https://learn.microsoft.com/en-us/dotnet/api/system.net.peertopeer.collaboration.contactmanager.presencechanged?view=netframework-4.8.1
ContactManager.PresenceChanged Event (System.Net.PeerToPeer.Collaboration) | Microsoft Learn
Raised whenever the presence status of a PeerContact in the ContactManager has changed.
event systempeertopeercollaborationmicrosoftlearn