Response.Cache.SetCacheability in Visual Studio .NET

Painting QR Code in Visual Studio .NET Response.Cache.SetCacheability

Some pages may not be eligible for caching because they have simple elements that must be dynamically generated. As an alternative to creating separate user controls for the dynamic element and configuring different caching policy for those user controls, you can use substitution. ASP.NET provides two cache substitution techniques:

You add static placeholders to your page in places where dynamic content is required, and then use the Response.WriteSubstitution method to specify a method that replaces portions of a cached page with dynamically generated content. To specify the substitution method, call WriteSubstitution and pass a callback method with an HttpResponseSubstitutionCallback signature.

You can t use substitution to update cached user controls where output caching is applied at the user control level.

Substitution controls are similar to Label controls, but Substitution controls are exempt from output caching. The only useful property is Substitution.MethodName, which you use to specify the method that generates the content that is inserted at the location of the Substitution control. The method specified by MethodName must accept an HttpContext parameter and return a String. The String value is inserted into the response at the Substitution control location when the cached page is returned to the user. The following code demonstrates how to specify a substitution method that displays the cur rent time in a Substitution control named Substitution1:

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'VB Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) ' Specify the callback method. Substitution1.MethodName = "GetCurrentDateTime" End Sub

Return DateTime.Now.ToString()

//C# void Page_Load(object sender, System.EventArgs e) {

Substitution1.MethodName = "GetCurrentDateTime";

public static string GetCurrentDateTime (HttpContext context)

return DateTime.Now.ToString();

The AdRotator control also performs post-cache substitution, by default, in order to constantly display new ads.

Often, you want to cache pages, but specific events might require you to stop using the cached page. For example, a page that displays results from a database query should only be cached until the results of the database query change. Similarly, a page that processes a file should be cached until the file is changed. Fortunately, ASP.NET gives you several ways to invalidate cached pages. The sections that follow describe how to make caching choices before returning a page and how to create a cache page output file dependency.

Determining Whether to Return a Cached Page Prior to Rendering To directly con trol whether a cached version of a page is used or whether the page is dynamically regenerated, respond to the ValidateCacheOutput event and set a valid for the HttpValidationStatus attribute. Then, from the Page.Load event handler, call the AddValidationCallback method and pass an HttpCacheValidateHandler object with your method.

The following example demonstrates how to create a method to handle the ValidatePage event:

'VB Public Shared Sub ValidatePage(ByVal context As HttpContext, _ ByVal data As [Object], ByRef status As HttpValidationStatus) If Not (context.Request.QueryString("Status") Is Nothing) Then Dim pageStatus As String = context.Request.QueryString("Status") If pageStatus = "invalid" Then status = HttpValidationStatus.Invalid ElseIf pageStatus = "ignore" Then status = HttpValidationStatus.IgnoreThisRequest Else status = HttpValidationStatus.Valid End If Else status = HttpValidationStatus.Valid End If End Sub //C# public static void ValidateCacheOutput(HttpContext context, Object data, ref HttpValidationStatus status) { if (context.Request.QueryString["Status"] != null) { string pageStatus = context.Request.QueryString["Status"]; if (pageStatus == "invalid") status = HttpValidationStatus.Invalid; else if (pageStatus == "ignore") status = HttpValidationStatus.IgnoreThisRequest; else status = HttpValidationStatus.Valid; } else status = HttpValidationStatus.Valid; }

Notice that this code sample uses logic to specify one of the HttpValidationStatus val ues to control how the page is cached:

Causes the cache to be invalidated so that the page is dynamically generated. The newly generated page is stored in the cache, replacing the earlier cached version.

Causes the current page request to be dynamically generated without invalidating the previously cached version of the page. The dynamically generated page output is not cached, and future requests