, but I guess that the index is then reset so that SW.2 is now where SW.1 was, and seeing as we have iterated past that item in the For Each...Next statement, the loop just moves to Other, missing out SW.2 altogether! Eek!

So to get round this I wrote the following function, which will properly delete all Session variables that begin with a specified substring:

'---------------------------------------
function SessionRemoveSelected(sItemPrefix)
'/////////////////////////////////////////////////
' Remove Selected Items starting with sItemPrefix
' from the Session. e.g. SS. will remove SS.ID and
' SS.NAME but not CustomerID Returns True or False
' depending on whether any items where removed.
'---------------------------------------
' sItemPrefix [string] : Item Prefix
'/////////////////////////////////////////////////
dim arySession()
dim lCount, lPrefixLength
dim SessionItem
dim blnResult

lCount = -1
lPrefixLength = len(sItemPrefix)
blnResult = false

' temporarily store in array items to remove
For Each SessionItem in Session.Contents
if left(SessionItem,lPrefixLength) = sItemPrefix then
lCount = lCount + 1
redim preserve arySession(lCount)
arySession(lCount) = SessionItem
end if
Next

' remove items
if IsArray(arySession)and lCount >= 0 then
for lCount = LBound(arySession) to UBound(arySession)
Session.Contents.Remove(arySession(lCount))
next
blnResult=true
end if

SessionRemoveSelected = blnResult
end function
'-------------------------------------------------