C# Tutorial "Visibility and Accessibility Modifiers" : Abstract Modifier

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Visibility and Accessibility Modifiers

Abstract Modifier

The abstract modifier is used in inheritance scenarios. When applied to a class, it forces that class to be inherited, you can't instantiate an abstract class. When applied to a type member, such as a method, it enforces that anything that inherits from the class has to implement the abstract member.

 
using System;
                    
// The message class must be inherited.
public abstract class Message
{
    public void Send()
    {
        string address = GetAddress();
        
        Console.WriteLine(address);
    }
​
    // This method must be implemented by something that inherits from Message
    public abstract string GetAddress();
}
​
public class EmailMessage: Message
{
    public override string GetAddress()
    {
        return "bill@microsoft.com";
    }
}
​
public class Program
{
    public static void Main(string[] args)
    {
        Message message = new EmailMessage();
        message.Send();
    }
}

In the above example, we've created a class called 'Message'. The Message class cannot be instantiated using the new keyword because it needs to be inherited:

public class EmailMessage : Message
{
    public override string GetAddress()
    {
        return "me@somewhere.com";
    }
}
 
public class EmailMessage : Message
{
    public override string GetAddress()
    {
        return "me@somewhere.com";
    }
}

The GetAddress method is also marked as abstract, and you'll notice that we haven't actually provided an implemention, we simply end the declaration with the ; semi-colon line terminator.

When we implement an abstract method in your inherited class, you must specify the override modifier as part of the declaration:

public override string GetAddress()
{

}
​x
 
public override string GetAddress()
{
​
}