https://learn.microsoft.com/ja-jp/dotnet/api/system.web.ui.control.apprelativetemplatesourcedirectory?view=netframework-4.8.1
Control.AppRelativeTemplateSourceDirectory Property (System.Web.UI) | Microsoft Learn
Gets or sets the application-relative virtual directory of the Page or UserControl object that contains this control.
property systemweb uicontrolmicrosoftlearn