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

You can define a class as static if you want to guarantee that it can't be instantiated, can't derive from or serve as the base for another type, and can contain only static members.

Because you can't create an instance of a static class, you can't use the new keyword to create a variable of the class type. You must access the members of a static class by using the class name itself. For example, if you have a static class that's named UtilityClass and that has a public method that's named MethodA, you call it as the following example shows:

UtilityClass.MethodA();

A static class can be used as a convenient container for sets of methods that just operate on input parameters and do not have to get or set any internal instance fields. For example, in the .NET Framework Class Library, the static Math class contains methods that perform mathematical operations, without any requirement to store or retrieve data that is unique to a particular instance of the Math class. That is, you apply the members of the class by specifying the class name and the method name, as shown in the following example.

double dub = -3.14;
Console.WriteLine(Math.Abs(dub));
Console.WriteLine(Math.Floor(dub));
Console.WriteLine(Math.Round(Math.Abs(dub)));

// Output:
// 3.14
// -4
// 3

As is the case with all class types, the type information for a static class is loaded by the .NET Framework common language runtime (CLR) when the program that references the class is loaded. The program cannot specify exactly when the class is loaded. However, it is guaranteed to be loaded and to have its fields initialized and its static constructor called before the class is referenced for the first time in your program. A static constructor is called only one time, and a static class remains in memory for the lifetime of the application domain in which your program resides. 

NoteNote

To create a non-static class that allows only one instance of itself to be created, see Implementing Singleton in C#.

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

  • It contains only static members.

  • It can't be instantiated.

  • It's sealed.

  • It can't contain Instance Constructors.

  • It can't be used in a local, instance, or static variable declaration, as a generic type argument, or as the element type of an array.

Creating a static class is therefore basically the same as creating a class that derives from Object, contains only static members, and is both sealed and abstract. The advantage of using a static class is that the compiler can verify that no instance members are accidentally added. The compiler guarantees that instances of the class can't be created.

Static classes are sealed and therefore can't be inherited. They can't inherit from any class except Object. A static class can't contain an instance constructor but can contain a static constructor. A non-static class should define a static constructor if the class contains static members that require non-trivial initialization. For more information, see Static Constructors (C# Programming Guide).

Here is an example of a static class that contains two methods that convert temperature from Celsius to Fahrenheit and from Fahrenheit to Celsius:

    public static class TemperatureConverter
    {
        public static double CelsiusToFahrenheit(string temperatureCelsius)
        {
            // Convert argument to double for calculations. 
            double celsius = Double.Parse(temperatureCelsius);

            // Convert Celsius to Fahrenheit. 
            double fahrenheit = (celsius * 9 / 5) + 32;

            return fahrenheit;
        }

        public static double FahrenheitToCelsius(string temperatureFahrenheit)
        {
            // Convert argument to double for calculations. 
            double fahrenheit = Double.Parse(temperatureFahrenheit);

            // Convert Fahrenheit to Celsius. 
            double celsius = (fahrenheit - 32) * 5 / 9;

            return celsius;
        }
    }

    class TestTemperatureConverter
    {
        static void Main()
        {
            Console.WriteLine("Please select the convertor direction");
            Console.WriteLine("1. From Celsius to Fahrenheit.");
            Console.WriteLine("2. From Fahrenheit to Celsius.");
            Console.Write(":");

            string selection = Console.ReadLine();
            double F, C = 0;

            switch (selection)
            {
                case "1":
                    Console.Write("Please enter the Celsius temperature: ");
                    F = TemperatureConverter.CelsiusToFahrenheit(Console.ReadLine());
                    Console.WriteLine("Temperature in Fahrenheit: {0:F2}", F);
                    break;

                case "2":
                    Console.Write("Please enter the Fahrenheit temperature: ");
                    C = TemperatureConverter.FahrenheitToCelsius(Console.ReadLine());
                    Console.WriteLine("Temperature in Celsius: {0:F2}", C);
                    break;

                default:
                    Console.WriteLine("Please select a convertor.");
                    break;
            }

            // Keep the console window open in debug mode.
            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
        }
    }
    /* Example Output:
        Please select the convertor direction
        1. From Celsius to Fahrenheit.
        2. From Fahrenheit to Celsius.
        :2
        Please enter the Fahrenheit temperature: 20
        Temperature in Celsius: -6.67
        Press any key to exit.
     */

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:

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:

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.

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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