Interview Questions on dot net cache

What is an application object ?

Application object can be used in situation where we want data to be shared across users globally.

What’s the difference between Cache object and application object ?

The main difference between the Cache and Application objects is that the Cache object provides cache-specific features, such as dependencies and expiration policies.

How can get access to cache object ?

The Cache object is defined in the System.Web.Caching namespace. You can get a reference to the Cache object by using the Cache property of the HttpContext class in the System.Web namespace or by using the Cache property of the Page object.

What are dependencies in cache and types of dependencies ?

When you add an item to the cache, you can define dependency relationships that can force that item to be removed from the cache under specific activities of dependencies.Example if the cache object is dependent on file and when the file data changes you want the cache object to be update. Following are the supported dependency :

File dependency : Allows you to invalidate a specific cache item when a disk based file or files change.

Time-based expiration : Allows you to invalidate a specific cache item depending on predefined time.

Key dependency :Allows you to invalidate a specific cache item depending when another cached item changes.


SAMPLE CODE

Public Sub displayAnnouncement()

Dim announcement As String

If Cache(“announcement”) Is Nothing Then

Dim file As New _
System.IO.StreamReader _
(Server.MapPath(“announcement.txt”))
announcement = file.ReadToEnd
file.Close()
Dim depends As New _
System.Web.Caching.CacheDependency _
(Server.MapPath(“announcement.txt”))
Cache.Insert(“announcement”, announcement, depends)

End If
Response.Write(CType(Cache(“announcement”), String))
End Sub

Private Sub Page_Init(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Init
displayAnnouncement()

End Sub
End Class

Above given method displayAnnouncement() displays banner text from Announcement.txt
file which is lying in application path of the web directory. Above method first checks whether the Cache object is nothing, if the cache object is nothing then it moves further to load the cache data from the file. Whenever the file data changes the cache object is removed and set to nothing.
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