C# – Abstract

Abstract

Abstract Class

• A class declared as abstract can never be instantiated. Instead, only its concrete subclasses can be instantiated.
• Specifies method signatures but lacks implementation.
• It is used as a template for derived classes(base class).
• Abstract class can have abstract and virtual members.
• The subclass must provide implementation unless declared abstract

Link to Example

Abstract Methods

• Abstract members cannot be private.
• Abstract methods are implicitly virtual.

using System;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           
           new Stock { SharesOwned = 200, CurrentPrice = 123.45M }.NetValue.Dump();
        }       

    }
    
        public abstract class Asset
        {
            public abstract decimal NetValue { get; }
        }

        public class Stock : Asset
        {
            public long SharesOwned;
            public decimal CurrentPrice;
            
            //override virtual method
            public override decimal NetValue => CurrentPrice * SharesOwned;
           
        }
    
}

Abstract Properties

//Abstract Class with abstract properties
abstract class absClass
{
    protected int myNumber;
    public abstract int numbers
    {
        get;
        set;
    }
}

class absDerived:absClass
{
    //Implementing abstract properties
    public override int numbers
    {
        get
        {
            return myNumber;
        }
        set
        {
            myNumber = value;
        }
    }
}

Rules:

• Abstract class cannot be sealed.
• Abstract methods are only allowed in abstract classes.
• Abstract methods cannot be private.
• Access modifier of the abstract method should be same in both abstract class and its derived class.
• Abstract member cannot be static.
• Abstract method cannot have virtual modifier.

Read more about Interface vs Abstract in Interface article.