Perché e quando dovrei rendere una classe 'statica'? Qual è lo scopo della parola chiave 'statica' sulle classi?

Rende ovvio agli utenti come viene utilizzata la classe. Ad esempio, sarebbe privo di senso scrivere il seguente codice:

Math m = new Math();

C # non deve vietarlo, ma poiché non serve a nulla, potrebbe anche dirlo all'utente. Alcune persone (incluso me) aderiscono alla filosofia secondo cui i linguaggi di programmazione (e le API ...) dovrebbero essere il più restrittivi possibile per renderli difficili da usare in modo errato: le sole operazioni consentite sono quelle che sono significative e (si spera) corrette.

StackOverflow ha un'ottima discussione su questo argomento . Per facilità di riferimento, copierò e incollerò qui, a nome del suo autore, Mark S. Rasmussen :

I wrote my thoughts of static classes in an earlier thread:

I used to love utility classes filled up with static methods. They made a great consolidation of helper methods that would otherwise lie around causing redundancy and maintenance hell. They're very easy to use, no instantiation, no disposal, just fire'n'forget. I guess this was my first unwitting attempt at creating a service oriented architecture - lots of stateless services that just did their job and nothing else. As a system grows however, dragons be coming.

Polymorphism

Say we have the method UtilityClass.SomeMethod that happily buzzes along. Suddenly we need to change the functionality slightly. Most of the functionality is the same, but we have to change a couple of parts nonetheless. Had it not been a static method, we could make a derivate class and change the method contents as needed. As it's a static method, we can't. Sure, if we just need to add functionality either before or after the old method, we can create a new class and call the old one inside of it - but that's just gross.

Interface woes

Static methods cannot be defined through interfaces for logic reasons. And since we can't override static methods, static classes are useless when we need to pass them around by their interface. This renders us unable to use static classes as part of a strategy pattern. We might patch some issues up by passing delegates instead of interfaces.

Testing

This basically goes hand in hand with the interface woes mentioned above. As our ability of interchanging implementations is very limited, we'll also have trouble replacing production code with test code. Again, we can wrap them up but it'll require us to change large parts of our code just to be able to accept wrappers instead of the actual objects.

Fosters blobs

As static methods are usually used as utility methods and utility methods usually will have different purposes, we'll quickly end up with a large class filled up with non-coherent functionality - ideally, each class should have a single purpose within the system. I'd much rather have a five times the classes as long as their purposes are well defined.

Parameter creep

To begin with, that little cute and innocent static method might take a single parameter. As functionality grows, a couple of new parameters are added. Soon further parameters are added that are optional, so we create overloads of the method (or just add default values, in languages that support them). Before long, we have a method that takes 10 parameters. Only the first three are really required, parameters 4-7 are optional. But if parameter 6 is specified, 7-9 are required to be filled in as well... Had we created a class with the single purpose of doing what this static method did, we could solve this by taking in the required parameters in the constructor, and allowing the user to set optional values through properties, or methods to set multiple interdependent values at the same time. Also, if a method has grown to this amount of complexity, it most likely needs to be in its own class anyways.

Demanding consumers to create an instance of classes for no reason

One of the most common arguments is, why demand that consumers of our class create an instance for invoking this single method, while having no use for the instance afterwards? Creating an instance of a class is a very very cheap operation in most languages, so speed is not an issue. Adding an extra line of code to the consumer is a low cost for laying the foundation of a much more maintainable solution in the future. And finally, if you want to avoid creating instances, simply create a singleton wrapper of your class that allows for easy reuse - although this does make the requirement that your class is stateless. If it's not stateless, you can still create static wrapper methods that handle everything, while still giving you all the benefits in the long run. Finally, you could also make a class that hides the instantiation as if it was a singleton: MyWrapper.Instance is a property that just returns new MyClass();

Only a Sith deals in absolutes

Of course, there are exceptions to my dislike of static methods. True utility classes that do not pose any risk to bloat are excellent cases for static methods - System.Convert as an example. If your project is a one-off with no requirements for future maintenance, the overall architecture really isn't very important - static or non static, doesn't really matter - development speed does, however.

Standards, standards, standards!

Using instance methods does not inhibit you from also using static methods, and vice versa. As long as there's reasoning behind the differentiation and it's standardised. There's nothing worse than looking over a business layer sprawling with different implementation methods.

Sono sorpreso che nessun altro abbia menzionato che le classi static consentono i metodi di estensione - estendere in modo sicuro un tipo esistente (incluso aggiungere definizioni di metodo alle interfacce ) che non possiedi. Ad esempio, ciò che in Scala è

trait MyFoo {
  def foo: Int
  def plusFoo(a: Int) = foo + a
}

può essere espresso in C # come

public interface IMyFoo {
  int Foo();
}

public static class MyFooExtensions {
  public static int PlusFoo(this IMyFoo f, int a) {
    return f.Foo() + a;
  }
}

Per me, una classe statica (in C #) è un po 'come richiamare una funzione.

Ad esempio

public static string DoSomething(string DoSomethingWithThisString){}

Ho passato una stringa e ho recuperato una stringa. Tutto è contenuto in quel metodo. Nessun accesso alla variabile membro, ecc.

Onestamente, per lo più il suo uso per me è stato quello di ridurre le linee di codice:

Perché:

MyClass class = new MyClass();
String s = class.DoSomething("test");

Quando posso farlo:

String s = MyClass.DoSomething("test");

Quindi, userei le classi statiche in quei casi, quando volevo fare qualcosa senza la necessità di una classe e significherebbe meno la digitazione.

I punti Mark S. sono validi, ma per me se ho qualche metodo di utilità è più semplice fingere di chiamare una funzione per farli riferimento come un solo liner.

Potrebbe essere semplicistico, ma è soprattutto il modo in cui ho usato static .