Programming Tutorials

Lifecycle of a web form in ASP.net

By: Dan Hurwitz and Jesse Liberty in Asp.net Tutorials on 2009-02-27  

A user sits at her browser and types in a URL. A web page appears, with text and images and buttons and so forth. She fills in a text box and clicks on a button. What is going on behind the scenes?

Every request made of the web server initiates a sequence of steps. These steps, from beginning to end, constitute the lifecycle of the page.

When a page is requested, it is loaded, processed, sent to the user, and unloaded. From one end of the lifecycle to the other, the goal of the page is to render appropriate HTML and other output back to the requesting browser. At each step, there are methods and events available to let you override the default behavior or add your own programmatic enhancements.

To fully understand the lifecycle of the page and its controls, it is necessary to recognize that the Page class creates a hierarchical tree of all the controls on the page. All the components on the page, except for any Page directives (described shortly), are part of this control tree. You can see the control tree for any page by adding trace="true" to the Page directive.

The Page itself is at the root of the tree. All the named controls are included in the tree, referenced by control ID. Static text, including whitespace, NewLines, and HTML tags, are represented in the tree as LiteralControls. The order of controls in the tree is strictly hierarchical. Within a given hierarchy level, the controls are ordered in the tree using the same sequence in which they appear in the page file.

Web components, including the Page, go through the entire lifecycle every time the page is loaded. (This involves a fair amount of performance overhead, which you can reduce by caching.) Events fire first on the Page, then recursively on every object in the control tree.

The following is a detailed description of each of the phases of the component lifecycle in a web form. There are two slightly different sequences of events in the lifecycle: on the first loading of the page and on subsequent postbacks. 

During the first page load, the lifecycle is composed of the following steps:

  1. Initialization

    The initialization phase is the first phase in the lifecycle for any page or control. The control tree is built during the initialization phase. In this phase, you can initialize any values needed for the duration of the request.

    The initialize phase is modified by handling the Init event with the OnInit method.
  2. Load

    User code runs and the form controls show client-side data.

    The load phase can be modified by handling the Load event with the OnLoad method.
  3. PreRender

    This is the phase just before the output is rendered. CreateChildControls is called, if necessary, to create and initialize server controls in the control tree. Modifications are made via the PreRender event, using the OnPreRender method.
  4. Save ViewState

    The view state is saved to a hidden variable on the page, persisting as a string object that will complete the round trip to the client. This can be overridden using the SaveViewState method.
  5. Render

    The page and its controls are rendered as HTML. You can override using the Render method. Within Render, CreateChildControls is called, if necessary, to create and initialize server controls in the control tree.
  6. Dispose

    This is the last phase of the lifecycle. It gives you an opportunity to do any final cleanup and release references to any expensive resources, such as database connections. This is important for scalability. It can be modified using the Dispose method.

During postback, the lifecycle is:

  1. Initialization

    Same as on first load.
  2. Load ViewState

    The ViewState property of the control is loaded from a hidden variable on the page. You can modify this behavior by overriding the LoadViewState method.
  3. Postback Data is loaded

    During this phase, the data sent to the server via the POST method is processed. Any updates to the view state necessitated by the postback are performed via the LoadPostData method.
  4. Load

    Same as on first load.
  5. Change events are raised

    If there are any state changes between the current state and the previous state, change events are raised via the RaisePostDataChangedEvent method. Again, the events are raised for the controls in the order in which the controls appear in the control tree.
  6. Handle postback events

    Exactly one user action caused the postback. That user action is handled now, after all the change events have been handled. The original client-side event that instigated the postback is handled in the RaisePostBackEvent method.
  7. PreRender

    Same as on first load.
  8. Save ViewState

    Same as on first load.
  9. Render

    Same as on first load.
  10. Dispose

    Same as on first load.





Add Comment

* Required information
1000

Comments

No comments yet. Be the first!

