HttpCachePolicy.AddValidationCallback Method (HttpCacheValidateHandler, Object)

 

Registers a validation callback for the current response.

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

Public Sub AddValidationCallback (
	handler As HttpCacheValidateHandler,
	data As Object
)

Parameters

handler
Type: System.Web.HttpCacheValidateHandler

The HttpCacheValidateHandler value.

data
Type: System.Object

The arbitrary user-supplied data that is passed back to the AddValidationCallback delegate.

Exception Condition
ArgumentNullException

The specified handler is null.

TheAddValidationCallback method provides a mechanism to check the response programmatically in the cache before the response is returned to the client by the output cache.

Before the response is served from the Web server cache, all registered handlers are queried to ensure resource validity. If any handler sets a flag indicating that the cached response is not valid, the entry is marked as not valid and expelled from the cache. In this case, as well as when any handler indicates that the cached response should be ignored for this request, the request is then handled as if it were a cache miss.

AddValidationCallback is introduced in the .NET Framework version 3.5. For more information, see .NET Framework Versions and Dependencies.

The following code example demonstrates how to add a delegate to validate a request based on query string values.

<%@ Page Language="VB" %>
<%@ OutputCache VaryByParam="none" Duration="600" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
   shared validationstate As String

   Public Sub Page_Load(sender As Object, e As EventArgs)
      Response.Cache.AddValidationCallback(new HttpCacheValidateHandler(AddressOf Me.ValidateCache), nothing)
      stamp.InnerHtml = DateTime.Now.ToString("r")
   End Sub

   Public Shared Sub ValidateCache(context As HttpContext, data As Object, ByRef status as HttpValidationStatus)
      If (context.Request.QueryString("Valid") = "false") Then
         status = HttpValidationStatus.Invalid
      Elseif (context.Request.QueryString("Valid") = "ignore") Then
         status = HttpValidationStatus.IgnoreThisRequest
      Else
         status = HttpValidationStatus.Valid
      End If
   End Sub

</script>

.NET Framework
Available since 1.1
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