When an ASP.NET page runs, the page
goes through a life cycle in which it performs a series of processing
steps. These include initialization, instantiating controls, restoring
and maintaining state, running event handler code, and rendering. The
following are the various stages or events of ASP.Net page life cycle.
PreInit
- Check the IsPostBack property to determine whether this is the first time the page is being processed.
- Create or re-create dynamic controls.
- Set a master page dynamically.
- Set the Theme property dynamically.
Note: If
the request is a postback then the values of the controls have not yet
been restored from the view state. If you set a control property at this
stage, its value might be overwritten in the next event.
Init
- This event fires after each control has been initialized.
- Each control's UniqueID is set and any skin settings have been applied.
- Use this event to read or initialize control properties.
- The
"Init" event is fired first for the bottom-most control in the
hierarchy, and then fired up the hierarchy until it is fired for the
page itself.
InitComplete
- Until
now the viewstate values are not yet loaded, hence you can use this
event to make changes to the view state that you want to ensure are
persisted after the next postback.
- Raised by the Page object.
- Use this event for processing tasks that require all initialization to be complete.
OnPreLoad
- Raised
after the page loads view state for itself and all controls, and after
it processes postback data that is included with the Request instance.
- Before
the Page instance raises this event, it loads view state for itself and
all controls, and then processes any postback data included with the
Request instance.
- Loads ViewState: ViewState data are loaded to controls.
- Loads Postback data: Postback data are now handed to the page controls.
Load
- The
Page object calls the OnLoad method on the Page object, and then
recursively does the same for each child control until the page and all
controls are loaded. The Load event of individual controls occurs after
the Load event of the page.
- This is the first place in the page lifecycle that all values are restored.
- Most code checks the value of IsPostBack to avoid unnecessarily resetting state.
- You may also call Validate and check the value of IsValid in this method.
- You can also create dynamic controls in this method.
- Use the OnLoad event method to set properties in controls and establish database connections.
Control PostBack Event(s)
- ASP.NET now calls any events on the page or its controls that caused the PostBack to occur.
- Use
these events to handle specific control events, such as a Button
control's Click event or a TextBox control's TextChanged event.
- In
a postback request, if the page contains validator controls, check the
IsValid property of the Page and of individual validation controls
before performing any processing.
- This is just an example of a control event. Here it is the button click event that caused the postback.
LoadComplete
- Raised at the end of the event-handling stage.
- Use this event for tasks that require that all other controls on the page be loaded.
OnPreRender
- Raised
after the Page object has created all controls that are required in
order to render the page, including child controls of composite
controls.
- The Page object
raises the PreRender event on the Page object, and then recursively
does the same for each child control. The PreRender event of individual
controls occurs after the PreRender event of the page.
- The PreRender event of individual controls occurs after the PreRender event of the page.
- Allows final changes to the page or its control.
- This event takes place before saving ViewState, so any changes made here are saved.
- For example: After this event, you cannot change any property of a button or change any viewstate value.
- Each data bound control whose DataSourceID property is set calls its DataBind method.
- Use the event to make final changes to the contents of the page or its controls.
OnSaveStateComplete
- Raised after view state and control state have been saved for the page and for all controls.
- Before this event occurs, ViewState has been saved for the page and for all controls.
- Any changes to the page or controls at this point will be ignored.
- Use this event perform tasks that require the view state to be saved, but that do not make any changes to controls.
Render Method
- This is a method of the page object and its controls (and not an event).
- The
Render method generates the client-side HTML, Dynamic Hypertext Markup
Language (DHTML), and script that are necessary to properly display a
control at the browser.
UnLoad
- This event is used for cleanup code.
- At this point, all processing has occurred and it is safe to dispose of any remaining objects, including the Page object.
- Cleanup can be performed on:
- Instances of classes, in other words objects
- Closing database connections.
- This event occurs for each control and then for the page.
- During
the unload stage, the page and its controls have been rendered, so you
cannot make further changes to the response stream.
- If you attempt to call a method such as the Response.Write method then the page will throw an exception.
EXAMPLES
Example 1: Control Values
In
the following code, I have assigned the values to the label control on
each event. When you run the code, you will see that in the
"Page_UnLoad", the values are not assigned to the label. WHY? Because,
during the unload stage, the page and its controls have been rendered,
so you cannot change the values.
Please observe the code comments and output. It will help you to clearly understand the concepts.
public partial class PageLiftCycle : System.Web.UI.Page
{
protected void Page_PreInit(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "PreInit";
}
protected void Page_Init(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "Init";
}
protected void Page_InitComplete(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "InitComplete";
}
protected override void OnPreLoad(EventArgs e)
{
//Work and It will assign the values to label.
//If the page is post back, then label contrl values will be loaded from view state.
