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Using MemoryCache in .Net 4.0

November 27, 2011 2 comments

Since ASP.Net first came, it came up with a very powerful feature of in-memory object cache (System.Web.Caching.Cache) to store commonly used expensive data on server side. Almost every ASP.Net application/site uses this feature now. But it suffered from few shortcomings like-

  • It is available in ASP.Net only leaving WinForms or WPF clients puzzled.
  • It is not extensible to accommodate other demands to store cache objects in disk or Sql Server. However, MS after realizing these shortcomings, Caching Application Block library was included in its Enterprise Library.
  • .Net developers have to look for their own mechanisms to create logical in-memory partitions called regions to group or organize cache objects in memory.

However, .Net 4.0 came up with a new set of caching APIs in System.Runtime.Caching namespace that addresses all the above shortcomings. This new namespace can be found in System.Runtime.Caching.dll assembly. But this assembly reference is available only target framework of .Net Framework 4, not .Net Framework 4 Client Profile.

The System.Runtime.Caching namespace contains two core set of classes:

  • Concrete implementation of System.Runtime.Caching.MemoryCache class to support in-memory object cache. MemoryCache is closely modeled after old System.Web.Caching.Cache of ASP.Net. However, one does not have to rely upon System.Web assembly to use MemoryCache (We will see it through an example shortly).
  • Abstract types to build custom cache implementation other than in-built MemoryCache.

Now let’s move to try out an example to leverage MemoryCache features. First of all we will create a class library type project. Add a class MyCache and assembly reference of System.Runtime.caching.dll.

MyCache Code sample:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.Caching;

namespace DotNetCachingWrapper
{
public enum MyCachePriority
{
Default,
NotRemovable
}

public class MyCache
{
// Gets a reference to the default MemoryCache instance.
private static ObjectCache cache = MemoryCache.Default;
private CacheItemPolicy policy = null;
private CacheEntryRemovedCallback callback = null;

public void AddToMyCache(String CacheKeyName, Object CacheItem, MyCachePriority MyCacheItemPriority, List<String> FilePath)
{
//
callback = new CacheEntryRemovedCallback(this.MyCachedItemRemovedCallback);
policy = new CacheItemPolicy();
policy.Priority = (MyCacheItemPriority == MyCachePriority.Default) ? CacheItemPriority.Default : CacheItemPriority.NotRemovable;
policy.AbsoluteExpiration = DateTimeOffset.Now.AddSeconds(10.00);
policy.RemovedCallback = callback;
policy.ChangeMonitors.Add(new HostFileChangeMonitor(FilePath));

// Add inside cache
cache.Set(CacheKeyName, CacheItem, policy);
}

public Object GetMyCachedItem(String CacheKeyName)
{
//
return cache[CacheKeyName] as Object;
}

public void RemoveMyCachedItem(String CacheKeyName)
{
//
if (cache.Contains(CacheKeyName))
{
cache.Remove(CacheKeyName);
}
}

private void MyCachedItemRemovedCallback(CacheEntryRemovedArguments arguments)
{
// Log these values from arguments list
String strLog = String.Concat("Reason: ", arguments.RemovedReason.ToString(), "
| Key-Name: ", arguments.CacheItem.Key, " | Value-Object: ",
arguments.CacheItem.Value.ToString());
}

}
}

Build this class library to generate assembly name of MyCachingWrapper.dll. Now we are ready to use this library in all ASP.Net web, WinForms and WPF applications.

Now let’s see examples of how this library can be commonly used in both ASP.Net Web Form and WinForm after adding reference of MyCachingWrapper.dll.

