https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.page.ispostback?view=netframework-4.8.1&redirectedfrom=MSDN
Page.IsPostBack Property (System.Web.UI) | Microsoft Learn
Gets a value that indicates whether the page is being rendered for the first time or is being loaded in response to a postback.
property systemweb uimicrosoftlearn