c# What is the different between static class and non-static (I am talking about the class itself not the field)

Question

The syntax maybe wrong

public static class Storage
{
    public static string filePath { get; set; }
}

And

public class Storage
{
    private void Storage () {};
    public static string filePath { get; set; }
}

I got this from an example on the internet. what is the use of the second one?

Solution 1

If you look at the IL code, the static class will be abstract and sealed which gives two important qualities:

• You cannot create instances from it
• It cannot be inherited

A consequence of the first point is that a static class cannot contain non-static members. There may be many uses of static members in a non-static class. One common use is to have a class factory:

public class SomeClass
{
    public int SomeInt { get; set; }

    public static SomeClass Create(int defaultValue)
    {
        SomeClass result = new SomeClass();
        result.SomeInt = defaultValue;
        return result;
    }
}

Solution 2

Here is the official/MSDN hot-spot to learn about static classes

The main features of a static class are:
* They only contain static members.
* They cannot be instantiated.
* They are sealed.
* They cannot contain Instance Constructors

Basically a static class is identical to a 'normal'/non-static class which has only static methods and a private ctor. Marking it as static helps clarify intent and helps the compiler do some compile-time checks to disallow certain things e.g. disallow instantiation.

Real-world uses I can think of: Use it to house or as a way to organize

• utility methods (methods not associated with any instance of a type) e.g. Math for Min and Max methods
• extension methods e.g. StopWatchExtensions for a Reset method on a StopWatch

Solution 3

Lots of classes have both instance and static methods. String for example has:

String.Format(string, arg0, arg1, arg2) // static method

And

String myString = "    Hello world!";
myString = myString.Substring(4);       // instance method

If you're asking why both the class and the method need the static keyword it's simply by design. I see what you're asking, if the class is static then of course all the methods are static as well, seems kind of redundant to put it there twice. I don't know if there's a good reason for that or not.

Solution 4

Static classes are only available from C#2 upwards. In C#1 you would have to seal your class and specify that it is not instantiable by added a private constructor to get this kind of behaviour.

Solution 5

When you declare a class as static:

• It is allowed to have only static members,
• It cannot be instantiated (it has no public constructor), and
• It cannot be inherited (it's sealed).

Any class which is not declared as static can be instantiated, inherited, and can have non-static members.

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Comments

[Lemon]

That isn't a static class though?

[Fredrik Mörk]

@Thorarin and @Svish: that is correct. It is serving to answer the last question of the use of static members in non-static types (as I interpreted the past question in the original post)