Interacting with C# program

I have VB6 program and C# program.
I should make it interact bewteen those two separate program.
For example, VB6 program send notification(or message) to the C# program to make it start some work.
After C# program finish that work, it should send notification to the VB6 program.
How could I implement this kind of interaction between separate program?
I will post this question for C# side methodology.
So I need just VB6 side methodology.