Passing Derived class as a parameter to a method when the parameter type is base class

Question

I am a newbie and trying to understand concepts of inheritance and design patterns.

I came across this pattern http://en.wikipedia.org/wiki/Strategy_pattern when I was going through some blog.

I found it interesting and wanted to learn more. So I developed the following program.

static void Main(string[] args) {     Context context;      // Three contexts following different strategies     context = new Context(new ConcreteStrategyAdd());     int resultA = context.executeStrategy(3, 4);      context = new Context(new ConcreteStrategySubtract());     int resultB = context.executeStrategy(3, 4);      context = new Context(new ConcreteStrategyMultiply());     int resultC = context.executeStrategy(3, 4);      Console.Read(); }        abstract class Strategy {     public abstract int execute(int a, int b);      public void Test()     {         Console.Write("tttt");     } }  class ConcreteStrategyAdd : Strategy {     public override int execute(int a, int b)     {         Console.WriteLine("Called ConcreteStrategyAdd's execute()");         return a + b;  // Do an addition with a and b     } }  class ConcreteStrategySubtract : Strategy {      public override int execute(int a, int b)     {         Console.WriteLine("Called ConcreteStrategySubtract's execute()");         return a - b;  // Do a subtraction with a and b     } }  class ConcreteStrategyMultiply : Strategy {     public override int execute(int a, int b)     {         Console.WriteLine("Called ConcreteStrategyMultiply's execute()");         return a * b;   // Do a multiplication with a and b     } }  class Context {     private Strategy strategy;      // Constructor     public Context(Strategy strategy)     {         this.strategy = strategy;     }      public int executeStrategy(int a, int b)     {         return strategy.execute(a, b);     } } 

The program compiles fine and is working. But my question is how is it possible to pass derived class as a parameter when the Context constructor is expecting base class as a parameter? Is the casting happening implicitly? Why does the compiler does not error?

context = new Context(new ConcreteStrategyAdd());    public Context(Strategy strategy) {     this.strategy = strategy; } 

Answer 1

Put really simply:

A Derived class (or subclass) is an instance of its base class.

So, when you pass an instance of ConcreteStrategyAdd into the constructor, you are essentially passing a Strategy object in.

There is no casting involved. The type hierarchy allows for this type of programming. It allows programmers to use polymorphism in their code.

Answer 2

There is no casting needed, since ConcreteStrategyAdd is a Strategy - it satisfies all the requirements of being a Strategy. This is the principle of Polymorphism.

Perhaps a more simplistic example is needed:

abstract class Fruit { }  class Apple : Fruit { } class Orange : Fruit { } class Melon : Fruit { }  class FruitBasket {     void Add(Fruit item) { ... } }  FruitBasket basket = new FruitBasket(); basket.Add(new Apple()); // Apple IS A fruit basket.Add(new Orange()); // Orange IS A fruit basket.Add(new Melon()); // Melon IS A fruit  class Potato : Vegetable { }  basket.Add(new Potato()); // ERROR! Potato IS NOT A fruit.