You may be wondering where to find Microsoft.SharePoint.ApplicationPages.dll , if you are creating a new SharePoint application page? But don’t worry, it resides in _app_bin folder of your SharePoint site’s virtual directory.
Assuming your IIS inetpub is at C then the exact path of Microsoft.SharePoint.ApplicationPages.dll is
C:\Inetpub\wwwroot\wss\VirtualDirectories\<Your Virtual Server>\_app_bin\Microsoft.SharePoint.ApplicationPages.dll
Here is the full list of assemblies at _app_bin folder:
Microsoft.Office.DocumentManagement.Pages.dll
Microsoft.Office.officialfileSoap.dll
Microsoft.Office.Policy.Pages.dll
Microsoft.Office.SlideLibrarySoap.dll
Microsoft.Office.Workflow.Pages.dll
Microsoft.Office.WorkflowSoap.dll
Microsoft.SharePoint.ApplicationPages.dll
STSSOAP.DLL
