The ASP.NET Page Life Cycle

Each time a request arrives at a Web server for an ASP.NET Web page, the first thing the Web server does is hand off the request to the ASP.NET engine. The ASP.NET engine then takes the request through a pipeline composed of numerous stages, which includes verifying file access rights for the ASP.NET Web page, resurrecting the user's session state, and so on. At the end of the pipeline, a class corresponding to the requested ASP.NET Web page is instantiated and the ProcessRequest() method is invoked (see Figure 1).

Figure 1. ASP.NET Page Handling

This life cycle of the ASP.NET page starts with a call to the ProcessRequest() method. This method begins by initializing the page's control hierarchy. Next, the page and its server controls proceed lock-step through various phases that are essential to executing an ASP.NET Web page. These steps include managing view state, handling postback events, and rendering the page's HTML markup. Figure 2 provides a graphical representation of the ASP.NET page life cycle. The life cycle ends by handing off the Web page's HTML markup to the Web server, which sends it back to the client that requested the page.

Note   A detailed discussion of the steps leading up to the ASP.NET page life cycle is beyond the scope of this article. For more information read Michele Leroux-Bustamante's Inside IIS & ASP.NET. For a more detailed look at HTTP handlers, which are the endpoints of the ASP.NET pipeline, check out my previous article onHTTP Handlers.

What is important to realize is that each and every time an ASP.NET Web page is requested it goes through these same life cycle stages (shown in Figure 2).

Figure 2. Events in the Page Life Cycle

Stage 0 - Instantiation

The life cycle of the ASP.NET page begins with instantiation of the class that represents the requested ASP.NET Web page, but how is this class created? Where is it stored?

ASP.NET Web pages, as you know, are made up of both an HTML portion and a code portion, with the HTML portion containing HTML markup and Web control syntax. The ASP.NET engine converts the HTML portion from its free-form text representation into a series of programmatically-created Web controls.

When an ASP.NET Web page is visited for the first time after a change has been made to the HTML markup or Web control syntax in the .aspx page, the ASP.NET engine auto-generates a class. If you created your ASP.NET Web page using the code-behind technique, this autogenerated class is derived from the page's associated code-behind class (note that the code-behind class must be derived itself, either directly or indirectly, from the System.Web.UI.Page class); if you created your page with an in-line, server-side<script> block, the class derives directly from System.Web.UI.Page. In either case, this autogenerated class, along with a compiled instance of the class, is stored in theWINDOWS\Microsoft.NET\Framework\version\Temporary ASP.NET Files folder, in part so that it doesn't need to be recreated for each page request.

The purpose of this autogenerated class is to programmatically create the page's control hierarchy. That is, the class is responsible for programmatically creating the Web controls specified in the page's HTML portion. This is done by translating the Web control syntax—<asp:WebControlName Prop1="Value1" ... />—into the class's programming language (C# or Microsoft® Visual Basic® .NET, most typically). In addition to the Web control syntax being converted into the appropriate code, the HTML markup present in the ASP.NET Web page's HTML portion is translated to Literal controls.

All ASP.NET server controls can have a parent control, along with a variable number of child controls. The System.Web.UI.Page class is derived from the base control class (System.Web.UI.Control), and therefore also can have a set of child controls. The top-level controls declared in an ASP.NET Web page's HTML portion are the direct children of the autogenerated Page class. Web controls can also be nested inside one another. For example, most ASP.NET Web pages contain a single server-side Web Form, with multiple Web controls inside the Web Form. The Web Form is an HTML control (System.Web.UI.HtmlControls.HtmlForm). Those Web controls inside the Web Form are children of the Web Form.

Since server controls can have children, and each of their children may have children, and so on, a control and its descendents form a tree of controls. This tree of controls is called the control hierarchy. The root of the control hierarchy for an ASP.NET Web page is the Page-derived class that is autogenerated by the ASP.NET engine.

