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Friday, May 27, 2011

Operator overloading and pairing rules in C#

What is the problem in the following code snippet?

namespace ConsoleApplication1
struct Point
int X;
int Y;

public static bool operator ==(Point p1, Point p2)
return ((p1.X == p2.X) && (p1.Y == p2.Y));

When you overload equality (==, !=) and comparison (<, >, <=, >=) operators, special pairing and overriding rules are enforced by C# compiler. For the above example, the compiler generates 2 warnings and the 1 error:

  • Warning: 'ConsoleApplication1.Point' defines operator == or operator != but does not override Object.GetHashCode()

  • Warning: 'ConsoleApplication1.Point' defines operator == or operator != but does not override Object.Equals(object o)

  • Error: The operator 'ConsoleApplication1.Point.operator ==(ConsoleApplication1.Point, ConsoleApplication1.Point)' requires a matching operator '!=' to also be defined   

Rule 1: If you overload the equality operators (== and !=), it is good practice to override the GetHashCode() and Equals() methods. Reason: Collections and HashTables rely on equality relations to work reliably.

Rule 2: If you overload one operation that is part of a pair (for example, == and !=), you are required to implement the other operator.

Rule 3: If you overload the comparison operators (< and >), it is a good practice to implement IComparable and IComparable<T> for the same reasons defined for rule 1.


  1. Replies
    1. Hi Nikhil,

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      In order to do design level changes, first of all design review needs to be done. The task becomes more difficult if the original architect of the application is not involved. All aspects of the design should be minutely analysed and then different approaches should be considered. Some general principles are listed below.
      Anyways great write up, your efforts are much appreciated.

      Many Thanks,

  2. Can we have a correct code for the above code snippet please?

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