Overloading Operators in VB.NET 2.0

Although you do not need to write overloaded operators every day, this feature certainly puts the latest version of VB.NET (whether you call it 2.0 or 8.0—there is some confusion in this area) on par with even the most powerful object-oriented languages. This article demonstrates a step-by-step process for writing custom operators, including a brief explanation for neophytes. Of course, if you didn't like VB.NET because of its differences from VB6, then you probably won't get too excited about the ability to overload operators in VB.NET 2.0. ; )

What Operators Are and Why You Overload Them

Programming has a rich history based in mathematics. Mathematics is steeped in meaningful symbols, operators, and operands. Operators are symbols that perform a function and operands are the things on which operators operate.

Operators typically indicate how many operands they use. Usually, you'll encounter unary, binary, and ternary operators. Unary means one operand, binary means two, and ternary means three. "Not" is an example of a unary operator; "+" is an example of a binary operator; and "?:" is an example of a ternary operator. (I have never seen a quaternary operator.)

Operators are convenient because they are quick to write and they are familiar, especially in arithmetic operations. For example, "&" and "+" are Boolean and arithmetic operators, but you can also used them to perform string concatenation for string operands. Clearly, operators have had multiple—or overloaded—meanings for some time. A natural evolution is this ability extending to us programmers.

Basic Guidelines for Overloading Operators

You should follow a few basic guidelines for overloading operators to either get past the compiler or avoid confusing people:

• Don't change an operator's semantic meaning. The result of an operator should be intuitive.
• Make certain overloaded operators are shared methods.
• Overload operators in pairs, if a symmetric pair exists. These are the pairs of operators: "=" and "<>"; ">" and "<"; ">=" and "<="; and IsTrue and IsFalse.
• You cannot overload assignment (e.g., the "=" operator in the preceding guideline is the equality-test operator.).
• You can overload only the operators listed in Table 1.
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  Table 1: Operators That Can Be Overloaded in .NET 2.0

  Operator	Operand Count	Description
  +	Unary	Positive
  -	Unary	Negative
  IsFalse	Unary	Is False test
  IsTrue	Unary	Is True test
  Not	Unary	Negation
  +	Binary	Addition
  -	Binary	Subtraction
  *	Binary	Multiplication
  /	Binary	Floating-point division
  \	Binary	Integer division
  &	Binary	Concatenation
  ^	Binary	Exponentiation
  >>	Binary	Shift right
  <<	Binary	Shift left
  =	Binary	Equality; assignment cannot be overloaded
  <>	Binary	Not equal
  >	Binary	Greater than
  <	Binary	Less than
  >=	Binary	Greater than or equal to
  <=	Binary	Less than or equal to
  And	Binary	Bitwise and
  Like	Binary	String pattern matching
  Mod	Binary	Modulo division
  Or	Binary	Bitwise or
  Xor	Binary	Bitwise xor
  CType	Unary	Type conversion

• Finally, don't assume that operators have been overloaded correctly or that others have defined all possible operators.

Defining a Named Method

Generic methods in VB.NET must be shared. They must have the same number of arguments as your operator will have. Thus, if you are overloading addition, you need a shared method with two arguments and a return type.

Suppose your problem domain needs a distance measured in feet and inches. (Don't worry, you could localize this class to switch to metric units, but that isn't relevant to this discussion.) To support your problem domain, you could easily define a class Distance that stores feet and inches as integers (see Listing 1).

Listing 1: A Distance Class That Stores Feet and Inches

Public Class Distance
  Private FFeet As Integer
  Private FInches As Integer

  Public Sub New(ByVal Feet As Integer, ByVal Inches As Integer)
    FFeet = Feet
    If (Inches > 12) Then
      FFeet += (Inches / 12)
      FInches = (Inches Mod 12)
    Else
      FInches = Inches
    End If
  End Sub

  Public Sub New(ByVal Inches As Integer)
    FFeet = (Inches / 12)
    FInches = (Inches Mod 12)
  End Sub

  Public Property Feet() As Integer
    Get
      Return FFeet
    End Get
    Set(ByVal value As Integer)
      FFeet = value
    End Set
  End Property

  Public Property Inches() As Integer
    Get
      Return FInches
    End Get
    Set(ByVal value As Integer)
      If (value > 12) Then
        FFeet += (value / 12)
        FInches = (value Mod 12)
      Else
        FInches = value
      End If
    End Set
  End Property

  Public Overrides Function ToString() As String
    Return FFeet & "ft " & FInches & "in"
  End Function

End Class

Listing 1 converts all inches greater or equal to 12 to feet. For example, if you set the inches field using the Inches property then 14 inches becomes 1 foot, 2 inches. Next, you can support basic arithmetic operations by defining addition and subtraction. Following your named methods guidelines, you get the named methods in Listing 2.

Listing 2: Operations Implemented as Named Methods Separate Algorithm from Syntax

  Public Shared Function Add(ByVal lhs As Distance, ByVal rhs As Distance) As Distance
    Return New Distance(lhs.Feet + rhs.Feet, lhs.Inches + rhs.Inches)
  End Function

  Public Shared Function Subtract(ByVal lhs As Distance, ByVal rhs As Distance) As Distance
    Return New Distance(lhs.Feet - rhs.Feet, lhs.Inches - rhs.Inches)
  End Function

After Listing 2, you can perform operations like Distance.Add(Distance1, Distance2). All that remains is implementing the overloaded operators.

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