Friday, July 06, 2007

Application State and Global Variables in ASP.NET

Application State

There are occasions when you need to define global variables in an application that are available to all modules of that application. In ASP.NET, it can be done by application state. You can directly add and remove items to the application state. The HttpApplicationState object represents the application state in ASP.NET. The HttpApplicationState object is created when a client requests any URL resource from within an ASP.NET application virtual directory. The reference to this object is exposed through the Application object.
The properties of HttpApplicationState class are listed in Table 1.
Table 1. The HttpApplicationState class properties
Property Description
AllKeys Gets the access keys available in an application state collection
Contents Gets the reference to an application state object
Count Total number of objects in an application state collection
Item Gets an item from the collection
StaticObjects Gets objects declared by <object> tag
The HttpApplicationState class provides methods to add, remove, lock and unlock items. Table 2 describes the HttpApplicationState class methods.
Table 2. The HttpApplicationState class methods
Method Description
Add Adds a new object to application state
Clear Removes all objects from an application state
Get Gets an application state object by name or index.
GetKey Gets an application state object name by index
Lock Locks an application state variable and make sure variable is not accessible by other modules of an application until it is unlocked.
Unlock Unlocks a locked variable
Remove Removes an object from the collection
RemoveAll Removes all objects from a collection
RemoveAt Removes an object from the collection by using an index
RemoveAt Updates the value of an object in a collection
              
Adding, Updating, and Removing Items to an Application State
As we saw in Table 2, the HttpApplicationState class provides methods to add and remove items to application state. It also provides Get and Set methods that can be used to get an object and update the values of an object.
The application state object is accessed through the Application object. The Application object is available in all pages or all modules of an application.
OK, now let's add an item to application state. You can either use the Add method or direct pass the item and its values. The following code snippet adds two items to application state:
Application("ConnectionString") = connectionString
Application("Title") = "Job Board"
Once an item is added to an item state, it is available to all pages and modules of the application. You can remove an item from an application state by using the Remove method of Application. The following code snippet removes the Title item:
Application.Remove("Title") 
You can also use the Clear or RemoveAll method to remove all items from an application state. The following code snippet removes all items from an application state:
Application.RemoveAll()
Caution: Use application state to store variable cautiously. Since variables are defined globally, these variables are available in the memory until the application closes. Using large objects such as lists, images, and large data may lead to some serious problems. Also use synchronization techniques to avoid any deadlock situations and maintain the integrity of data. Synchronization technique for application state is discussed in the following section.
Locking and Unlocking Application State Items

All items added to the application state are available to all modules and pages of an application. There may be cases when an item can be accessed and updated by multiple users simultaneously, which may lead to data integrity violation. Even using complex objects and updating simultaneously may cause serious damage to the data and may lead to deadlocks or access violations. For example, say a Web site has a counter, which advances when a new users visits the site. Now say current value of counter is 100 and 2 users visit the site simultaneously. In this case each user will have 101.
The HttpApplicationState class provides Lock and Unlock methods that provides a way to resolve this situation. The Lock method locks an item and a locked item won't be available to other users until an item is not unlocked. The following code snippet locks a counter, updates its value and unlocks it when a user is done updating it.
Application.Lock()
Application("counter") += 1
Application.UnLock()
Caution: Locking and Unlocking application state items may degrade the performance of an application.


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