generate qr code in c# The PreLoad Event in C#

Encode QR-Code in C# The PreLoad Event

The PreLoad Event
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The PreLoad event merely indicates that the page has terminated the system-level initialization phase and is going to enter the phase that gives user code in the page a chance to further configure the page for execution and rendering. This event is raised only for pages.
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The Load Event
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The Load event is raised for the page first and then recursively for all child controls. At this time, controls in the page tree are created and their state fully reflects both the previous state and any data posted from the client. The page is ready to execute any initialization code related to the logic and behavior of the page. At this time, access to control properties and view state is absolutely safe.
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Handling Dynamically Created Controls
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When all controls in the page have been given a chance to complete their initialization before display, the page processor makes a second try on posted values that haven t been matched to existing controls. The behavior described earlier in the Processing Posted Data
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Part II
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ASP.NET Pages and Server Controls
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section is repeated on the name/value pairs that were left over previously. This apparently weird approach addresses a specific scenario the use of dynamically created controls. Imagine adding a control to the page tree dynamically for example, in response to a certain user action. As mentioned, the page is rebuilt from scratch after each postback, so any information about the dynamically created control is lost. On the other hand, when the page s form is submitted, the dynamic control there is filled with legal and valid information that is regularly posted. By design, there can t be any server control to match the ID of the dynamic control the first time posted data is processed. However, the ASP.NET framework recognizes that some controls could be created in the Load event. For this reason, it makes sense to give it a second try to see whether a match is possible after the user code has run for a while. If the dynamic control has been re-created in the Load event, a match is now possible and the control can refresh its state with posted data.
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Handling the Postback
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The postback mechanism is the heart of ASP.NET programming. It consists of posting form data to the same page using the view state to restore the call context that is, the same state of controls existing when the posting page was last generated on the server. After the page has been initialized and posted values have been taken into account, it s about time that some server-side events occur. There are two main types of events. The first type of event signals that certain controls had the state changed over the postback. The second type of event executes server code in response to the client action that caused the post.
Detecting Control State Changes
The whole ASP.NET machinery works around an implicit assumption: there must be a one-toone correspondence between some HTML input tags that operate in the browser and some other ASP.NET controls that live and thrive in the Web server. The canonical example of this correspondence is between <input type= text > and TextBox controls. To be more technically precise, the link is given by a common ID name. When the user types some new text into an input element and then posts it, the corresponding TextBox control that is, a server control with the same ID as the input tag is called to handle the posted value. I described this step in the Processing Posted Data section earlier in the chapter. For all controls that had the LoadPostData method return true, it s now time to execute the second method of the IPostBackDataHandler interface: the RaisePostDataChangedEvent method. The method signals the control to notify the ASP.NET application that the state of the control has changed. The implementation of the method is up to each control. However, most controls do the same thing: raise a server event and give page authors a way to kick
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