ASP.Net Page_Load:

protected void Page_Load(object sender, EventArgs e)
{
//
MyCache objCache = new MyCache();
String strUserName = objCache.GetMyCachedItem("USER_NAME") as String;
if (String.IsNullOrEmpty(strUserName))
{
List<String> lstFiles = new List<string>();
lstFiles.Add(HttpContext.Current.Request.MapPath("~/XmlFiles/ListOfUsers.xml"));

XElement x = XElement.Load(HttpContext.Current.Request.MapPath("~/XmlFiles/ListOfUsers.xml"));
var qry = from u in x.Elements("Users") where u.Element("UserCode").Value == "101" select u;
strUserName = qry.First().Element("UserName").Value;

// Add inside cache
objCache.AddToMyCache("USER_NAME", strUserName, MyCachePriority.Default, lstFiles);
}
this.lblUserName.Text = strUserName;
}

WinForm Form_Load:

private void frmDefault_Load(object sender, EventArgs e)
{
//
MyCache objCache = new MyCache();
String strFilePath =System.IO.Path.Combine(Environment.CurrentDirectory, "../../XmlFiles/ListOfUsers.xml");
String strUserName = objCache.GetMyCachedItem("USER_NAME") as String;
if (String.IsNullOrEmpty(strUserName))
{
List<String> lstFiles = new List<string>();
lstFiles.Add(strFilePath);

XElement x = XElement.Load(strFilePath);
var qry = from u in x.Elements("Users") where u.Element("UserCode").Value == "101" select u;
strUserName = qry.First().Element("UserName").Value;

// Add inside cache
objCache.AddToMyCache("USER_NAME", strUserName, MyCachePriority.Default, lstFiles);
}
this.lblUserName.Text = strUserName;
}

Sample ListOfUsers.xml File:

<?xml version="1.0"?>
<ListOfUsers xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance&quot; xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
<Users>
<UserCode>101</UserCode>
<UserName>Dan Brown</UserName>
</Users>
<Users>
<UserCode>102</UserCode>
<UserName>da Vinci</UserName>
</Users>
<Users>
<UserCode>103</UserCode>
<UserName>Monalisa</UserName>
</Users>
<Users>
<UserCode>104</UserCode>
<UserName>Shakespeare</UserName>
</Users>
<Users>
<UserCode>105</UserCode>
<UserName>William Wordsworth</UserName>
</Users>
</ListOfUsers>

Now we can debug above code and see other interesting things. You will see the usage of MemoryCache methods and its techniques to cache objects are similar to what we have been doing in ASP.Net cache so far. Also, we see how the same MyCache library is usable in both web and windows applications.

MemoryCache points worth mentioning:

  • MemoryCache.Default returns the single and same instance of in-memory ObjectCache through a static read-only property.

    public static MemoryCache Default { get; }

  • CacheEntryRemovedCallback signature is different than what we have seen in prior versions of ASP.Net. Refer to example. In the current callback method, one can see the following details when cache item is expired after 10 seconds.

CallbackArgumentsDetails

  • CacheItemPriority enum in .Net 4.0 is cleaner than what we have seen in prior versions of ASP.Net. Now, it is only Default and NotRemovable.
  • In prior version of ASP.Net, CacheDependency was used to monitor changes in any underlying objects like files, Sql database tables, rows, columns, etc. Now .Net 4.0 provides ChangeMonitor class which is ASP.Net neutral and has wider scope of monitoring dependent objects to expire cache items. However, CacheDependency is still there in ASP.Net 4.0. We should use other implementation of ChangeMonitor like HostFileChangeMontor in the above example.
  • The default implementation of MemoryCache does not give us flexibility to add regions along with keys. In order to use regions, you should extend MemoryCache.

Hope you all will appreciate .Net 4.0 caching feature.

Testing and Monitoring WCF Service using soapUI and Fiddler

September 3, 2011 Leave a comment

This article is not about understanding the details of .Net WCF service, soapUI and Fiddler as all these three are quite popular things among .Net developers. And tools like soapUI and Fiddler may already be in the arsenal of many web developers and QA engineers. Those who are new to these two tools should refer to following articles.

However, we will take a look into how Fiddler can be used to Monitor SOAP Request and SOAP Response transmitting behind the soapUI.

Let’s start with a simple WCF service in place. No need to have a WCF .Net Client as of now to test and monitor HTTP traffic of WCF service calls.

