The Object
class (which
is the root of every class in C#) offers a virtual method called
Equals( )
. (Virtual methods are
discussed in Chapter 11.) If
you overload the equals operator (==
), it is recommended that you also override
the Equals( )
method.
Overriding the Equals( )
method
allows your class to be compatible with other .NET languages that do not
overload operators (but do support method overloading).
The Object
class implements the
Equals( )
method with this
signature:
public virtual bool Equals(object o)
From this signature, you can see that your override of this method
will take an object as a parameter, and return a bool
(true if the two objects are equal, where
“equality” is defined by the creator of the class).
By overriding this method, you allow your Fraction
class to act polymorphically with all
other objects. For example, anywhere you can call Equals( )
on two Objects
, you can call Equals( )
on two Fractions
.
Inside the body of Equals( )
,
you need to ensure that you are comparing one Fraction
object with another Fraction
object. If the other object is not a
fraction, they cannot be equal, and you’ll return false.
public override bool Equals(object o) {if ( ! (o is Fraction) ) { return false; } return this == (Fraction) o; }
The is
operator is used to
check whether the runtime type of an object is compatible with the
operand (in this case, Fraction
).
Thus o is Fraction
evaluates true if
o
is, in fact, a Fraction
or a type derived from Fraction
.
If you are comparing two Fractions
, you can delegate the decision as to
their equality to the overloaded operator (operator==
) that you’ve already written. This
allows you to avoid duplicate code.
public override bool Equals(object o)
{
if (! (o is Fraction) )
{
return false;
}return this == (Fraction) o;
}
In this way, the Equals( )
method determines only that you do in fact have two fractions. If so, it
delegates deciding if the two fractions are truly equal to the already
implemented operator ==
.
The complete modification of the Fraction
class is shown in Example 12-2, followed by the
analysis.
Example 12-2. Implementing equality operators
using System; public class Fraction { private int numerator; private int denominator; // create a fraction by passing in the numerator // and denominator public Fraction( int numerator, int denominator ) { this.numerator = numerator; this.denominator = denominator; } // overloaded operator+ takes two fractions // and returns their sum public static Fraction operator +( Fraction lhs, Fraction rhs ) { // like fractions (shared denominator) can be added // by adding thier numerators if ( lhs.denominator == rhs.denominator ) { return new Fraction( lhs.numerator + rhs.numerator, lhs.denominator ); } // simplistic solution for unlike fractions // 1/2 + 3/4 == (1*4) + (3*2) / (2*4) == 10/8 // this method does not reduce. int firstProduct = lhs.numerator * rhs.denominator; int secondProduct = rhs.numerator * lhs.denominator; return new Fraction( firstProduct + secondProduct, lhs.denominator * rhs.denominator ); }// test whether two Fractions are equal 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; } // delegates to operator == public static bool operator !=( Fraction lhs, Fraction rhs ) { return !( lhs == rhs ); } // tests for same types, then delegates public override bool Equals( object o ) { if ( !( o is Fraction ) ) { return false; } return this == (Fraction)o; } // return a string representation of the fraction public override string ToString( ) { String s = numerator.ToString( ) + "/" + denominator.ToString( ); return s; } } public class Tester { public void Run( ) { Fraction f1 = new Fraction( 3, 4 ); Console.WriteLine( "f1: {0}", f1.ToString( ) ); Fraction f2 = new Fraction( 2, 4 ); Console.WriteLine( "f2: {0}", f2.ToString( ) ); Fraction f3 = f1 + f2; Console.WriteLine( "f1 + f2 = f3: {0}", f3.ToString( ) ); Fraction f4 = new Fraction( 5, 4 ); if ( f4 == f3 ) { Console.WriteLine( "f4: {0} == F3: {1}", f4.ToString( ), f3.ToString( ) ); } if ( f4 != f2 ) { Console.WriteLine( "f4: {0} != F2: {1}", f4.ToString( ), f2.ToString( ) ); } if ( f4.Equals( f3 ) ) { Console.WriteLine( "{0}.Equals({1})", f4.ToString( ), f3.ToString( ) ); } } static void Main( ) { Tester t = new Tester( ); t.Run( ); } }
The output looks like this:
f1: 3/4 f2: 2/4 f1 + f2 = f3: 5/4 f4: 5/4 == F3: 5/4 f4: 5/4 != F2: 2/4 5/4.Equals(5/4)
Example 12-2
starts by implementing the overloaded equals operator, operator==
. If the fractions have the same
denominator, you test whether the numerators are equal. If they are, you
return true; otherwise, you return false.
This is a mathematical simplification to keep the example readable. Testing for true equality (such as 3/4 = 6/8) is left as an exercise for the reader.
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; }
This method is invoked in the Run( )
method when you write:
if (f4 == f3)
The if
statement expects a
Boolean value, which is what operator==
returns.
It is quite common to overload the equals operator (==
) to test whether two objects are equal. C#
insists that if you overload the equals operator, you must also overload
the not-equals operator (!=
).
Similarly, the less than (<
) and
greater than (>
) operators must be
paired, as must the less than or equal to (<=
) and greater than or equal to (>=
) operators.
It is good programming practice to have the inequality operator delegate its work to the equality operator, so that if you change the definition of equality, you are assured that the inequality operator will use the new definition.
public static bool operator !=(Fraction lhs, Fraction rhs) { return !(lhs==rhs); }
This operator is invoked in Run( )
when you write:
if (f4 != f2)
Put a breakpoint on this line of code and run to this line in Visual Studio .NET, as shown in Figure 12-1. (For more about breakpoints, see Chapter 9.)
Press F11 to step into the method call—you’ll step into the
!=
operator at the return
statement. Press F11 again and step
into the ==
operator. Whatever value
is returned by the ==
operator is
negated when it is returned by the !=
operator. If false is returned by ==
,
then true is returned by !=
.
You can make the reversal of equality explicit by adding a
temporary Boolean variable named “equality.” Rewrite the !=
operator as follows:
public static bool operator !=(Fraction lhs, Fraction rhs) { bool equality = lhs == rhs; return !(equality); }
You can now put a breakpoint on the second line of this method and
examine the value returned by operator==
, as shown in Figure 12-2.
You can see in the Autos window that the value of equality
(shown circled and highlighted) is
false. The fractions have been expanded to show their values (5/4 and
2/4), and they are not equal. The value returned by the !=
operator is the opposite of false; that is,
true.
In addition to implementing the ==
and !=
operator, you implement the Equals( )
method, for the reasons explained previously.
public override bool Equals( object o ) { if ( !( o is Fraction ) ) { return false; } return this == (Fraction)o; }
If the two objects are not both Fractions, you return false;
otherwise, you delegate to the ==
operator, casting o
to a Fraction
type. Put a breakpoint on the return line, and you’ll find that you step
back into operator==
. The value
returned by operator==
is the value
returned by the Equals( )
method if
both objects are fractions.