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CacheItemRemovedCallback Delegate

Defines a callback method for notifying applications when a cached item is removed from the Cache.

Namespace:  System.Web.Caching
Assembly:  System.Web (in System.Web.dll)

'Declaration
Public Delegate Sub CacheItemRemovedCallback ( _
	key As String, _
	value As Object, _
	reason As CacheItemRemovedReason _
)

Parameters

key
Type: System.String
The key that is removed from the cache.
value
Type: System.Object
The Object item associated with the key removed from the cache.
reason
Type: System.Web.Caching.CacheItemRemovedReason
The reason the item was removed from the cache, as specified by the CacheItemRemovedReason enumeration.

The following code example demonstrates a page that shows users the value assigned to an item in the cache and then notifies them when the item is removed from the cache. It creates a RemovedCallback method, which uses the signature of the CacheItemRemovedCallback delegate, to notify users when the cache item is removed and uses the CacheItemRemovedReason enumeration to tell them why it was removed. In addition, it uses the Cache.Item property to add objects to the cache and retrieve the value of those objects. In the AddItemToCache method, it uses the Cache.Add method to add an item to the cache. To use the CacheItemRemovedCallback delegate, you must add an item to the cache with this method or the Cache.Insert method so that that ASP.NET can automatically make the proper method calls when the item is removed. The custom RemoveItemFromCache method uses the Cache.Remove method to explicitly delete the item from the cache, causing the RemovedCallback method to be invoked.


<%@ Page Language="VB" %>

<html>
 <Script runat=server>
    Shared itemRemoved As boolean = false
    Shared reason As CacheItemRemovedReason
    Dim onRemove As CacheItemRemovedCallback

    Public Sub RemovedCallback(k As String, v As Object, r As CacheItemRemovedReason)
      itemRemoved = true
      reason = r
    End Sub

    Public Sub AddItemToCache(sender As Object, e As EventArgs)
        itemRemoved = false

        onRemove = New CacheItemRemovedCallback(AddressOf Me.RemovedCallback)

        If (IsNothing(Cache("Key1"))) Then
          Cache.Add("Key1", "Value 1", Nothing, DateTime.Now.AddSeconds(60), Cache.NoSlidingExpiration, CacheItemPriority.High, onRemove)
        End If
    End Sub

    Public Sub RemoveItemFromCache(sender As Object, e As EventArgs)
        If (Not IsNothing(Cache("Key1"))) Then
          Cache.Remove("Key1")
        End If
    End Sub
 </Script>

 <body>
  <Form runat="server">
    <input type=submit OnServerClick="AddItemToCache" value="Add Item To Cache" runat="server"/>
    <input type=submit OnServerClick="RemoveItemFromCache" value="Remove Item From Cache" runat="server"/>
  </Form>
<%
If (itemRemoved) Then
    Response.Write("RemovedCallback event raised.")
    Response.Write("<BR>")
    Response.Write("Reason: <B>" + reason.ToString() + "</B>")
Else
    Response.Write("Value of cache key: <B>" + Server.HtmlEncode(CType(Cache("Key1"),String)) + "</B>")
End If
%>
 </body>
</html>


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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