静态与公共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.

Answer 2

A public class must be called in application by another class, for exampel this may be a class of data access (called by businnes layer).

A static class need not necessarily the creation of an instance for example tracing or logging class.

Answer 3

One (perhaps over) simplified example for thinking about static is the following:

If you have the class Dog; you could instantiate the class to create Dog Poodle = new Dog(); and Dog Labrador = new Dog(); If the Dog class has a variable hairColor, then for Poodle and Labrador, hairColor could be different. The two different instances are seperate.

If however, you added a static variable to Dog called numberOfDogs, and incremented the variable every time a new Dog was instantiated (you could do this is the constructor for example), then the variable would count the total number of Dogs, and would be the same number regardless of which instance of Dog you checked. This is useful (and dangerous) depending on how you use it.