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:
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abstractshape.cs: the Shape class that contains an abstract Area property.
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shapes.cs: The subclasses of the Shape class.
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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); } }
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Modifiers on the property are placed on the property declaration itself. For example:
public abstract double Area
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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 */
