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How to: Define Abstract Properties (C# Programming Guide)

The following example shows how to define abstract properties. An abstract property declaration does not provide an implementation of the property accessors -- it declares that the class supports properties, but leaves the accessor implementation to derived classes. The following example demonstrates how to implement the abstract properties inherited from a base class.

This sample consists of three files, each of which is compiled individually and its resulting assembly is referenced by the next compilation:

  • abstractshape.cs: the Shape class that contains an abstract Area property.

  • shapes.cs: The subclasses of the Shape class.

  • shapetest.cs: A test program to display the areas of some Shape-derived objects.

To compile the example, use the following command:

csc abstractshape.cs shapes.cs shapetest.cs

This will create the executable file shapetest.exe.

This file declares the Shape class that contains the Area property of the type double.

// compile with: csc /target:library abstractshape.cs 
public abstract class Shape
{
    private string name;

    public Shape(string s)
    {
        // calling the set accessor of the Id property.
        Id = s;
    }

    public string Id
    {
        get
        {
            return name;
        }

        set
        {
            name = value;
        }
    }

    // Area is a read-only property - only a get accessor is needed: 
    public abstract double Area
    {
        get;
    }

    public override string ToString()
    {
        return Id + " Area = " + string.Format("{0:F2}", Area);
    }
}
  • Modifiers on the property are placed on the property declaration itself. For example:

    public abstract double Area
    
  • When declaring an abstract property (such as Area in this example), you simply indicate what property accessors are available, but do not implement them. In this example, only a get accessor is available, so the property is read-only.

The following code shows three subclasses of Shape and how they override the Area property to provide their own implementation.

// compile with: csc /target:library /reference:abstractshape.dll shapes.cs 
public class Square : Shape
{
    private int side;

    public Square(int side, string id)
        : base(id)
    {
        this.side = side;
    }

    public override double Area
    {
        get
        {
            // Given the side, return the area of a square: 
            return side * side;
        }
    }
}

public class Circle : Shape
{
    private int radius;

    public Circle(int radius, string id)
        : base(id)
    {
        this.radius = radius;
    }

    public override double Area
    {
        get
        {
            // Given the radius, return the area of a circle: 
            return radius * radius * System.Math.PI;
        }
    }
}

public class Rectangle : Shape
{
    private int width;
    private int height;

    public Rectangle(int width, int height, string id)
        : base(id)
    {
        this.width = width;
        this.height = height;
    }

    public override double Area
    {
        get
        {
            // Given the width and height, return the area of a rectangle: 
            return width * height;
        }
    }
}

The following code shows a test program that creates a number of Shape-derived objects and prints out their areas.

// compile with: csc /reference:abstractshape.dll;shapes.dll shapetest.cs 
class TestClass
{
    static void Main()
    {
        Shape[] shapes =
        {
            new Square(5, "Square #1"),
            new Circle(3, "Circle #1"),
            new Rectangle( 4, 5, "Rectangle #1")
        };

        System.Console.WriteLine("Shapes Collection");
        foreach (Shape s in shapes)
        {
            System.Console.WriteLine(s);
        }
    }
}
/* Output:
    Shapes Collection
    Square #1 Area = 25.00
    Circle #1 Area = 28.27
    Rectangle #1 Area = 20.00
*/

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