使用新功能继承C＃

Question

I'm making a game in C#. I created a base class Shape and seven other classes ( Shape1 , Shape2 , ...) that inherit from Shape . In my main program I keep track of the shape currently in play by using

Shape Current;

Then I randomise its value to be equal to one of the shapes, for example:

Current = new Shape1()

The problem is, after I randomise I want to draw the shape with

Current.Draw()

Each shape has its own Draw function, and I want it to use that function specificly, and not the function in Shape . How can I do that?

Answer 1

The concept you are describing is called polymorphism (treating different object types as one).

In C#, you do this via the virtual , abstract and override keywords. In the base class, you mark the polymorphic method as either virtual or abstract , with the difference being abstract methods must be defined by derived classes:

public abstract void Draw();

Note that virtual methods must define a body, while abstract methods must not . Then in the derived class you define the same method with the override keyword:

public override void Draw()
{
   //whatever implementation
}

For far more information, see MSDN

Answer 2

I must say @BradleyDotNet explained it very well, I'm just adding a practical example that may help to clarify the use of it. Notice how I used all virtual, abstract and override keywords.

using System;

namespace ShapeExample
{
    class Program
    {
        public static void Main(string[] args)
        {
            for (int i = 0; i < 20; i++)
            {
                var shape = GetRandomShape();
                shape.Draw();
            }    
        }

        public static Random Random = new Random();

        public static Shape GetRandomShape()
        {
            var d = Random.Next(3);
            switch (d)
            {
                case 1:
                    return new Shape1();
                case 2:
                    return new Shape2();
                default:
                    return new Shape3();
            }
        }
    }

    public abstract class Shape
    {
        public virtual void Draw()
        {
            //Console.WriteLine("General Shape");
            Console.WriteLine("  _");
            Console.WriteLine(" / \\ ");
            Console.WriteLine("/___\\");
        }
    }

    public class Shape1 : Shape
    {
        public override void Draw()
        {
            //Console.WriteLine("I want a square instead");
            Console.WriteLine(" ____");
            Console.WriteLine("|    |");
            Console.WriteLine("|____|");
        }
    }

    public class Shape2 : Shape
    {
        public override void Draw()
        {
            //Console.WriteLine("I want a complicated circle instead");
            double r = 3.0;
            double r_in = r - 0.4;
            double r_out = r + 0.4;

            for (double y = r; y >= -r; --y)
            {
                for (double x = -r; x < r_out; x += 0.5)
                {
                    double value = x * x + y * y;
                    if (value >= r_in * r_in && value <= r_out * r_out)
                    {
                        Console.Write('*');
                    }
                    else
                    {
                        Console.Write(' ');
                    }
                }
                Console.WriteLine();
            }
        }
    }

    public class Shape3 : Shape
    {
        //I don't care how I look like, I'm using the regular shape drawing.

        //but I have some particular info that is not part of a regular Shape
        public int ParticularField { get; }

        public Shape3()
        {
            ParticularField = -100;
        }
    }   
}