My name
is
Jon Skeet

Why do we assign child class object to parent class reference variable?

I have the following code.

public class Parent
{
    public void Print()
    {
        Console.WriteLine ("Parent Method");
    }
}

public class Child : Parent
{
    public new void Print()
    {
        Console.WriteLine ("Child Method");
    }
}

public class Program
{
    public static void Main()
    {
        Child C = new Child();
        C.Print();
    }
}

If I run this code, I get the result "Child Method" But if I do the following, why do I get the result "Parent Method"?

public class Parent
{
    public void Print()
    {
        Console.WriteLine ("Parent Method");
    }
}

public class Child : Parent
{
    public new void Print()
    {
        Console.WriteLine ("Child Method");
    }
}

public class Program
{
    public static void Main()
    {
        Parent P = new Child();   // The only difference is this.
        P.Print();
    }
}

The only difference is as below

Child C = new Child();   
Parent P = new Child();

I thought new Child() means that we create the instance of Child class. And I thought both, C and P, are just object reference variables that hold the location of instance of Child class.
Can you correct me if I'm wrong or tell me if I miss something because I don't understand why in the above cases I get different results?

It's because you've redeclared the Print method in Child. So at compile time, P.Print() resolves to Parent.Print, but C.Print() resolves to Child.Print(). If you had a virtual method which was overridden in Child instead, they'd both print "Child Method":

public class Parent
{
    // Declare this as virtual, allowing it to be overridden in
    // derived classes. The implementation will depend on the
    // execution-time type of the object it's called on.
    public virtual void Print()
    {
        Console.WriteLine ("Parent Method");
    }
}

public class Child : Parent
{
    // Override Parent.Print, so if Print is called on a reference
    // with compile-time type of Parent, but at execution it
    // refers to a Child, this implementation will be executed.
    public override void Print()
    {
        Console.WriteLine ("Child Method");
    }
}

What's the difference between override and new? (C# FAQ entry on MSDN - turns out to have been written by me, but I'd forgotten!)
Knowing when to use the override and new keywords (MSDN)

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