1. Open soapUI and set the following settings. Make sure the service’s wsdl path is correct.

Loading wsdl definition

2. After loading the definition of WSDL, service definitions will appear like shown below.

Service requests definitions

3. Double click on Request # node and navigate to the request window on the right.

Submitting the request

4. Fill the method parameters denoted by “?”.

5. Open Fiddler and make sure it is ready to capture HTTP(S) traffic.

6. Submit request to the specified endpoint and SOAP Response can be seen on the right pane.

Service Request-Response in soapUI

7. If you see the Fiddler window, there is no traffic captured. This is really frustrating if you have been testing your services using soapUI when you do not see the underlying details of the SOAP Request and SOAP Response in the wire.

8. All you need to do following proxy settings in the soapUI File –> Preferences –> Proxy Settings window.

soapUI Proxy Settings

9. Re-submit web service request on soapUI.

10. Now you see WCF Request-Response traffic in the Fiddler window Smile. You are now all in your territory to view the details of service request-response headers, body, and many more!

SOAP Request-Response view in Fiddler

But why did we add Port No. 8888 in the proxy settings of soapUI? It is because Fiddler by default listens on port no. 8888.

Fiddler default listening port no.

If you have configured your Fiddler proxy tool to listen on some other port no., then you should use that one.

I hope you enjoyed this testing tip. No need to reiterate how blissful it is to test web services using soapUI and Fiddler if you are a web service developer/provider to remote clients.

ASP.Net 4.0 New Features

ViewStateMode – ViewState for Individual Controls
ASP.Net 4.0 allows view state in a page to be more controllable from page to its child controls level. That is, view state of a control can be enabled or disabled irrespective of its parent control’s view state. Even if view state of a page is disabled, controls of the page can have their own view state individually enabled or disabled or even inherited from the page’s view state mode property.
This property if utilized properly can certainly boost performance of a page.

For example, we can individually enable or disable user control’s view state in a page.

ViewStateMode

By default, ViewStateMode is enabled for a page object, while controls have inherit mode.

Page.MetaKeywords and Page.MetaDescription – SEO Optimization Feature
ASP.Net 4.0 has come up with these two properties that will help developers add meta tags for keywords and description in the aspx pages in easier fashion. Web Search Engines really need these two meta tags for search indexing of any pages. These two properties can be used in a page in various ways. Inside <head> tag or in the code behind or even at <%@Page%> directive level.

However, setting meta keywords or description in code behind will be more useful when we have to add keywords and descriptions dynamically from source like database.

MetaKeyWords is used to store few useful keywords that will briefly highlight important information of a page by tags. From SEO perspective, meta keywords should contain keywords separated by spaces.

MetaDescription is used to add page description in short that will help Search Engines to quickly describe about the page links in search pages.

Prior to ASP.Net 4.0, we have to add meta tags using HtmlMeta control (public class HtmlMeta : HtmlControl) adding into page header as:

protected void Page_Load(object sender, EventArgs e)
{
//
HtmlMeta metakey = new HtmlMeta();
metakey.Name = "keywords";
metakey.Content = "ASP.Net 2.0 3.5";
HtmlMeta metadesc = new HtmlMeta();
metadesc.Name = "description";
metadesc.Content = "ASP.Net 2.0 3.5 Page Description...";
//Add to page header
Page.Header.Controls.Add(metakey);
Page.Header.Controls.Add(metadesc);
}


In ASP.Net 4.0, we can add in many ways.

protected void Page_Load(object sender, EventArgs e)
{
//Adding Page meta tags information
this.Page.MetaKeywords = "ASP.Net 4.0 SEO Meta Tag";
this.Page.MetaDescription = "Serializing and Deserializing Thoughts..";
}

Or,

<head runat="server">
  
<title>Feature: ViewStateMode</title>
<meta name="keywords" content ="ASP.Net 4.0 ViewStateMode"/>
<meta name="description" content="ViewStateMode feature in ASP.Net 4.0" />
</head>

Or inside Page directive,

MetaDescription

Response.RedirectPermanent – Search Engine Friendly Webpage Redirection

In classic ASP or ASP.Net earlier than 4.0, we used to redirect to new pages or links by setting Response.StatusCode to 301 before calling Response.AddHeader method. Now ASP.Net 4.0 has provided Response.RedirectPermanent method to redirect to new pages or links with StatusCode of 301 implicitly set. Search Engines use this 301 code to understand permanent redirection from old pages links.

