In VB6, it was a simple matter to write applications that served up COM classes, the application would run as a system-wide singleton and would be able to serve COM classes to multiple processes. I believe this is called a COM LocalServer.
What's the equivalent in .NET? How can I make a single executable that handles requests from multiple applications or processes?
The nearest equivalent would be WCF (Windows Communication Foundation)
Learn The ABCs Of Programming Windows Communication Foundation
Instance Management Techniques For Developing WCF Apps
The singleton service is the ultimate shareable service. When a service is configured as a singleton, all clients get connected to the same single well-known instance independently of each other, regardless of which endpoint of the service they connect to. The singleton service lives forever, and is only disposed of once the host shuts down. The singleton is created exactly once when the host is created.