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How to: Define a Conversion Operator (Visual Basic)

If you have defined a class or structure, you can define a type conversion operator between the type of your class or structure and another data type (such as Integer, Double, or String).

Define the type conversion as a CType Function (Visual Basic) procedure within the class or structure. All conversion procedures must be Public Shared, and each one must specify either Widening (Visual Basic) or Narrowing (Visual Basic).

Defining an operator on a class or structure is also called overloading the operator.

The following example defines conversion operators between a structure called digit and a Byte.

Public Structure digit
Private dig As Byte 
    Public Sub New(ByVal b As Byte)
        If (b < 0 OrElse b > 9) Then Throw New System.ArgumentException(
            "Argument outside range for Byte")
        Me.dig = b
    End Sub 
    Public Shared Widening Operator CType(ByVal d As digit) As Byte 
        Return d.dig
    End Operator 
    Public Shared Narrowing Operator CType(ByVal b As Byte) As digit
        Return New digit(b)
    End Operator 
End Structure

You can test the structure digit with the following code.

Public Sub consumeDigit()
    Dim d1 As New digit(4)
    Dim d2 As New digit(7)
    Dim d3 As digit = CType(CByte(3), digit)
    Dim s As String = "Initial 4 generates " & CStr(CType(d1, Byte)) &
          vbCrLf & "Initial 7 generates " & CStr(CType(d2, Byte)) &
          vbCrLf & "Converted 3 generates " & CStr(CType(d3, Byte))
    Try 
        Dim d4 As digit
        d4 = CType(CType(d1, Byte) + CType(d2, Byte), digit)
    Catch e4 As System.Exception
        s &= vbCrLf & "4 + 7 generates " & """" & e4.Message & """" 
    End Try 
    Try 
        Dim d5 As digit = CType(CByte(10), digit)
    Catch e5 As System.Exception
        s &= vbCrLf & "Initial 10 generates " & """" & e5.Message & """" 
    End Try
    MsgBox(s)
End Sub
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