For example,

Classic ASP method:

<%@ Language=VBScript %>
<%
Response.Status="301 Moved Permanently"
Response.AddHeader "Location","http://www.new-page-url.com/"
%>


ASP.Net method prior to 4.0:

<script runat="server">
private void Page_Load(object sender, System.EventArgs e)
{
Response.Status = "301 Moved Permanently";
Response.AddHeader("Location","http://www.new-page-url.com");
}
</script>


ASP.Net 4.0 method:

Response.RedirectPermanent("http://www.new-page-url.com ");


Web.Config Refactoring – Custom HttpHandlers and HttpModules

Web.config now looks cleaner as most of the settings are controlled from machine.config file as ASP.Net 4.0 is all set to benefit from IIS 7 and IIS 7.5 features. When IIS is set to use .Net 4.0 and Integrated Pipeline mode, <compilation> element holds .Net version attribute. And the traditional <httpHandlers> and <httpModules> section is now shifted out of <system.web> and added inside new section <system.webserver>. All the custom handlers are added inside <handlers>, and all the modules inside <modules> section.

<system.webServer>
<!-- Add the module for Integrated mode applications -->
<modules runAllManagedModulesForAllRequests="true">
<add name="MyModule" type="WebAppModule.MyCustomModule, WebAppModule" />
</modules>
<!-- Add the handler for Integrated mode applications -->
<handlers>
<add name="MyHandler" path="svrtime.tm" verb="GET" 
<type="WebAppModule.MyCustomHandler, WebAppModule"
preCondition="integratedMode" /> </handlers> </system.webServer>


Also,

<system.web>
<compilation debug="true" targetFramework="4.0" />


Interesting point is, when we add custom handlers and modules this way, we do not have to manually configure handlers and modules in IIS again. IIS will automatically refresh itself.

Designing Business Logic Layer: Some Guidelines

Business Logic Layer is a very crucial layer for any data base applications. A timely thought when applied to this layer from the beginning of application layers design can save lots of time and complexity. Software architects divide the software into modules, then different layers, and core functioning layers for important application features. But when actual development work starts, complexity of different layers and modules start crawling into code gradually.
Reason being:
• We try to mix business rules of different modules wishfully
• Writing methods with abundant codes
• Not clearly separating responsibilities of presentation and data access layers
• Creating code duplicity, i.e.; writing same set of code or methods at various places

Results are:
• Difficult to debug
• Difficult to understand the flow
• Difficult to maintain and modify business rules correctly when such rules exist across layers and modules
• Difficult to write Unit Tests

We can avoid these things if we take care of these things when writing codes.
• Write methods that do single meaningful task with one call. Do not mix other code logic with the methods. For example, if we write SavePayment() method, then this method should only focus on save task, and not update or delete or check connection status or read Xml files, etc. This is what we call Single Responsibility Principle.
• Encourage use of factory methods for object creation instead of writing lots of If-else constructs based upon some input type values.
• When you need data or result sets (DTO) of other modules, then preferably call business logic layer methods of that module instead of writing that module code logic into yours. This is quite important aspect for any business logic layer.
• Classes in this layer should be loosely coupled. For this different injection patterns like dependency injection or inversion of control, etc can help. Sometimes even a simple Enum type can come to a great rescue.
• Write business methods that accept valid entity class object or DTO object in business rather than single valued parameters like integer or string or array or even optional params. This ensures business logic layer code function even unmodified when there are database table and entity or DTO class changes in behind.
• Avoid lots of business rules in stored procedures or even in presentation UI.
• Business logic layer methods should not be aware of presentation UI controls’ properties or values. These methods should accept values in integer or string instead.

