you can use a named pipe
NamedPipeServerStream
NamedPipeClientStream
or something else
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c# IPC (interprocess communications)
I would write my own plugin
enable notification and start my winform by a logon event received from the notification interface
( this winform would run a named pipe server )
than i would start a thread in my plugin that is connecting to the winform named pipe server and push my infos to it.
On a logoff event i would terminate the thread and on an shutdown event too (shutdown events are not supported in pgina).
But for a simple "how long" timer i would start a stopwatch on the form itself and retrieve the max time from a regkey or any other way.
If you take the "how long" time from radius than you need to modify the plugin to push the state somewhere.