Static Classes and Static Class Members (C# Programming Guide)

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Static Constructors (C# Programming Guide)

The following list provides the main features of a static class:

• Contains only static members.
• Cannot be instantiated.
• Is sealed.
• Cannot contain Instance Constructors.

Creating a static class is therefore basically the same as creating a class that contains only static members and a private constructor. A private constructor prevents the class from being instantiated. The advantage of using a static class is that the compiler can check to make sure that no instance members are accidentally added. The compiler will guarantee that instances of this class cannot be created.
Static classes are sealed and therefore cannot be inherited. They cannot inherit from any class except Object. Static classes cannot contain an instance constructor; however, they can contain a static constructor. Non-static classes should also define a static constructor if the class contains static members that require non-trivial initialization. For more information, see Static Constructors (C# Programming Guide).
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Static Members

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A non-static class can contain static methods, fields, properties, or events. The static member is callable on a class even when no instance of the class has been created. The static member is always accessed by the class name, not the instance name. Only one copy of a static member exists, regardless of how many instances of the class are created. Static methods and properties cannot access non-static fields and events in their containing type, and they cannot access an instance variable of any object unless it is explicitly passed in a method parameter.
It is more typical to declare a non-static class with some static members, than to declare an entire class as static. Two common uses of static fields are to keep a count of the number of objects that have been instantiated, or to store a value that must be shared among all instances.
Static methods can be overloaded but not overridden, because they belong to the class, and not to any instance of the class.
Although a field cannot be declared as static const, a const field is essentially static in its behavior. It belongs to the type, not to instances of the type. Therefore, const fields can be accessed by using the same ClassName.MemberName notation that is used for static fields. No object instance is required.
C# does not support static local variables (variables that are declared in method scope).
You declare static class members by using the static keyword before the return type of the member, as shown in the following example:

C#

public class Automobile
{
    public static int NumberOfWheels = 4;
    public static int SizeOfGasTank
    {
        get
        {
            return 15;
        }
    }
    public static void Drive() { }
    public static event EventType RunOutOfGas;

    // Other non-static fields and properties...
}

Static members are initialized before the static member is accessed for the first time and before the static constructor, if there is one, is called. To access a static class member, use the name of the class instead of a variable name to specify the location of the member, as shown in the following example:

C#

Automobile.Drive();
int i = Automobile.NumberOfWheels;

If your class contains static fields, provide a static constructor that initializes them when the class is loaded.
A call to a static method generates a call instruction in Microsoft intermediate language (MSIL), whereas a call to an instance method generates a callvirt instruction, which also checks for a null object references. However, most of the time the performance difference between the two is not significant.