Learning C# 3.0

The Equals Operator

Although we’ve just warned you away from wildly implementing overloaded arithmetic operators, the comparison operators, especially ==, are another story. It’s very common to overload the == operator to determine whether two objects are equal. What “equal” means is up to you, although your criteria should be reasonable. You might decide that two Employee objects are equal if they have the same name, or you may decide that simply having the same employee ID is sufficient.

Overloading the == operator works the same as overloading any other operator. Simply use the keyword operator with the == symbol, and place your code inside the method. The == operator always returns a Boolean (true or false), so you’ll need to declare the operator as a public static bool. For example, for the Fraction class, your operator might look like this:

public static bool operator== ( Fraction lhs, Fraction rhs )
{
   if ( lhs.denominator == rhs.denominator &&
       lhs.numerator == rhs.numerator )
   {
      return true;
   }
   // code here to handle unlike fractions
   return false;
}

Notice that there’s no else clause here. If the numerator and denominator are equal, the operator returns true, and exits. If they’re not equal, the return statement after the if is executed, so there’s no need for an else.

C# insists that if you overload the equals operator, you must also overload the not-equals operator (!=). It’s good programming practice to have the inequality operator delegate its work to the equality operator, ...