C# Topics: Static Classes

To create a static class, precede the class keyword with the static keyword. Based on the above two rules, here is an example:

using System;

public static class Square
{
    public static double side;

    public static double Perimeter()
    {
        return side * 4;
    }

    public static double Area()
    {
        return side * side;
    }
}

public class Exercise
{
    public static int Main()
    {
        Square.Side = 36.84;

        Console.WriteLine("Square Characteristics");
        Console.Write("Side:      ");
        Console.WriteLine(Square.side);
        Console.Write("Perimeter: ");
        Console.WriteLine(Square.Perimeter());
        Console.Write("Area:      ");
        Console.WriteLine(Square.Area());

        return 0;
    }
}

This would produce:

Square Characteristics
Side:      36.84
Perimeter: 147.36
Area:      1357.1856
Press any key to continue . . .

If you create a class marked as static, you cannot derive a class from it.

Like a normal method, a constructor can be made static. There are rules you must follow. If you want to use a static constructor, you must explicitly create it (the compiler doesn't create a static constructor for you).

The static constructor must become the default constructor. That is, you must create a constructor that doesn't take any argument. Here is an example:

public class Person
{
    public string firstName;

    static Person()
    {
    }
}

If you create a static constructor, you cannot directly access the non-static fields of the class:

You can still access any field of the class as you see fit. Here are examples:

using System;

public class Person
{
    public string firstName;

    static Person()
    {
    }
}

public class Exercise
{
    public static int Main()
    {
        Person pers = new Person();

        pers.firstName = "Gertrude";

        Console.WriteLine("Personal Identification");
        Console.Write("Name: ");
	Console.WriteLine(pers.firstName);

        return 0;
    }
}

To use a static constructor, you have various options. To initialize a member variable in the static constructor, you can declare a variable for the class and access the member variable. Here is an example:

public class Person
{
    public string firstName;

    static Person()
    {
        Person pers = new Person();

        pers.firstName = "Gertrude";
    }
}

In reality, one of the reasons for using a static constructor is to initialize the static fields of the class or take any action that would be shared by all instances of the class. Therefore, another option to use a static constructor is to initialize the static member variables. After doing this, when accessing the initialized static field(s), it(they) would hold the value(s) you gave it(them). Here is an example:

using System;

public class Person
{
    public static string firstName;

    static Person()
    {
        firstName = "Gertrude";
    }
}

public class Exercise
{
    public static int Main()
    {
        Console.WriteLine("Personal Identification");
        Console.Write("Name: ");
	Console.WriteLine(Person.firstName);

        return 0;
    }
}

This would produce:

Personal Identification
Name: Gertrude
Press any key to continue . . .

Because the constructor is static, you cannot access it by declaring a variable for the class.

Another rule to observe with a static constructor is that you must not add an access modifier to it. The following will result in an error:

public class Person
{
    public static Person()
    {
    }
}

You can create a class that uses a combination of a static constructor and one or more other (non-static) constructors. Here is an example:

public class Person
{
    public string firstName;
    public string lastName;

    static Person()
    {
    }

    public Person(string first, string last)
    {
        firstName = first;
        lastName = last;
    }
}

If you create such a class and if you want to declare a variable for it, the default constructor doesn't exist or is not available. If you want to declare a variable, you must use a constructor that takes an argument. Here is an example:

using System;

public class Person
{
    public string firstName;
    public string lastName;

    static Person()
    {
    }

    public Person(string first, string last)
    {
        firstName = first;
        lastName = last;
    }
}

public class Exercise
{
    public static int Main()
    {
        Person pers = new Person("Gertrude", "Monay");

        Console.WriteLine("Personal Identification");
        Console.Write("First Name: ");
        Console.WriteLine(pers.firstName);
        Console.Write("Last Name:  ");
        Console.WriteLine(pers.lastName);

        return 0;
    }
}

This would produce:

Personal Identification
First Name: Gertrude
Last Name:  Monay
Press any key to continue . . .

Based on this, you should create a static constructor only if you have a good reason.