Base classes

A class declaration may specify a base class by following the class name and type parameters with a colon and the name of the base class. Omitting a base class specification is the same as deriving from type object. In the following example, the base class of Point3D is Point, and the base class of Point is object:

public class Point
{
public int x, y;

public Point(int x, int y) {
this.x = x;
this.y = y;
}
}

public class Point3D: Point
{
public int z;

public Point3D(int x, int y, int z): base(x, y) {
this.z = z;
}
}

A class inherits the members of its base class. Inheritance means that a class implicitly contains all members of its base class, except for the instance and static constructors, and the destructors of the base class. A derived class can add new members to those it inherits, but it cannot remove the definition of an inherited member. In the previous example, Point3D inherits the x and y fields from Point, and every Point3D instance contains three fields, x, y, and z.

An implicit conversion exists from a class type to any of its base class types. Therefore, a variable of a class type can reference an instance of that class or an instance of any derived class. For example, given the previous class declarations, a variable of type Point can reference either a Point or a Point3D:

Point a = new Point(10, 20);
Point b = new Point3D(10, 20, 30);

Fields

A field is a variable that is associated with a class or with an instance of a class.

A field declared with the static modifier defines a static field. A static field identifies exactly one storage location. No matter how many instances of a class are created, there is only ever one copy of a static field.

A field declared without the static modifier defines an instance field. Every instance of a class contains a separate copy of all the instance fields of that class.

In the following example, each instance of the Color class has a separate copy of the r, g, and b instance fields, but there is only one copy of the Black, White, Red, Green, and Blue static fields:

public class Color
{
public static readonly Color Black = new Color(0, 0, 0);
public static readonly Color White = new Color(255, 255, 255);
public static readonly Color Red = new Color(255, 0, 0);
public static readonly Color Green = new Color(0, 255, 0);
public static readonly Color Blue = new Color(0, 0, 255);

private byte r, g, b;

public Color(byte r, byte g, byte b) {
this.r = r;
this.g = g;
this.b = b;
}
}

As shown in the previous example, read-only fields may be declared with a readonly modifier. Assignment to a readonly field can only occur as part of the field’s declaration or in a constructor in the same class.