Let me explain all these points by one example. I worked in an accounting module of a project where customers can make payments of their bills in various ways. They can make either full payment or in-partial or even in installments. For each payment mode, there were different rules and validations. So this module had clear separation of implementation with rules of each mode functioning without depending upon others. This way our lots of coding and debugging time got saved.
Let’s see the code snippets.

Enum showing different Payment Mode

public enum PaymentMode

{
Normal,
Part,
Installment
}

Custome Bill DTO

public class CustomerBillDTO

{
private Int64 intBillNo;
private Int16 intBillMonth;
private Int16 intBillYear;
private double dblBillAmount;
private string strCustomerID;
private Int64 intPayAmount;
//And many other fields…
}

Payment Processor factory class

interface IPaymentProcessorFactory
{
//
IPaymentProcessor GetPaymentProcessor(PaymentMode mode);
}

public class PaymentProcessorFactory : IPaymentProcessorFactory
{
//
private IPaymentProcessor objPaymentProcessor = null;

public IPaymentProcessor GetPaymentProcessor(PaymentMode mode)
{
//
switch (mode)
{
case PaymentMode.Normal:
objPaymentProcessor = new NormalPaymentProcessor();
break;

case PaymentMode.Part:
objPaymentProcessor = new PartPaymentProcessor();
break;
case PaymentMode.Installment:
objPaymentProcessor = new InstallmentPaymentProcessor();
break;
}
return objPaymentProcessor;
}
}

Different Payment Processor class

public interface IPaymentProcessor

{
//
bool SavePayment(CustomerBillDTO Bill);
}

public class NormalPaymentProcessor:IPaymentProcessor
{
//
public bool SavePayment(CustomerBillDTO Bill)
{
return true;
}
}

public class PartPaymentProcessor : IPaymentProcessor
{
//
public bool SavePayment(CustomerBillDTO Bill)
{
return true;
}
}

public class InstallmentPaymentProcessor : IPaymentProcessor
{
//
public bool SavePayment(CustomerBillDTO Bill)
{
return true;
}
}

Main class that processes each payment

class PaymentProcess

{
private IPaymentProcessorFactory objProcessor = null;
public PaymentProcess(IPaymentProcessorFactory Processor)
{
//
this.objProcessor = Processor;
}

public bool ProcessPayment(CustomerBillDTO Bill, PaymentMode mode)
{
//
IPaymentProcessor objPaymentProcessor = this.objProcessor.GetPaymentProcessor(mode);
return objPaymentProcessor.SavePayment(Bill);
}
}

At the calling end, we simply make a generous call as:

private void BtnSave_Click(object sender, EventArgs e)
{
//
PaymentProcessorFactory objFactory = new PaymentProcessorFactory();
PaymentProcess objProcess = new PaymentProcess(objFactory);
bool result = objProcess.ProcessPayment(objCustomerBillDTO(), PaymentMode.Normal);
}

As we see this is how we have clearly separated each logical functioning of a SavePayment() method.
Even in future, if Part or Installment payment mode is stopped, we do not have to modify the code logic to add any If-else construct to branch out or skip any code flows. In case a new payment mode is added, then writing a new XModePaymenetProcessor class, adding one more Enum value and finally one more object instantiation code in factory class will do enough.

Adding or removing any Bill or Customer related fields in CustomerBillDTO do not even pose threat to this business logic layer.

Finally, one should always keep in mind that you write class and class methods for others. So you should be very clear here: what the class should offer and how.

Thanks.

Calling Method in Parent Page from User Control

October 10, 2009 Leave a comment

In ASP.Net, we develop custom user control as a reusable server control independent of any containing parent aspx page. User control has its own public properties, methods, delegates, etc that can be used by parent aspx page. When a user control is embedded or loaded into a page, the page can access public properties, methods, delegates, etc that are in user control. After loading the user control, there a situation may arise like calling methods in page itself. But when a user control is developed, it has no knowledge of containing page. So it becomes a trick to call the page method.

In .Net, Delegate class has one method DynamicInvoke. DynamicInvoke method is used to invoke (late-bound) method referenced by delegate. We can use this method to call a method in parent page from user control. Let’s try with this example.

