4.5 Operator Overloading

C# lets you overload operators to work with operands that are custom classes or structs using operators. An operator is a static method with the keyword operator preceding the operator to overload (instead of a method name ), parameters representing the operands, and return types representing the result of an expression. Table 4-1 lists the available overloadable operators.

Table 4-1. Overloadable operators

+	-	!	~	++
--	+	-	* (binary only)	/
%	& (binary only)		^	<<
>>	= =	!=	>	<
>=	<=

Literals that also act as overloadable operators are true and false .

4.5.1 Implementing Value Equality

A pair of references exhibit referential equality when both references point to the same object. By default, the = = and != operators will compare two reference-type variables by reference. However, it is occasionally more natural for the = = and != operators to exhibit value equality, whereby the comparison is based on the value of the objects that the references point to.

Whenever overloading the = = and != operators, you should always override the virtual Equals method to route its functionality to the = = operator. This allows a class to be used polymorphically (which is essential if you want to take advantage of functionality such as the collection classes). It also provides compatibility with other .NET languages that don't overload operators.

A good guideline for knowing whether to implement the = = and != operators is if it is natural for the class to overload other operators too, such as < , > , + , or - ; otherwise , don't bother ”just implement the Equals method. For structs, overloading the = = and != operators provides a more efficient implementation than the default one.

 class Note {   int value;   public Note(int semitonesFromA) {     value = semitonesFromA;   }   public static bool operator =  =(Note x, Note y) {     return x.value =  = y.value;   }   public static bool operator !=(Note x, Note y) {     return x.value != y.value;   }   public override bool Equals(object o) {     if(!(o is Note))       return false;     return this =  =(Note)o;   } } Note a = new Note(4); Note b = new Note(4); Object c = a; Object d = b;    // To compare a and b by reference Console.WriteLine((object)a =  =(object)b; // false     //To compare a and b by value: Console.WriteLine(a =  = b); // true    //To compare c and d by reference: Console.WriteLine(c =  = d); // false    //To compare c and d by value: Console.WriteLine(c.Equals(d)); // true 

4.5.2 Logically Paired Operators

The C# compiler enforces operators that are logical pairs to both be defined. These operators are = = != , < > , and <= >= .

4.5.3 Custom Implicit and Explicit Conversions

As explained in the discussion on types, the rationale behind implicit conversions is they are guaranteed to succeed and do not lose information during the conversion. Conversely, an explicit conversion is required either when runtime circumstances will determine whether the conversion will succeed or if information may be lost during the conversion. In this example, we define conversions between our musical Note type and a double (which represents the frequency in hertz of that note):

 ... // Convert to hertz public static implicit operator double(Note x) {   return 440*Math.Pow(2,(double)x.value/12); }    // Convert from hertz(only accurate to nearest semitone) public static explicit operator Note(double x) {   return new Note((int)(0.5+12*(Math.Log(x/440)/Math.Log(2)))); } ...    Note n =(Note)554.37; // explicit conversion double x = n; // implicit conversion 

4.5.4 Three-State Logic Operators

The true and false keywords are used as operators when defining types with three-state logic to enable these types to work seamlessly with constructs that take boolean expressions ”namely, the if , do , while , for , and conditional (?:) statements. The System.Data.SQLTypes.SQLBoolean struct provides this functionality:

 public struct SQLBoolean ... {   ...   public static bool operator true(SQLBoolean x) {     return x.value =  = 1;   }   public static bool operator false(SQLBoolean x) {     return x.value =  = -1;   }   public static SQLBoolean operator !(SQLBoolean x) {     return new SQLBoolean(- x.value);   }   public bool IsNull {     get { return value =  = 0;}   }   ... } class Test {   void Foo(SQLBoolean a) {     if (a)       Console.WriteLine("True");     else if (! a)       Console.WriteLine("False");     else       Console.WriteLine("Null");    } } 

4.5.5 Indirectly Overloadable Operators

The && and operators are automatically evaluated from & and , so they do not need to be overloaded. The [ ] operators can be customized with indexers (see Section 3.1.5 in Chapter 3). The assignment operator = cannot be overloaded, but all other assignment operators are automatically evaluated from their corresponding binary operators (e.g., += is evaluated from + ).