Question

I did create a XDateTime class that is able to handle inaccurate date and time.

This class has all the CType operators required to convert to and from a string and it has been fully tested during the last months.

    Public Shared Widening Operator CType(ByVal xDateTime As FrameworkBL.XDateTime) As String
    Dim retrunValue As String = Nothing

    If xDateTime Is Nothing Then
        retrunValue = Nothing
    Else
        retrunValue = xDateTime.StringValue
    End If

    Return retrunValue
End Operator

Public Shared Narrowing Operator CType(ByVal value As String) As FrameworkBL.XDateTime
    Dim returnValue As FrameworkBL.XDateTime = Nothing


    If String.IsNullOrEmpty(value) Then
        returnValue = Nothing
    Else
        returnValue = New FrameworkBL.XDateTime(value)
    End If

    Return returnValue

End Operator

However, when a ByRef object parameter return a string, my CType operators seems to be ignored and a cast Exception is raised.

    Private Sub Test()
    Dim myXDateTime As FrameworkBL.XDateTime

    myXDateTime = "200101010000007" 'Ok
    Me.Temp1(myXDateTime) 'Ok
    Me.Temp2(myXDateTime) 'Ok
    Me.Temp3(myXDateTime) 'Unable to cast object of type 'System.String' to type 'FrameworkBL.XDateTime'

End Sub

Private Sub Temp1(ByRef myObject As String)
    myObject = "200201010000007"
End Sub

Private Sub Temp2(ByRef myObject As XDateTime)
    myObject = "200301010000007"
End Sub

Private Sub Temp3(ByRef myObject As Object)
    myObject = "200401010000007"
End Sub

This kind of problem is documented by Microsoft but I couldn’t find a working solution to fix this problem. Am I in a dead end or is there an option that would allow me to keep my ByRef Object parameter??

Answer 1

The solution is not to write this code. In fact, your code shouldn’t even compile, and never mind the ByRef. You simply cannot assign a string to an Object, and rightly so. You should enable Option Strict On in your project settings.

This is the correct and expected behaviour.

Apart from that, your operator implementations can (and should) be drastically shortened by taking advantage of the VB If operator. This makes the redundant initialisation and assignments go away:

Public Shared Widening Operator CType(ByVal xDateTime As XDateTime) As String
    Return If(xDateTime Is Nothing, Nothing, xDateTime.StringValue)
End Operator

Same for the other way round. That said, I don’t think conversion operators should ever work on Nothing values. This just makes Nothing creep into code where it shouldn’t.