How Do I Create an Extension of a Single Class Property - vb.net

Question

I have a primitive Class that looks like this:
Public Class BaseGeoData
Property GeoOrigin As String
Property GeoDestination As String
Property TravelDistance As Double?
Property TravelTime As Double?
Public Sub New()
End Sub
End Class
Public Class GeoData
Inherits BaseGeoData
Public Sub New(geoOrigStr As String, geoDestStr As String)
GeoOrigin = geoOrigStr
GeoDestination = geoDestStr
TravelDistance = 5000 'in meters
TravelTime = 360 'in minutes
End Sub
End Class
I want to be able to add 2 extensions that will return converted values like this:
TravelDistance.ToMiles()
TravelTime.ToHours()
When I add a Module to extend the class, it offers the extension to the entire class, most properties of which will never use the extension. How can I just offer the extensions to the properties that need them?

Answer 1

Introduce own type of "Unit" for measurement values
Public MustInherit Class Unit
Public ReadOnly Property Value As Double
Public MustOverride ReadOnly Property Name As String
Public Sub New(value As Double)
Me.Value = value
End Sub
Public Overrides Function ToString() As String
Return $"{Value} {Name}"
End Function
End Class
Public Class Meter
Inherits Unit
Public Sub New(value As Double)
MyBase.New(value)
End Sub
Public Overrides ReadOnly Property Name As String
Get
Return "m"
End Get
End Property
End Class
Public Class Mile
Inherits Unit
Public Sub New(value As Double)
MyBase.New(value)
End Sub
Public Overrides ReadOnly Property Name As String
Get
Return "mi"
End Get
End Property
End Class
And extension methods for creating unit and convertions
Public Module UnitConversions
<Extension>
Public Function Meters(value As Integer) As Meter
Return New Meter(value)
End Function
<Extension>
Public Function Miles(value As Integer) As Mile
Return New Mile(value)
End Function
<Extension>
Public Function ToMiles(meters As Meter) As Mile
Dim miles = meters.Value * 0.00062137
Return New Mile(miles)
End Function
<Extension>
Public Function ToMeters(miles As Mile) As Meter
Dim meters = miles.Value * 1609.344
Return New Meter(meters)
End Function
End Module
Then you can use value in more readable manner
TravelDistance = 5000.Meters() ' meters
' Conversion
geoData.TravelDistance.ToMiles() ' miles
Console.WriteLine(geoData.TravelDistance) ' print 3.10685 mi

Answer 2

You can only add extension methods into types (i.e. classes).
TravelDistance is of type Double? so you have to add an extention method into Double?.
Note that it would make the method available for every Double?, which may not be something you want.

Answer 3

I really like Plutonix's resolution and is the same one I would go for first.
Its simple and resolves your initial problem.
Public Class BaseGeoData
Property GeoOrigin As String
Property GeoDestination As String
Property TravelDistance As Double?
Property TravelTime As Double?
Public Sub New()
End Sub
End Class
Public Class GeoData
Inherits BaseGeoData
Public Sub New(geoOrigStr As String, geoDestStr As String)
GeoOrigin = geoOrigStr
GeoDestination = geoDestStr
TravelDistance = 5000 'in meters
TravelTime = 360 'in minutes
End Sub
Function DistanceMiles() As Double
DistanceMiles = (TravelDistance/1609.344)
End Function
Function TimeHours() As Double
DistanceMiles = (TravelTime /60)
End Function
End Class