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.

Abstract classes have the following features:

1. An abstract class cannot be instantiated.
2. An abstract class may contain abstract methods and accessors.
3. It is not possible to modify an abstract class with the sealed modifier, which means that the class cannot be inherited.
4. A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.
5. 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 braces ({ }) following the signature. For example:public abstract void MyMethod();
• The implementation is provided by an overriding method, 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.
• 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.

Example:

interface I
{
void M();
}

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

In this example, the class MyDerivedC is derived from an abstract class MyBaseC. The abstract class contains an abstract method, MyMethod(), and two abstract properties, GetX() and GetY().

Example:
// abstract_keyword.cs
// Abstract Classes

using System;
abstract class MyBaseC // Abstract class
{
protected int x = 100;
protected int y = 150;
public abstract void MyMethod(); // Abstract method

public abstract int GetX // Abstract property
{
get;
}

public abstract int GetY // Abstract property
{
get;
}
}

class MyDerivedC: MyBaseC
{
public override void MyMethod()
{
x++;
y++;
}

public override int GetX // overriding property
{
get
{
return x+10;
}
}

public override int GetY // overriding property
{
get
{
return y+10;
}
}

public static void Main()
{
MyDerivedC mC = new MyDerivedC();
mC.MyMethod();
Console.WriteLine(“x = {0}, y = {1}”, mC.GetX, mC.GetY);
}
}

Output
x = 111, y = 161

In the preceding example, if you attempt to instantiate the abstract class by using a statement like this:
MyBaseC mC1 = new MyBaseC(); // Error

you will get the following error message:
Cannot create an instance of the abstract class ‘MyBaseC’.

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