# 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.

## Example

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 = {Area:F2}";
}
}

• 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.

## Example

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
{

: base(id)
{
}

public override double Area
{
get
{
// Given the radius, return the area of a circle:
}
}
}

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;
}
}
}


## Example

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
*/