The Life and Times of a Server Control

• Introduction
• What is a Server Control?
• Control Properties
• Control Methods
• Control Events
• Conclusions

What is a Server Control? In ASP.Net, any class which can render an HTML interface in an ASPX page (i.e. write HTML to the output stream) is classified under the Namespace System.Web.UI.Control. This includes the Page class, Literal Controls, HTML Controls, Web Controls, User Controls, and Custom Controls. This means that all Server Controls inherit System.Web.UI.Control. That means that all Server Controls have certain properties, methods, and events in common. Let's take a look:

Control Properties:

• ClientID - The ClientID is the "ID" attribute assigned to the HTML object which this Control represents in the page. It is used for client-side operations such as JavaScript functions. As any Server Control which can maintain state and do PostBacks must have a client side HTML "ID" attribute in order to use the JavaScript state and event management client-side functions, this property automatically supplies one if the developer does not.
• Controls - This is a Collection of the Controls contained inside this one. Like a Windows Form, an ASPX page can host multiple Controls, and is, in fact, a Control. In practical terms, a Control is contained inside another control if the HTML for the control is between the beginning and ending tags of another HTML Control. Example:
  
  <form id="Form1" method="POST" runat="server">
  <asp:label id="Message1" font-size="16" font-bold="true" forecolor="red" runat=server>This is Message One</asp:label>
  </form>
  
  In the example above, the Label WebControl is a member of the Controls Collection of the Form HTMLControl.
• EnableViewState - This Boolean value indicates whether or not the Control should "remember" it's state between PostBacks₁.
• ID - This can be a bit confusing, as it looks a lot like ClientID. The ID property is used on the server-side to access the Control programmatically (in the CodeBehind class).
• NamingContainer - This property references the parent Control of this Control and implements the iNamingContainer Interface. The purpose of the iNamingContainer Interface is to make sure that the ID property of all Controls is unique. Any Control contained in a Control which implements iNamingContainer will have a Unique ID regardless of whether it has the same ID as another Control. The ID is derived from the assigned ID and the ID of the parent Container.
• Page - This property references the Page object in which this Control resides.
• Parent - This property references the Control which immediately contains this one in it's hierarchy of containers.
• Site - This property contains information about the Control's Web Site.
• TemplateSourceDirectory - returns the virtual directory of the Page or User Control which contains the Control.
• UniqueID - Returns the unique, hierarchically-qualified ID of the Control. This is different from ID and ClientID, in that multiple Controls can have the same ID, but not the same UniqueID. When hosted inside a NamingContainer, the UniqueID is a combination of the ID of the NamingContainer and the ID of the Control.
• Visible - This Boolean value indicates whether or not the control will be rendered in the page. When set to False, no HTML for the Control (or any child controls contained therein) will appear in the Page.

Control Methods

• DataBind - Binds data from a Data Source to a Server Control. This method is used most often with templated data-bound Controls.
• Dispose - Frees up resources used by a Control when it is no longer needed.
• Equals (inherited from Object) - Used to determine whether 2 object instances are equal.
• FindControl - Searches the current naming container for a specific Control
• GetHashCode (inherited from Object) - Returns a type-specific hash code for an Object
• GetType (inherited from Object) - Returns a System.Type object with Type metadata regarding that Type
• HasControls - This method returns a Boolean value that indicates whether the Control contains other Controls
• RenderControl - This is the method which writes the HTML for the Control to the output stream.
• ResolveUrl - This method resolves a relative URL to an absolute URL, according to the TemplateSourceDirectory property.
• ToString (inherited from Object) - This method returns a string that represents this Object.

Control Events

Because HTTP is stateless, a Server Control has a very short lifetime. It exists from the time the Page is requested until the Page is sent to the browser, at which time the Page, and its' entire contents, are disposed of. During this lifetime, the control must be re-instantiated and started first. Then Events from the client browser can be processed by the control. Finally, all controls in a Page render their HTML, and the Page does it's clean-up. At any point in that LifeCycle, the developer may want to add code to do something. Therefore, the various "insertion points" for code are made available via a number of Events which fire during a Control's lifetime:

• Init - This event occurs as the control is being instantiated.
• LoadViewState - Once all controls are instantiated, the ViewState can be loaded for each control. This is handled automatically, and is used to maintain state of controls which can do so. As you should recall from my first article₁, ViewState is maintained on the client in a hidden form field. The Control reads the hidden "__VIEWSTATE" field's data from the form, and loads the ViewState data from the last Request into the ViewState object (Collection) on the server.
• LoadPostData (if iPostBackDataHandler is implemented for this Control) - The control processes PostBack data, and updates its' properties accordingly. The LoadPostData Sub takes 2 arguments: The PostDataKey and the PostCollection. The PostDataKey (string) contains the ID of the Control which caused the PostBack. The PostDataCollection is similar to the Request.Form Collection; it contains all the data posted from the form. This Sub is used to do any post data processing that may be necessary at this point. It returns a Boolean value. If it returns True, the RaisePostDataChangedEvent is fired.
• Load - This Event Handler performs functions that are common to all requests - generally where the "meat" of your code usually goes. The controls have now been initialized. ViewState has been restored, and the state of the controls is now as it was on the client.
• RaisePostDataChangedEvent (if iPostBackDataHandler is implemented for this Control) - This event is raised when the LoadPostData Sub returns True. It is generally usually used to raise any events from the Control that need to be fired as a result of Post Data changing.
• RaisePostBackEvent (if iPostBackEventHandler is implemented for this Control) - This event is used by Server Controls which process PostBack events coming from Server Controls. Notice that this event occurs just after the RaisePostDataChangedEvent. That is so that any RaisePostDataChangedEvents can be fired first, allowing other Controls to react to them in this Event Handler.
• PreRender - This Event Fires just before the SaveViewState method is called. It can be used to make any changes to the control which will still be needed for the current Request, and must be made after handling PostBack events. At this point, the Control is ready to be rendered (written) to the HTML output stream.
• SaveViewState - This Method writes the new (modified) ViewState value to the output document's hidden "__VIEWSTATE" field.
• Render - This Method writes the HTML output of the Control to the output stream.
• Dispose - Perform any final cleanup prior to unloading.
• Unload - Fires just prior to the Control unloading.

Conclusions: As HTTP is stateless, Server Controls have a very short lifetime. To the user, they see the same HTML interface with each PostBack, and it looks as if they are working with the same Controls with each Page refresh. In truth, however, Server Controls must be rebuilt with each request, and put into their last state prior to processing PostBack data, and performing any new operations.

Understanding the sequence of Events that occur within the LifeCycle of a Server Control is important to writing effective ASP.Net applications.