Implementing a CallBack when a Cached

The capability to call a method whenever an item expires from the cache allows you to

build systems that can intelligently add data back into the cache.

<Script runat=server>

Shared reason As CacheItemRemovedReason

Dim onRemove As CacheItemRemovedCallback

Public Shared Sub ItemRemovedCallback(key As String,

value As Object,

removedReason As CacheItemRemovedReason)

' Reason why the item was removed

reason = removedReason

' Reinsert the item into the Cache, but change the value

Cache.Insert(key, "Item re-inserted")

End Sub

Public Sub Page_Load(sender As Object, e As EventArgs)

onRemove = New CacheItemRemovedCallback(

AddressOf Me.ItemRemovedCallback)

' Add an item to the Cache with the callback

If Is Nothing(Cache("Key1")) Then

Cache.Insert("Key1", "In the Cache", Nothing,

DateTime.Now.AddSeconds(10),

TimeSpan.Zero, CacheItemPriority.Low, onRemove)

End If

' Set the label value

Label1.Text = Cache("Key1")

End Sub

</Script>

Value of the Cache: <asp:label id="Label1" runat="server" />

Comments

The capability of a function to be called when an item is removed from the cache is

very powerful. It allows you to reinsert the item or to report a reason as to why the item

was removed.

The only caveat is that during the time between when the item is removed from the

cache, yet before the callback function is called, another request might ask for the item

from the cache and would not find it. Regardless, it's still a very powerful technique for

controlling how items are removed from the cache.