Operator Overloads

Article
11/03/2006

Operator overloads allow types to be combined and compared using operators such as "+", "-", "=", and "!=". By adding operator overloads to a type, you allow developers to use the type as though it were a built-in primitive type. Operator overloading should be done only when the meaning of the operation is intuitive for the type (for example, to support adding two instances of a type that represents a numerical value). Operator overloading should not be used to provide a syntactic shortcut for non-intuitive operations.

The following example shows the signature for the addition operation of the DateTime class.

[Visual Basic]

Public Shared Function op_Addition(ByVal d As DateTime, _
ByVal t As TimeSpan _
) As DateTime

[C#]

public static DateTime op_Addition(
DateTime d, 
TimeSpan t
);

Avoid defining operator overloads, except in types that should work like primitive (built-in) types.

Consider defining operator overloads in a type that should work like a primitive type.

For example, String defines operator== and operator!=.

Do define operator overloads in structures that represent numbers (such as System.Decimal).

Do not be clever when defining operator overloads. Operator overloading is useful in cases where it is immediately obvious what the result of the operation will be. For example, it makes sense to be able to subtract one System.DateTime object from another System.DateTime object and get a System.TimeSpan object. However, it is not appropriate to use the logical union operator to union two database queries, or to use the shift operator to write to a stream.

Do not provide operator overloads unless at least one of the operands is of the type defining the overload.

The C# compiler enforces this guideline.

Do overload operators in a symmetric fashion.

For example, if you overload the equality operator, you should also overload the inequality operator. Similarly, if you overload the less-than operator, you should also overload the greater-than operator.

Consider providing methods with friendly names corresponding to each overloaded operator.

You must comply with this guideline to be CLS-compliant. The following table contains a list of operator symbols and their corresponding alternative methods and operator names.

C# operator symbol	Name of alternative method	Name of operator
Not defined	ToXxx or FromXxx	op_Implicit
Not defined	ToXxx or FromXxx	op_Explicit
+ (binary)	Add	op_Addition
- (binary)	Subtract	op_Subtraction
* (binary)	Multiply	op_Multiply
/	Divide	op_Division
%	Mod	op_Modulus
^	Xor	op_ExclusiveOr
& (binary)	BitwiseAnd	op_BitwiseAnd
\|	BitwiseOr	op_BitwiseOr
&&	And	op_LogicalAnd
\|\|	Or	op_LogicalOr
=	Assign	op_Assign
<<	LeftShift	op_LeftShift
>>	RightShift	op_RightShift
Not defined	LeftShift	op_SignedRightShift
Not defined	RightShift	op_UnsignedRightShift
==	Equals	op_Equality
>	CompareTo	op_GreaterThan
<	CompareTo	op_LessThan
!=	Equals	op_Inequality
>=	CompareTo	op_GreaterThanOrEqual
<=	CompareTo	op_LessThanOrEqual
*=	Multiply	op_MultiplicationAssignment
-=	Subtract	op_SubtractionAssignment
^=	Xor	op_ExclusiveOrAssignment
<<=	LeftShift	op_LeftShiftAssignment
%=	Mod	op_ModulusAssignment
+=	Add	op_AdditionAssignment
&=	BitwiseAnd	op_BitwiseAndAssignment
\|=	BitwiseOr	op_BitwiseOrAssignment
,	Comma	op_Comma
/=	Divide	op_DivisionAssignment
--	Decrement	op_Decrement
++	Increment	op_Increment
- (unary)	Negate	op_UnaryNegation
+ (unary)	Plus	op_UnaryPlus
~	OnesComplement	op_OnesComplement

For more information on design guidelines, see the "Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries" book by Krzysztof Cwalina and Brad Abrams, published by Addison-Wesley, 2005.

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