Whew! Those last few paragraphs may have been a bit confusing, as this is not the easiest subject to discuss or digest. To clear out any potential confusion, let's look at a quick example. Imagine you have an ASP.NET Web page with the following HTML portion:

<html>
<body>
  <h1>Welcome to my Homepage!</h1>
  <form runat="server">
    What is your name?
    <asp:TextBox runat="server" ID="txtName"></asp:TextBox>
    <br />What is your gender?
    <asp:DropDownList runat="server" ID="ddlGender">
      <asp:ListItem Select="True" Value="M">Male</asp:ListItem>
      <asp:ListItem Value="F">Female</asp:ListItem>
      <asp:ListItem Value="U">Undecided</asp:ListItem>
    </asp:DropDownList>
    <br />
    <asp:Button runat="server" Text="Submit!"></asp:Button>
  </form>
</body>
</html>

When this page is first visited, a class will be autogenerated that contains code to programmatically build up the control hierarchy. The control hierarchy for this example can be seen in Figure 3.

Figure 3. Control Hierarchy for sample page

This control hierarchy is then converted to code that is similar to the following:

Page.Controls.Add( 
  new LiteralControl(@"<html>\r\n<body>\r\n
    <h1>Welcome to my Homepage!</h1>\r\n"));
HtmlForm Form1 = new HtmlForm();
Form1.ID = "Form1";
Form1.Method = "post";
Form1.Controls.Add(
  new LiteralControl("\r\nWhat is your name?\r\n"));
TextBox TextBox1 = new TextBox();
TextBox1.ID = "txtName";
Form1.Controls.Add(TextBox1);
Form1.Controls.Add(
  new LiteralControl("\r\n<br />What is your gender?\r\n"));
DropDownList DropDownList1 = new DropDownList();
DropDownList1.ID = "ddlGender";
ListItem ListItem1 = new ListItem();
ListItem1.Selected = true;
ListItem1.Value = "M";
ListItem1.Text = "Male";
DropDownList1.Items.Add(ListItem1);
ListItem ListItem2 = new ListItem();
ListItem2.Value = "F";
ListItem2.Text = "Female";
DropDownList1.Items.Add(ListItem2);
ListItem ListItem3 = new ListItem();
ListItem3.Value = "U";
ListItem3.Text = "Undecided";
DropDownList1.Items.Add(ListItem3);
Form1.Controls.Add(
  new LiteralControl("\r\n<br /> \r\n"));
Button Button1 = new Button();
Button1.Text = "Submit!";
Form1.Controls.Add(Button1);
Form1.Controls.Add(
  new LiteralControl("\r\n</body>\r\n</html>"));
Controls.Add(Form1);

Note   The C# source code above is not the precise code that is autogenerated by the ASP.NET engine. Rather, it's a cleaner and easier to read version of the autogenerated code. To see the full autogenerated code—which won't win any points for readability—navigate to the

WINDOWS\Microsoft.NET\Framework\Version\Temporary ASP.NET Files

folder and open one of the

.cs

or

.vb

files.

One thing to notice is that, when the control hierarchy is constructed, the properties that are explicitly set in the declarative syntax of the Web control are assigned in the code. (For example, the Button Web control has its Text property set to "Submit!" in the declarative syntax – Text="Submit!" – as well as in the autogenerated class—Button1.Text = "Submit!";.

Stage 1 - Initialization

After the control hierarchy has been built, the Page, along with all of the controls in its control hierarchy, enter the initialization stage. This stage is marked by having the Page and controls fire their Init events. At this point in the page life cycle, the control hierarchy has been constructed, and the Web control properties that are specified in the declarative syntax have been assigned.

We'll look at the initialization stage in more detail later in this article. With regards to view state it is important for two reasons; first, server controls don't begin tracking view state changes until right at the end of the initialization stage. Second, when adding dynamic controls that need to utilize view state, these controls will need to be added during thePage's Init event as opposed to the Load event, as we'll see shortly.

Stage 2 - Load View State

The load view state stage only happens when the page has been posted back. During this stage, the view state data that had been saved from the previous page visit is loaded and recursively populated into the control hierarchy of the Page. It is during this stage that the view state is validated. As we'll discuss later in this article, the view state can become invalid due to a number of reasons, such as view state tampering, and injecting dynamic controls into the middle of the control hierarchy.

Stage 3 - Load Postback Data

The load postback data stage also only happens when the page has been posted back. A server control can indicate that it is interested in examining the posted back data by implementing the IPostBackDataHandler interface. In this stage in the page life cycle, the Page class enumerates the posted back form fields, and searches for the corresponding server control. If it finds the control, it checks to see if the control implements the IPostBackDataHandler interface. If it does, it hands off the appropriate postback data to the server control by calling the control's LoadPostData() method. The server control would then update its state based on this postback data.

