Question

Hello from C# and OOP newbie.

How can I avoid change of class on assigning derived class object to base class object in c#?
After i run code bellow i get this response

obj1 is TestingField.Two
obj2 is TestingField.Two

I expected that i will lose access to derived methods and properties (which I did) after assigning reference but I did not expect change of class in midcode :S

using System;

namespace TestingField
{
    class Program
    {
        static void Main(string[] args)
        {
            One obj1 = new One();
            Two obj2 = new Two();
            obj1 = obj2;
            Console.WriteLine("obj1 is {0}", obj1.GetType());
            Console.WriteLine("obj2 is {0}", obj2.GetType());
            Console.ReadLine();
        }
    }

    class One
    {
    }

    class Two : One
    {
        public void DoSomething()
        {
            Console.WriteLine("Did Something.");
        }

    }
}

Answer 1

While you are right, you will lose access to members declared in the derived type, the object won’t suddenly change it’s type or implementation. You can access only members declared on the base type, but the implementation of the derived type is used in the case of overriden members, which is the case with GetType, which is a compiler generated method which automatically overrides the base class’s implementation.

Extending your example:

class One
{
   public virtual void SayHello()
   {
      Console.WriteLine("Hello from Base");
   }
}

class Two : One
{
    public void DoSomething()
    {
        Console.WriteLine("Did Something.");
    }
    public override void SayHello()
    {
      Console.WriteLine("Hello from Derived");
    }

}

Given:

One obj = new Two();
obj.SayHello(); // will return "Hello from Derived"