It is often useful to know how an ASP.NET Web page was invoked: whether by an original request (an HTTP GET), a postback (an HTTP POST), a cross-page post from another page (an HTTP POST), or a transfer from another page using the Transfer method or using a callback from the browser. The Page class exposes a set of properties that you can use to determine how a page was invoked.
Examine the values of the following Page class properties, and then refer the table to determine how the page was invoked:
IsPostBack
PreviousPage
IsCrossPagePostBack
IsCallback
The following table lists ways in which a page can be invoked and the corresponding Page property values.
Invocation method
Property values
Original request
IsPostBack is set to false.
PreviousPage is set to null (Nothing in Visual Basic).
IsCallback is set to false.
Postback
IsPostBack is set to true.
Cross-page posting
PreviousPage references the source page.
IsCrossPagePostBack is set to true.
Server transfer
IsCrossPagePostBack that is referenced in the PreviousPage is set to false.
Callback
IsCallback is set to true.
Be sure you test the IsCrossPagePostBack property of the page that is referenced in PreviousPage. The IsCrossPagePostBack property of the current page always returns false.