Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.
Assembly: System.Web (in System.Web.dll)
For an explanation of the difference between postbacks and callbacks, see Implementing Client Callbacks Programmatically Without Postbacks in ASP.NET Web Pages.
The following example shows how to test the value of the property when the page is loaded in order to determine whether the page is being rendered for the first time or is responding to a postback. If the page is being rendered for the first time, the code calls the Page.Validate method.
The page markup (not shown) contains RequiredFieldValidator controls that display asterisks if no entry is made for a required input field. Calling Page.Validate causes the asterisks to be displayed immediately when the page is rendered, instead of waiting until the user clicks the Submit button. After a postback, you do not have to call Page.Validate, because that method is called as part of the Page life cycle.