Static vs. public C#

Question

I know that it's been answered multiple times here on SO, but I still don't get the nuts of bolts of what exactly it means to instantiate a class. I read this and it did help my understanding.

I know that static classes like Console cannot be used with the new expression like Console c = new Console() because there aren't any instance variables in that class. I also know that static classes provide on 'generic' methods and are generally used for Math functions. I know that once you instantiate a class like Double d = new Double(); you are now given access to whatever methods are inside of the Double class.

I know these facts but I feel like I don't really understand what they actually MEAN. Could someone give an example of where a static class is absolutely necessary and one where creating an instance of a class is absolutely necessary?

Answer 1

Think of a class like a set of blueprints. Instantiating a class is like taking the blueprints and building the item. When an engineer designs a car, he comes up with the design. That would be the class. After the car is designed, the plans are handed off to the assembly line to be built. Each car that rolls off the line would be an instance of that design.

When the car is still just a design, you can't really do anything with it. You can't open its door if there's no car. Once you have the instance of a car, you can manipulate it. You can open the door, start the engine, etc. The same goes for a class like Double. Once you have the instance, you can manipulate it.

A static class, like Console, are classes that don't have instances. They're more like a way to group useful related functionality. In the case of Console, the functionality is used to interact with the command line. Math is used to group mathematics related code. Configuration is used to read/manipulate configuration files. None of these things require you to create anything unique for them to work.