C Sharp - Discriminated Unions in C Sharp

The way I code is being heavily influenced by the functional paradigm. There's like a shopping list of items that it corresponds to, but one of them that I do an awful lot is this idea that once you've set a variable, you don't change it again. It would be like, I'll write line one, and then line two is based on line one and then line three. And you can't get you, well, you could get yourself a rubber and rub out the previous line and change it. But then. That's an eraser, kids. Sorry any race.