EN CSharp Keyword abstract

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C#: abstract

Handbook C# · Practical Guide

Indicates a class that is not instantiated directly, or a non-implemented member that is required for a non-abstract descendant.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
Shape shape = new Square();
System.Console.WriteLine(shape.Area);
abstract class Shape { public abstract int Area { get; } }
class Square : Shape { public override int Area => 4; }

Result:

4

Syntax

abstract class Base { public abstract void Run(); }

Example 1: simple form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Entity entity = new Player("Archer");
        Console.WriteLine(entity.Describe());
        Console.WriteLine(entity.Code);
    }
}
public interface ILabel { string Label { get; } }
public abstract class Entity : ILabel
{
    private readonly string name;
    protected Entity(string name) { this.name = name; }
    protected string Name => name;
    internal int Code { get; set; } = 7;
    public virtual string Label => Name;
    public abstract string Describe();
}
public sealed class Player : Entity
{
    public Player(string name) : base(name) { }
    public override string Label => "Player: " + base.Label;
    public override string Describe() => Label;
}

Example 2: another practical form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Shape shape = new Square(3);
        Console.WriteLine(shape.Area);
    }
}
public abstract class Shape { public abstract int Area { get; } }
public sealed class Square(int side) : Shape { public override int Area => side * side; }

Official source: Microsoft Learn.