Overloading Operators in VB.NET 2.0
Defining an Operator MethodAt this point, you have your new class: Distance. You have defined fields, properties, and named methods for operations you'd like to perform on your class. Finally, you can create the operator methods by copying and pasting the named methods and changing the keyword Function to Operator and the method name to operator(symbol), where operator is a literal and symbol represents the actual operator (in this example, "+" and "-"). To complete the implementation, simply call the named methods in the operator methods. Listing 3 shows the additional operator methods.
Listing 3: The Operator+ and Operator: Methods for Distance
Public Shared Operator +(ByVal lhs As Distance, ByVal rhs As Distance) As Distance Return Add(lhs, rhs) End Operator Public Shared Operator -(ByVal lhs As Distance, ByVal rhs As Distance) As Distance Return Subtract(lhs, rhs) End Operator Tip: If you are very comfortable with operator overloading, skip the create
named methods step. As for me, after 16 years of OOP programming this is still a useful
Listing 4 shows the entire listing of the Distance class and a Main method in a Console application to show you how to invoke the overloaded operators. In this example, "+" and "-" work as you would expect.
Listing 4: The Distance Class with Overloaded Operators and a Sample Main Subroutine
Module Module1 Sub Main() Dim i, j, k As Integer i = 5 j = 7 k = i And j Console.WriteLine(k) Dim a As Distance = New Distance(5, 3) Dim b As Distance = New Distance(6, 13) Console.WriteLine(a.ToString()) Console.WriteLine(b.ToString()) Dim c As Distance = a + b Console.WriteLine(c.ToString()) Dim d As Distance = c - b Console.WriteLine(d.ToString()) Dim e As Distance = d Console.WriteLine(e.ToString()) Console.ReadLine() End Sub End Module 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 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 Operator +(ByVal lhs As Distance, ByVal rhs As Distance) As Distance Return Add(lhs, rhs) End Operator 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 Public Shared Operator -(ByVal lhs As Distance, ByVal rhs As Distance) As Distance Return Subtract(lhs, rhs) End Operator End Class
Intuitive Operators for Your ClassesOperator overloading is harder in C++ because C++ supports overloading tricky member-of operators like member-of (.) and function call (). In addition, C++ supports static and instance operators, but using name methods makes overloading even esoteric operators like () easier.
In VB.NET (and C#) overloaded operators must be shared (static), and operator methods must have the same count as the operator you are overloading. Finally, in VB.NET the special word Operator is used in place of Sub or Function, which makes it very clear that the method you are looking at is an operator statement.
Overloading operators can provide your classes with intuitive operators, and they are easy to implement and use, as long as you don't change the underlying behavior of the operator and use named methods to separate syntax from implementation.
About the Author
Paul Kimmel is the VB Today columnist for www.codeguru.com and has written several books on object-oriented programming and .NET. Check out his upcoming book UML DeMystified from McGraw-Hill/Osborne (Spring 2005) and Expert One-on-One Visual Studio 2005 from Wrox (Fall 2005). Paul is also the founder and chief architect for Software Conceptions, Inc, founded 1990. He is available to help design and build software worldwide. You may contact him for consulting opportunities or technology questions at firstname.lastname@example.org.
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