abstract (C# Reference)

The abstract modifier indicates that the thing being modified has a missing or incomplete implementation. The abstract modifier can be used with classes, methods, properties, indexers, and events. Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes, not instantiated on its own. Members marked as abstract must be implemented by non-abstract classes that derive from the abstract class.

Example 1

In this example, the class Square must provide an implementation of GetArea because it derives from Shape:

abstract class Shape
{
    public abstract int GetArea();
}

class Square : Shape
{
    private int _side;

    public Square(int n) => _side = n;

    // GetArea method is required to avoid a compile-time error.
    public override int GetArea() => _side * _side;

    static void Main()
    {
        var sq = new Square(12);
        Console.WriteLine($"Area of the square = {sq.GetArea()}");
    }
}
// Output: Area of the square = 144

Abstract classes have the following features:

• An abstract class cannot be instantiated.

• An abstract class may contain abstract methods and accessors.

• It is not possible to modify an abstract class with the sealed modifier because the two modifiers have opposite meanings. The sealed modifier prevents a class from being inherited and the abstract modifier requires a class to be inherited.

• A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.

Use the abstract modifier in a method or property declaration to indicate that the method or property does not contain implementation.

Abstract methods have the following features:

• An abstract method is implicitly a virtual method.

• Abstract method declarations are only permitted in abstract classes.

• Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no curly braces ({ }) following the signature. For example:

  public abstract void MyMethod();  
  

  The implementation is provided by a method override, which is a member of a non-abstract class.

• It is an error to use the static or virtual modifiers in an abstract method declaration.

Abstract properties behave like abstract methods, except for the differences in declaration and invocation syntax.

• It is an error to use the abstract modifier on a static property.

• An abstract inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.

For more information about abstract classes, see Abstract and Sealed Classes and Class Members.

An abstract class must provide implementation for all interface members.

An abstract class that implements an interface might map the interface methods onto abstract methods. For example:

interface I
{
    void M();
}

abstract class C : I
{
    public abstract void M();
}

Example 2

In this example, the class DerivedClass is derived from an abstract class BaseClass. The abstract class contains an abstract method, AbstractMethod, and two abstract properties, X and Y.

// Abstract class
abstract class BaseClass
{
    protected int _x = 100;
    protected int _y = 150;

    // Abstract method
    public abstract void AbstractMethod();

    // Abstract properties
    public abstract int X { get; }
    public abstract int Y { get; }
}

class DerivedClass : BaseClass
{
    public override void AbstractMethod()
    {
        _x++;
        _y++;
    }

    public override int X   // overriding property
    {
        get
        {
            return _x + 10;
        }
    }

    public override int Y   // overriding property
    {
        get
        {
            return _y + 10;
        }
    }

    static void Main()
    {
        var o = new DerivedClass();
        o.AbstractMethod();
        Console.WriteLine($"x = {o.X}, y = {o.Y}");
    }
}
// Output: x = 111, y = 161

In the preceding example, if you attempt to instantiate the abstract class by using a statement like this:

BaseClass bc = new BaseClass();   // Error  

You will get an error saying that the compiler cannot create an instance of the abstract class 'BaseClass'.

Nonetheless, it is possible to use an abstract class constructor, as in the example below

Example 3

public abstract class Shape
{
    public string Color { get; set; }

    // Constructor of the abstract class
    protected Shape(string color)
    {
        Color = color;
        Console.WriteLine("Created a shape with color {color}.");
    }

    // Abstract method that must be implemented by derived classes
    public abstract double CalculateArea();
}

public class Square : Shape
{
    public double Side { get; set; }

    // Constructor of the derived class calling the base class constructor
    public Square(string color, double side) : base(color)
    {
        Side = side;
    }

    public override double CalculateArea()
    {
        return Side * Side;
    }
}

public class Program
{
    public static void Main(string[] args)
     {
            Square square = new Square("red", 5);
            Console.WriteLine("Area of the square: {square.CalculateArea()}");            
     }
}

The Shape class is declared abstract, which means it cannot be instantiated directly. Instead, it serves as a blueprint for other classes.

• Even though you can't create objects of an abstract class, it can still have a constructor. This constructor is typically protected, meaning it can only be accessed from derived classes. In this case, the Shape constructor takes a color parameter and initializes the Color property. It also prints a message to the console. The public Square(string color, double side) : base(color) part calls the base class's constructor (Shape) and passes the color argument to it.
• In the Shape class, the defined constructor takes a color as a parameter protected Shape(string color). This means there's no longer a default parameterless constructor automatically provided by C# thus derived classes must use the : base(color) expression to invoke the base constructor. Setting the default value to color protected Shape(string color="green") will allow to omit the : base(color) expression in derived classes, still such constructor protected Shape(string color="green") will be invoked, setting the color to green.

C# Language Specification

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

See also

• virtual
• override
• C# Keywords