Responding to Postback Events
The
<asp:button>
objects
automatically postback when clicked. You need not write
any code to handle that event unless you want to do something more
than postback to the server. If you take no other action, the page
will simply be resent to the client.
Normally, when a page is redrawn, each control is redrawn from
scratch. The Web is stateless, and if you want to manage the state of
a control (e.g., redraw the user’s text in the text
box), you must do so yourself. In classic ASP, the programmer was
responsible for managing this state, but
ASP.NET provides some
assistance. When the page is posted, a hidden element named
ViewState
is automatically added to the page:
<input type="hidden" name="_ _VIEWSTATE" value="YTB6LTI5MTE3ODE1N19hMHpfaHo1ejF4X2Ewel9oejV6NXhfYTB6YTB6YTB6aHpSZXBlYX RMYXlvdXRfU3lzdGVtLldlYi5VSS5XZWJDb250cm9scy5SZXBlYXRMYXlvdXR6VGFibGV4X0RhdGF WYWx1ZUZpZWxkX1NoaXBwZXJJRF9EYXRhVGVceHRGaWVsZF9Db21wYW55TmFtZXhfX3hfYTB6YTB6 YXpTcGVlZHkgRVx4cHJlc3NfMV94X2F6VW5pdGVkIFBhY2thZ2VfMl94X2F6RmVkZXJhbCBTaGlwc GluZ18zX3hfeF94X3hfX3h4X3h4X3hfX3hcdDUwX1N5c3RlbS5TdHJpbmc=a15204ed" />
This element represents the state of the form (the values are already chosen by the user). When the page is redrawn on the client, ASP.NET uses the view state to return the controls to their previous state.
When the user clicks the Order button, the page is posted and the event handler assigned to that button is invoked:
public void Order_Click (object sender, System.EventArgs ...
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