First create a user control called CustomUserCtrl. Its code will look some thing like this:

public partial class CustomUserCtrl : System.Web.UI.UserControl
{
private System.Delegate _delWithParam;
public Delegate PageMethodWithParamRef
{
set { _delWithParam = value; }
}

private System.Delegate _delNoParam;
public Delegate PageMethodWithNoParamRef
{
set { _delNoParam = value; }
}

protected void Page_Load(object sender, EventArgs e)
{
}

protected void BtnMethodWithParam_Click(object sender, System.EventArgs e)
{
//Parameter to a method is being made ready
object[] obj = new object[1];
obj[0] = "Parameter Value" as object;
_delWithParam.DynamicInvoke(obj);
}

protected void BtnMethowWithoutParam_Click(object sender, System.EventArgs e)
{
//Invoke a method with no parameter
_delNoParam.DynamicInvoke();
}
}

Then add this user control into an aspx page. The code behind of this page is as:

public partial class _Default : System.Web.UI.Page
{
delegate void DelMethodWithParam(string strParam);
delegate void DelMethodWithoutParam();
protected void Page_Load(object sender, EventArgs e)
{
DelMethodWithParam delParam = new DelMethodWithParam(MethodWithParam);
//Set method reference to a user control delegate
this.UserCtrl.PageMethodWithParamRef = delParam;
DelMethodWithoutParam delNoParam = new DelMethodWithoutParam(MethodWithNoParam);
//Set method reference to a user control delegate
this.UserCtrl.PageMethodWithNoParamRef = delNoParam;
}

private void MethodWithParam(string strParam)
{
Response.Write("<br/>It has parameter: " + strParam);
}

private void MethodWithNoParam()
{
Response.Write("<br/>It has no parameter.");
}
}

BtnMethodWithParam and BtnMethowWithoutParam are two different buttons on the user control that are invoking the methods in the parent page. On Page_Load of the page, we are setting the references of page class methods to delegate type properties in the user control. Click different buttons of user control, you will see MethodWithParam(string strParam) and MethodWithNoParam() methods called.

This is all we have to do to call page class methods from user control in ASP.Net.

Load ASP.Net User Control Dynamically Using jQuery

October 10, 2009 5 comments

Today we will explore the way of loading ASP.Net user control at run time using jQuery. jQuery has one method load(fn) that will help here. This load(fn) method has following definition.

load (url, data, callback): A GET request will be performed by default – but if any extra parameters are passed, then a POST will occur.
url (string): URL of the required page
data (map – key/value pair): key value pair data that will be sent to the server
callback (callback method): call back method, not necessarily success

Now comes custom HttpHandler that will load the required user control from the URL given by this load(fn) method. We all know that it is either in-built or custom HttpHandler that is the end point for any request made in ASP.Net.

Let’s see by example. In the ASP.Net application, add one aspx page and user control. Then, add one more class derived from IHttpHandler. The aspx html markup will look something like this.

<html xmlns=”http://www.w3.org/1999/xhtml&#8221; >
<head runat=”server”>
<title>Load ASP.Net User Control</title>
<script src=”jquery-1.2.6.js”></script>
<script>
$(document).ready(function() {
$(“#BtnLoadUserCtrl”).click(function() {
$(“#UserCtrl”).load(“SampleUserCtrl.ascx”);
});
});
</script>
</head>
<body>
<form runat=”server”>
<div>
<br />
<input value=”Load User Control” /> <br />
<div id=”UserCtrl”></div>
</div>
</form>
</body>
</html>

The code is quite readable. On the click event of BtnLoadUserCtrl button, SampleUserCtrl.ascx user control is being tried to load in the <div> element having id UserCtrl.

Then, write our custom Httphandler called jQueryHandler as below.

public class jQueryHandler:IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
// We add control in Page tree collection
using(var dummyPage = new Page())
{
dummyPage.Controls.Add(GetControl(context));
context.Server.Execute(dummyPage, context.Response.Output, true);
}
}

private Control GetControl(HttpContext context)
{
// URL path given by load(fn) method on click of button
string strPath = context.Request.Url.LocalPath;
UserControl userctrl = null;
using(var dummyPage = new Page())
{
userctrl = dummyPage.LoadControl(strPath) as UserControl;
}
// Loaded user control is returned
return userctrl;
}

public bool IsReusable
{
get { return true; }
}
}

Do not miss to add this HttpHandler in the web.config.

