Chapter 13 Creating Interfaces and Defining Abstract Classes

255

Summarizing Keyword Combinations

The following table summarizes the various valid (yes), invalid (no), and mandatory (required) keyword combinations when creating classes and interfaces.

Keyword		Interface	Abstract class	Class	Sealed class	Structure
abstract		no	yes	no	no	no
new		yes1	yes	yes	yes	no2
override		no	yes	yes	yes	no3
private		no	yes	yes	yes	yes
protected		no	yes	yes	yes	no4
public		no	yes	yes	yes	yes
sealed		no	yes	yes	required	no
virtual		no	yes	yes	no	no
1	An interface can extend another interface and introduce a new method with the same signature.
2	A structure implicitly derives from System.Object, which contains methods that the structure can hide.
3	A structure implicitly derives from System.Object, which contains no virtual methods.

4 A structure is implicitly sealed and cannot be derived from.

256	Part II Understanding the C# Language

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Chapter 13 Quick Reference

To	Do this
Declare an interface	Use the interface keyword. For example:
	interface IDemo
	{
	string Name();
	string Description();
	}
Implement an interface	Declare a class using the same syntax as class inheritance, and then
	implement all the member functions of the interface. For example:
	class Test : IDemo
	{
	public string IDemo.Name()
	{
	...
	}
	public string IDemo.Description()
	{
	...
	}
	}

Create an abstract class that can be used only as a base class, containing abstract methods

Declare the class using the abstract keyword. For each abstract method, declare the method with the abstract keyword and without a method

body. For example:

abstract class GrazingMammal

{

abstract void DigestGrass();

...

}

Create a sealed class that cannot be used as a base class

Declare the class using the sealed keyword. For example:

sealed class Horse

{

...

}