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Classes and Structs (C# Programming Guide)

Updated: July 2008

Classes and structs are two of the basic constructs of the common type system in the .NET Framework. Each is essentially a data structure that encapsulates a set of data and behaviors that belong together as a logical unit. The data and behaviors are the members of the class or struct, and they include its methods, properties, and events, and so on, as listed later in this topic.

A class or struct declaration is like a blueprint that is used to create instances or objects at run time. If you define a class or struct called Person, Person is the name of the type. If you declare and initialize a variable p of type Person, p is said to be an object or instance of Person. Multiple instances of the same Person type can be created, and each instance can have different values in its properties and fields.

A class is a reference type. When an object of the class is created, the variable to which the object is assigned holds only a reference to that memory. When the object reference is assigned to a new variable, the new variable refers to the original object. Changes made through one variable are reflected in the other variable because they both refer to the same data.

A struct is a value type. When a struct is created, the variable to which the struct is assigned holds the struct's actual data. When the struct is assigned to a new variable, it is copied. The new variable and the original variable therefore contain two separate copies of the same data. Changes made to one copy do not affect the other copy.

In general, classes are used to model more complex behavior, or data that is intended to be modified after a class object is created. Structs are best suited for small data structures that contain primarily data that is not intended to be modified after the struct is created.

For more information, see Classes (C# Programming Guide), Objects (C# Programming Guide), and Structs (C# Programming Guide).

In the following example, MyCustomClass is defined with three members at the top level of the ProgrammingGuide namespace. An instance (object) of MyCustomClass is created in the Main method in the Program class, and the object’s methods and properties are accessed by using dot notation.

namespace ProgrammingGuide
{
    // Class definition. 
    public class MyCustomClass
    {
        // Class members: 
        // Property. 
        public int Number { get; set; }

        // Method. 
        public int Multiply(int num)
        {
            return num * Number;
        }

        // Instance Constructor. 
        public MyCustomClass()
        {
            Number = 0;
        }
    }
    // Another class definition. This one contains 
    // the Main method, the entry point for the program. 
    class Program
    {
        static void Main(string[] args)
        {
            // Create an object of type MyCustomClass.
            MyCustomClass myClass = new MyCustomClass();

            // Set the value of a public property.
            myClass.Number = 27;

            // Call a public method. 
            int result = myClass.Multiply(4);
        }
    }
}

Encapsulation is sometimes referred to as the first pillar or principle of object-oriented programming. According to the principle of encapsulation, a class or struct can specify how accessible each of its members is to code outside of the class or struct. Methods and variables that are not intended to be used from outside of the class or assembly can be hidden to limit the potential for coding errors or malicious exploits.

For more information about classes, see Classes (C# Programming Guide) and Objects (C# Programming Guide). For more information about how to design classes, see Type Design Guidelines.

Members

All methods, fields, constants, properties, and events must be declared within a type; these are called the members of the class or struct. In C#, there are no global variables or methods as there are in some other languages. Even a program's entry point, the Main method, must be declared within a class or struct. The following list includes all the various kinds of members that may be declared in a class or struct.

Accessibility

Some methods and properties are meant to be called or accessed from code outside your class or struct, known as client code. Other methods and properties might be only for use in the class or struct itself. It is important to limit the accessibility of your code so that only the intended client code can reach it. You specify how accessible your types and their members are to client code by using the access modifierspublic , protected , internal, protected internal, and private . The default accessibility is private. For more information, see Access Modifiers (C# Programming Guide).

Inheritance

Classes (but not structs) support the concept of inheritance. A class that derives from another class (the base class) automatically contains all the public, protected, and internal members of the base class except its constructors and destructors. For more information, see Inheritance (C# Programming Guide) and Polymorphism (C# Programming Guide).

Classes may be declared as abstract, which means that one or more of their methods have no implementation. Although abstract classes cannot be instantiated directly, they can serve as base classes for other classes that provide the missing implementation. Classes can also be declared as sealed to prevent other classes from inheriting from them. For more information, see Abstract and Sealed Classes and Class Members (C# Programming Guide).

Interfaces

Classes and structs can inherit multiple interfaces. To inherit from an interface means that the type implements all the methods defined in the interface. For more information, see Interfaces (C# Programming Guide).

Generic Types

Classes and structs can be defined with one or more type parameters. Client code supplies the type when it creates an instance of the type. For example The List<T> class in the System.Collections.Generic namespace is defined with one type parameter. Client code creates an instance of a List<string> or List<int> to specify the type that the list will hold. For more information, see Generics (C# Programming Guide).

Static Types

Classes (but not structs) can be declared as static. A static class can contain only static members and cannot be instantiated with the new keyword. One copy of the class is loaded into memory when the program loads, and its members are accessed through the class name. Both classes and structs can contain static members. For more information, see Static Classes and Static Class Members (C# Programming Guide).

Nested Types

A class or struct can be nested within another class or struct. For more information, see Nested Types.

Partial Types

You can define part of a class, struct or method in one code file and another part in a separate code file. For more information, see Partial Classes and Methods.

Object Initializers

You can instantiate and initialize class or struct objects, and collections of objects, without explicitly calling their constructor. For more information, see Object and Collection Initializers (C# Programming Guide).

Anonymous Types

In situations where it is not convenient or necessary to create a named class, for example when you are populating a list with data structures that you do not have to persist or pass to another method, you use anonymous types. For more information, see Anonymous Types (C# Programming Guide).

Extension Methods

You can "extend" a class without creating a derived class by creating a separate type whose methods can be called as if they belonged to the original type. For more information, see Extension Methods (C# Programming Guide).

Implicitly Typed Local Variables

Within a class or struct method, you can use implicit typing to instruct the compiler to determine the correct type at compile time. For more information, see Implicitly Typed Local Variables (C# Programming Guide).

For more information, see the following sections in the C# Language Specification:

  • 1.6 Classes and Objects

  • 1.7 Structs

  • 3.4.4 Class Members

  • 4.2.1 Class Types

  • 10 Classes

  • 11 Structs

Date

History

Reason

July 2008

Added information and definitions to the introduction.

Information enhancement.

July 2008

Added information to the paragraph about accessibility.

Information enhancement.

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