How to imlement the code from graph?

I'm going to develop an application that should have a graphical view so users would be able to drag and drop nodes (functions) and connect outputs of one node to inputs of one another. Also users should be able to write c# codes inside nodes, Therefore the application can be extended and more nodes can be created for future uses.
And the Question is: How is this possible, actually!?

I've been surfing the web for the past few weeks and I have found some possible scenarios already. Could you please guide me whether the structure and main approach of the application should be like one of the following methods or not? To cut the long story short, How do Rhino and Grasshopper work? I'm going to build an application like those.

• Do they use VPL technologies? Do they take the graph and totally convert it to a c# code(Or a script), and then run it?
• Or Each node is a separate runnable module? Component based programming techniques? Do they use black box modeling and data flow programming for interaction between nodes? An every node is a separate program. Something like piping in Unix?

Thanks for any help