To help clarify things, let's look at a simple example. One nice thing about ASP.NET is that the Web controls in a Web Form remember their values across postback. That is, if you have a TextBox Web control on a page and the user enters some value into the TextBox and posts back the page, the TextBox's Text property is automatically updated to the user's entered value. This happens because the TextBox Web control implements the IPostBackDataHandler interface, and the Page class hands off the appropriate value to the TextBox class, which then updates its Text property.

To concretize things, imagine that we have an ASP.NET Web page with a TextBox whose ID property is set to txtName. When the page is first visited, the following HTML will be rendered for the TextBox: <input type="text" id="txtName" name="txtName" />. When the user enters a value into this TextBox (such as, "Hello, World!") and submits the form, the browser will make a request to the same ASP.NET Web page, passing the form field values back in the HTTP POST headers. These include the hidden form field values (such as __VIEWSTATE), along with the value from the txtName TextBox.

When the ASP.NET Web page is posted back in the load postback data stage, the Page class sees that one of the posted back form fields corresponds to theIPostBackDataHandler interface. There is such a control in the hierarchy, so the TextBox's LoadPostData() method is invoked, passing in the value the user entered into the TextBox ("Hello, World!"). The TextBox's LoadPostData() method simply assigns this passed in value to its Text property.

Notice that in our discussion on the load postback data stage, there was no mention of view state. You might naturally be wondering, therefore, why I bothered to mention the load postback data stage in an article about view state. The reason is to note the absence of view state in this stage. It is a common misconception among developers that view state is somehow responsible for having TextBoxes, CheckBoxes, DropDownLists, and other Web controls remember their values across postback. This is not the case, as the values are identified via posted back form field values, and assigned in the LoadPostData() method for those controls that implement IPostBackDataHandler.

Stage 4 - Load

This is the stage with which all ASP.NET developers are familiar, as we've all created an event handler for a page's Load event (Page_Load). When the Load event fires, the view state has been loaded (from stage 2, Load View State), along with the postback data (from stage 3, Load Postback Data). If the page has been posted back, when the Load event fires we know that the page has been restored to its state from the previous page visit.

Stage 5 - Raise Postback Event

Certain server controls raise events with respect to changes that occurred between postbacks. For example, the DropDownList Web control has a SelectedIndexChanged event, which fires if the DropDownList's SelectedIndex has changed from the SelectedIndex value in the previous page load. Another example: if the Web Form was posted back due to a Button Web control being clicked, the Button's Click event is fired during this stage.

There are two flavors of postback events. The first is a changed event. This event fires when some piece of data is changed between postbacks. An example is the DropDownListsSelectedIndexChanged event, or the TextBox's TextChanged event. Server controls that provide changed events must implement the IPostBackDataHandler interface. The other flavor of postback events is the raised event. These are events that are raised by the server control for whatever reason the control sees fit. For example, the Button Web control raises the Click event when it is clicked, and the Calendar control raises the VisibleMonthChanged event when the user moves to another month. Controls that fire raised events must implement the IPostBackEventHandler interface.

Since this stage inspects postback data to determine if any events need to be raised, the stage only occurs when the page has been posted back. As with the load postback data stage, the raise postback event stage does not use view state information at all. Whether or not an event is raised depends on the data posted back in the form fields.

Stage 6 - Save View State

In the save view state stage, the Page class constructs the page's view state, which represents the state that must persist across postbacks. The page accomplishes this by recursively calling the SaveViewState() method of the controls in its control hierarchy. This combined, saved state is then serialized into a base-64 encoded string. In stage 7, when the page's Web Form is rendered, the view state is persisted in the page as a hidden form field.

Stage 7 - Render

In the render stage the HTML that is emitted to the client requesting the page is generated. The Page class accomplishes this by recursively invoking the RenderControl() method of each of the controls in its hierarchy.

These seven stages are the most important stages with respect to understanding view state. (Note that I did omit a couple of stages, such as the PreRender and Unload stages.) As you continue through the article, keep in mind that every single time an ASP.NET Web page is requested, it proceeds through these series of stages.