Most Viewed Articles (in Asp.net )

Getting values from appsettings.json ASP.NET

Microsoft.Identity vs Microsoft.IdentityModel.Clients.ActiveDirectory

HttpError is not found in Asp,net core project

Directives in ASP.net

Severity Code Description Project File Line Suppression State Error CS0103 The name 'JsonConvert' does not exist in the current context.

Button that is only clickable when the checkbox is checked

Severity Code Description Project File Line Suppression State Error CS0246 The type or namespace name 'JToken' could not be found.

Downloading and Installing the AWS SDK for .NET

Passing a model globally to all Views in your Asp.net webapp

What is Code-Behind in ASP.net

Things to note when changing a function to async in your controller

Severity Code Description Project File Line Suppression State Error CS1061 'string[]' does not contain a definition for 'Any' and no accessible extension method 'Any' accepting a first argument of type 'string[]' could be found (are you missing a using directive or an assembly reference?)

Pagination in ASP.net core application

Severity Code Description Project File Line Suppression State Warning Found conflicts between different versions of the same dependent assembly.

Severity Code Description Project File Line Suppression State Error CS0103 The name 'Encoding' does not exist in the current context

Latest Articles (in Asp.net)

Things to note when changing a function to async in your controller

AmbiguousMatchException: The request matched multiple endpoints.

Call an Action in a controller when user clicks a button in View

Button that is only clickable when the checkbox is checked

Pass the same model to multiple views within the same controller

Severity Code Description Project File Line Suppression State Error CS0103 The name 'Encoding' does not exist in the current context

Severity Code Description Project File Line Suppression State Error CS0103 The name 'JsonConvert' does not exist in the current context.

Passing a model globally to all Views in your Asp.net webapp

Severity Code Description Project File Line Suppression State Error CS0246 The type or namespace name 'JToken' could not be found.

Severity Code Description Project File Line Suppression State Error CS0308 The non-generic type 'List' cannot be used with type arguments.

One client credential type required either: ClientSecret, Certificate, ClientAssertion or AppTokenProvider must be defined when creating a Confidential Client. Only specify one

Severity Code Description Project File Line Suppression State Warning Found conflicts between different versions of the same dependent assembly.

Severity Code Description Project File Line Suppression State Error CS1061 'string[]' does not contain a definition for 'Any' and no accessible extension method 'Any' accepting a first argument of type 'string[]' could be found (are you missing a using directive or an assembly reference?)

Pagination in ASP.net core application

Microsoft.Identity vs Microsoft.IdentityModel.Clients.ActiveDirectory

Related Tutorials

Things to note when changing a function to async in your controller

AmbiguousMatchException: The request matched multiple endpoints.

Call an Action in a controller when user clicks a button in View

Button that is only clickable when the checkbox is checked

Pass the same model to multiple views within the same controller

Severity Code Description Project File Line Suppression State Error CS0103 The name 'Encoding' does not exist in the current context

Severity Code Description Project File Line Suppression State Error CS0103 The name 'JsonConvert' does not exist in the current context.

Passing a model globally to all Views in your Asp.net webapp

Severity Code Description Project File Line Suppression State Error CS0246 The type or namespace name 'JToken' could not be found.

Severity Code Description Project File Line Suppression State Error CS0308 The non-generic type 'List' cannot be used with type arguments.

One client credential type required either: ClientSecret, Certificate, ClientAssertion or AppTokenProvider must be defined when creating a Confidential Client. Only specify one

Severity Code Description Project File Line Suppression State Warning Found conflicts between different versions of the same dependent assembly.

Severity Code Description Project File Line Suppression State Error CS1061 'string[]' does not contain a definition for 'Any' and no accessible extension method 'Any' accepting a first argument of type 'string[]' could be found (are you missing a using directive or an assembly reference?)

Pagination in ASP.net core application

Microsoft.Identity vs Microsoft.IdentityModel.Clients.ActiveDirectory