//E.g: If you string str = lblName.Text, then str will contain viewstate values.
lblName.Text = lblName.Text + "<br/>" + "PreLoad";
}
protected void Page_Load(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "Load";
}
protected void btnSubmit_Click(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "btnSubmit_Click";
}
protected void Page_LoadComplete(object sender, EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "LoadComplete";
}
protected override void OnPreRender(EventArgs e)
{
//Work and It will assign the values to label.
lblName.Text = lblName.Text + "<br/>" + "PreRender";
}
protected override void OnSaveStateComplete(EventArgs e)
{
//Work and It will assign the values to label.
//But "SaveStateComplete" values will not be available during post back. i.e. View state.
lblName.Text = lblName.Text + "<br/>" + "SaveStateComplete";
}
protected void Page_UnLoad(object sender, EventArgs e)
{
//Work and it will not effect label contrl, view stae and post back data.
lblName.Text = lblName.Text + "<br/>" + "UnLoad";
}
}
Output
The first time the Page Load is output:
When you click on the Submit Button output:
During the first time of the page load with EnableViewState="false":
When you click on the Submit Button output with EnableViewState="false":
Example 2: ViewState Values
Please observe the code comments and output. It will help you to clearly understand the concepts.
public partial class PageLiftCycle : System.Web.UI.Page
{
protected void Page_PreInit(object sender, EventArgs e)
{
//Work and It will assign the values to label.
//Note : If page is post back or first time call and you have not set any values to ViewState["value"], then
//Convert.ToString(ViewState["value"]) is always empty.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreInit";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void Page_Init(object sender, EventArgs e)
{
//Work and It will assign the values to label.
//Note : If page is post back or first time call and you have not set any values to ViewState["value"] in privious events, then
//Convert.ToString(ViewState["value"]) is always empty.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "Init";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void Page_InitComplete(object sender, EventArgs e)
{
//Work and It will assign the values to label.
//Note : If page is post back or first time call and you have not set any values to ViewState["value"] in privious events, then
//Convert.ToString(ViewState["value"]) is always empty.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "InitComplete";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected override void OnPreLoad(EventArgs e)
{
//Work and It will assign the values to label.
//Note : If page is post back and you have set or not set any values to ViewState["value"] in privious events, then
//Convert.ToString(ViewState["value"]) will always have post back data.
//E.g: If you string str = Convert.ToString(ViewState["value"]), then str will contain post back values.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreLoad";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void Page_Load(object sender, EventArgs e)
{
//Work and It will assign the values to label.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "Load";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void btnSubmit_Click(object sender, EventArgs e)
{
//Work and It will assign the values to label.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "btnSubmit_Click";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void Page_LoadComplete(object sender, EventArgs e)
{
//Work and It will assign the values to label.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "LoadComplete";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected override void OnPreRender(EventArgs e)
{
//Work and It will assign the values to label.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreRender";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected override void OnSaveStateComplete(EventArgs e)
{
//Work and It will assign the values to label.
//But "SaveStateComplete" values will not be available during post back. i.e. View state.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "SaveStateComplete";
lblName.Text = Convert.ToString(ViewState["value"]);
}
protected void Page_UnLoad(object sender, EventArgs e)
{
//Work and it will not effect label contrl values, view state and post back data.
ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "UnLoad";
lblName.Text = Convert.ToString(ViewState["value"]);
}
}
Output
During the first the Time Page Load is output:
When you click on the Submit Button output:
During the first time the page loads with EnableViewState="false":
When you click on the Submit Button the output with EnableViewState="false":
Example 3: ViewState Values
Please observe the code comments and output. It will help you to clearly understating the concepts.
public partial class PageLiftCycle : System.Web.UI.Page
{
protected void Page_PreInit(object sender, EventArgs e)
{
Response.Write("<br/>" + "PreInit");
}
protected void Page_Init(object sender, EventArgs e)
{
Response.Write("<br/>" + "Init");
}
protected void Page_InitComplete(object sender, EventArgs e)
{
Response.Write("<br/>" + "InitComplete");
}
protected override void OnPreLoad(EventArgs e)
{
Response.Write("<br/>" + "PreLoad");
}
protected void Page_Load(object sender, EventArgs e)
{
Response.Write("<br/>" + "Load");
}
protected void Page_LoadComplete(object sender, EventArgs e)
{
Response.Write("<br/>" + "LoadComplete");
}
protected override void OnPreRender(EventArgs e)
{
Response.Write("<br/>" + "PreRender");
}
protected override void OnSaveStateComplete(EventArgs e)
{
Response.Write("<br/>" + "SaveStateComplete");
}
protected void Page_UnLoad(object sender, EventArgs e)
{
//Runtime Error : Response is not available in this context.
//Response.Write("<br/>" + "UnLoad"); //Error
}
}
Output
Note:
If you write Response.Write("<br/>" + "UnLoad"); in the
Page_UnLoad event, then it will genenrate the Runtime Error "Response is
not available in this context".
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