Equals()

This is a discussion on Equals() within the C# Programming forums, part of the General Programming Boards category; I was reading MSDN: Code: class TwoDPoint : System.Object { public readonly int x, y; public TwoDPoint( int x, int ...

  1. #1
    System Novice siavoshkc's Avatar
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    Question Equals()

    I was reading MSDN:
    Code:
    class TwoDPoint : System.Object
    {
        public readonly int x, y;
    
        public TwoDPoint(int x, int y)  //constructor
        {
            this.x = x;
            this.y = y;
        }
    
        public override bool Equals(System.Object obj)
        {
            // If parameter is null return false.
     if (obj == null)
            {
                return false;
            }
    
            // If parameter cannot be cast to Point return false.
            TwoDPoint p = obj as TwoDPoint;
            if ((System.Object)p == null)
            {
                return false;
            }
    
            // Return true if the fields match:
     return (x == p.x) && (y == p.y);
        }
    
        public bool Equals(TwoDPoint p)
        {
            // If parameter is null return false:
     if ((object)p == null)
            {
                return false;
            }
    
            // Return true if the fields match:
     return (x == p.x) && (y == p.y);
        }
    
        public override int GetHashCode()
        {
            return x ^ y;
        }
    }

    Code:
    class ThreeDPoint : TwoDPoint
    {
        public readonly int z;
    
        public ThreeDPoint(int x, int y, int z)
            : base(x, y)
        {
            this.z = z;
        }
    
        public override bool Equals(System.Object obj)
        {
            // If parameter cannot be cast to ThreeDPoint return false:
            ThreeDPoint p = obj as ThreeDPoint;
            if ((object)p == null)
            {
                return false;
            }
    
            // Return true if the fields match:
     return base.Equals(obj) && z == p.z;
        }
    
        public bool Equals(ThreeDPoint p)
        {
            // Return true if the fields match:
      return base.Equals((TwoDPoint)p) && z == p.z;    }
    
        public override int GetHashCode()
        {
            return base.GetHashCode() ^ z;
        }
    }
    Look at the bold line. How it casts ThreeDPoint class to TwoDPoint? ThreeDPoint class has three fields, while TwoDLine has two.
    Last edited by siavoshkc; 09-26-2006 at 10:00 AM.
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  2. #2
    and the Hat of Clumsiness GanglyLamb's Avatar
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    2d class is the base class, 3d is the derived class with 2d as a base class.

    When you create a 3d class object , in fact you pass 2 of the 3 arguments to the constructor of the base class, this is not a one way street, so going back from a 3d object to a 2d object is fine, as long as 3d is derived from 2d ( of course all the properties/methods of the 3d class will not be available to the downcasted object which now is a 2d object ).

    Now in the 3d class you can use any of the base class methods that are public.

    I would not write the Equals method of the 3d class like that, but that's just something personal I think.

    So there's nothing wrong with what the code is doing.

  3. #3
    Cat without Hat CornedBee's Avatar
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    Yeah, there is. There is no IS-A relationship between a 2d- and a 3d-point. Deriving one from the other is a mistake.
    All the buzzt!
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    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  4. #4
    System Novice siavoshkc's Avatar
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    So if 3d was not derived from 2d that downcasting wouldn't work, correct?
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  5. #5
    Code Goddess Prelude's Avatar
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    >So if 3d was not derived from 2d that downcasting wouldn't work, correct?
    Ask yourself this: when does a cast work between two independent types?
    My best code is written with the delete key.

  6. #6
    System Novice siavoshkc's Avatar
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    Ask yourself this: when does a cast work between two independent types?
    Till now I didn't know it works.

    I found this today:
    Code:
    public class A
    {
        public A() { }
    }
    
    public class B : A
    {
        public B() { }
    }
    The new class—the derived class—then gains all the non-private data and behavior of the base class in addition to any other data or behaviors it defines for itself. The new class then has two effective types: the type of the new class and the type of the class it inherits.

    In the example above, class B is effectively both B and A. When you access a B object, you can use the cast operation to convert it to an A object. The B object is not changed by the cast, but your view of the B object becomes restricted to A's data and behaviors. After casting a B to an A, that A can be cast back to a B. Not all instances of A can be cast to B—just those that are actually instances of B. If you access class B as a B type, you get both the class A and class B data and behaviors. The ability for an object to represent more than one type is called polymorphism. For more information, see Polymorphism (C# Programming Guide). For more information on casting, see Casting (C# Programming Guide).
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