<httpHandlers>
<add verb=”*” path=”*.ascx” type=”JQUserControl.jQueryHandler, JQUserControl”/>
</httpHandlers>

This web.config configuration tells that jQueryHandler will process request for file type having .ascx extension and methods all (GET, POST, etc). The type attribute value is something like:
type=”Namespace.TypeName, Assembly name where Handler can be found”

Now we are ready to test our sample. Run the page, and see on the click of button, the sampleusertCtrl.ascx is loaded.

I hope we can now extend this concept to fit any such programming requirement in future.
Happy Coding!

CacheItemRemovedCallback Example in ASP.Net

September 20, 2009 Leave a comment

Notify When an Item is Removed from Cache in ASP.Net

While adding or inserting an item into cache object, we add dependency object as well to ensure the cache is automatically invalidated if any change is detected in that dependent object like file, for example. Then, we again read or update that item from the original source to make sure the item is still fresh with data. This is one of the main reasons that really appeal the use of ASP.Net caching feature where one can decide about dependencies and expiry time policy. There are other properties also that can be used in combination to set the scope of cached object within time frame and location. See also HttpCacheability .

But today we will explore the CacheItemRemovedCallback delegate provided by ASP.Net. It is used to notify the application about cache removal or deletion with some reason. CacheItemRemovedReason enumeration is used as a parameter in call back method to tell the appropriate reason of removal.

Let’s take an example to know more about the CacheItemRemovedCallback delegate.

protected void Page_Load(object sender, EventArgs e)
{
//Fetch item list from cache
ArrayList cacheditems = CachedItemList();
}

private static CacheItemRemovedCallback OnCachedItemRemoved = null;
private ArrayList CachedItemList()
{
//
OnCachedItemRemoved = new CacheItemRemovedCallback(this.CachedItemRemovedCallback);
ArrayList cacheditems = HttpContext.Current.Cache.Get("CACHE_KEY") as ArrayList;

// Found in cache
if (cacheditems != null)
{
return cacheditems;
}
else
{
// Not found in cache
cacheditems = ItemList();
HttpContext.Current.Cache.Insert("CACHE_KEY", cacheditems, new System.Web.Caching.CacheDependency(Server.MapPath("~//CacheDependentFile.txt")), Cache.NoAbsoluteExpiration, Cache.NoSlidingExpiration, CacheItemPriority.Default, OnCachedItemRemoved);

return cacheditems;
}
}

private void CachedItemRemovedCallback(string key, Object val, CacheItemRemovedReason reason)
{
//
if (reason == CacheItemRemovedReason.DependencyChanged)
{
// Log the cache key name, reason and time details
// when the cached object was removed from the cache

}
}

private ArrayList ItemList()
{
//
ArrayList lst = new ArrayList();
lst.Add("First Item");
lst.Add("Second Item");
lst.Add("Third Item");
lst.Add("Fourth Item");
lst.Add("Fifth Item");
return lst;
}

Let’s see carefully this call back method:

private void CachedItemRemovedCallback(string key, Object val, CacheItemRemovedReason reason)
{
//

}

The first parameter specifies the cache key name that we used to store an item (an ArrayList collection values). Second parameter is the object that we stored in the cache. The third parameter is an enumeration which has enum values like Removed, Expired, Underused, and DependencyChanged.

In the above example, if any change is made in CacheDependentFile.txt file, the call back method is automatically fired, and the reason captured will be DependencyChanged. Try and see.

Important Point: When using a CacheItemRemovedCallback make sure that you make the callback method ("OnCachedItemRemoved " in the sample above) a static method.

This feature can be used in many cases like logging the reason why any cached item was removed from the cache, and many others depending upon the scenario.