When to use static classes in C# [duplicate]

Question

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• Class with single method — best approach? 14 answers

Here's what MSDN has to say under When to Use Static Classes:

static class CompanyInfo
{
    public static string GetCompanyName() { return "CompanyName"; }
    public static string GetCompanyAddress() { return "CompanyAddress"; }
    //...
}

Use a static class as a unit of organization for methods not associated with particular objects. Also, a static class can make your implementation simpler and faster because you do not have to create an object in order to call its methods. It is useful to organize the methods inside the class in a meaningful way, such as the methods of the Math class in the System namespace.

To me, that example doesn't seem to cover very many possible usage scenarios for static classes. In the past I've used static classes for stateless suites of related functions, but that's about it. So, under what circumstances should (and shouldn't) a class be declared static?

Answer 1

I use static classes as a means to define "extra functionality" that an object of a given type could use under a specific context. Usually they turn out to be utility classes.

Other than that, I think that "Use a static class as a unit of organization for methods not associated with particular objects." describe quite well their intended usage.

Answer 2

I do tend to use static classes for factories. For example, this is the logging class in one of my projects:

public static class Log
{
   private static readonly ILoggerFactory _loggerFactory =
      IoC.Resolve<ILoggerFactory>();

   public static ILogger For<T>(T instance)
   {
      return For(typeof(T));
   }

   public static ILogger For(Type type)
   {
      return _loggerFactory.GetLoggerFor(type);
   }
}

You might have even noticed that IoC is called with a static accessor. Most of the time for me, if you can call static methods on a class, that's all you can do so I mark the class as static for extra clarity.

Answer 3

When deciding whether to make a class static or non-static you need to look at what information you are trying to represent. This entails a more 'bottom-up' style of programming where you focus on the data you are representing first. Is the class you are writing a real-world object like a rock, or a chair? These things are physical and have physical attributes such as color, weight which tells you that you may want to instantiate multiple objects with different properties. I may want a black chair AND a red chair at the same time. If you ever need two configurations at the same time then you instantly know you will want to instantiate it as an object so each object can be unique and exist at the same time.

On the other end, static functions tend to lend more to actions which do not belong to a real-world object or an object that you can easily represent. Remember that C#'s predecessors are C++ and C where you can just define global functions that do not exist in a class. This lends more to 'top-down' programming. Static methods can be used for these cases where it doesn't make sense that an 'object' performs the task. By forcing you to use classes this just makes it easier to group related functionality which helps you create more maintainable code.

Most classes can be represented by either static or non-static, but when you are in doubt just go back to your OOP roots and try to think about what you are representing. Is this an object that is performing an action (a car that can speed up, slow down, turn) or something more abstract (like displaying output).

Get in touch with your inner OOP and you can never go wrong!

Answer 4

Static classes are very useful and have a place, for example libraries.

The best example I can provide is the .Net Math class, a System namespace static class that contains a library of maths functions.

It is like anything else, use the right tool for the job, and if not anything can be abused.

Blankly dismissing static classes as wrong, don't use them, or saying "there can be only one" or none, is as wrong as over using the them.

C#.Net contains a number of static classes that is uses just like the Math class.

So given the correct implementation they are tremendously useful.

We have a static TimeZone class that contains a number of business related timezone functions, there is no need to create multiple instances of the class so much like the Math class it contains a set of globally accesible TimeZone realated functions (methods) in a static class.

Answer 5

This is another old but very hot question since OOP kicked in. There are many reasons to use(or not) a static class, of course and most of them have been covered in the multitude of answers.

I will just add my 2 cents to this, saying that, I make a class static, when this class is something that would be unique in the system and that would really make no sense to have any instances of it in the program. However, I reserve this usage for big classes. I never declare such small classes as in the MSDN example as "static" and, certainly, not classes that are going to be members of other classes.

I also like to note that static methods and static classes are two different things to consider. The main disadvantages mentioned in the accepted answer are for static methods. static classes offer the same flexibility as normal classes(where properties and parameters are concerned), and all methods used in them should be relevant to the purpose of the existence of the class.

A good example, in my opinion, of a candidate for a static class is a "FileProcessing" class, that would contain all methods and properties relevant for the program's various objects to perform complex FileProcessing operations. It hardly has any meaning to have more than one instance of this class and being static will make it readily available to everything in your program.

Answer 6

For C# 3.0, extension methods may only exist in top-level static classes.