对象被创建为超类，想要将其变成子类

Question

I was given a library with generic data objects in it, Projects, Tables, Rows and Fields. In my program I have a single Project called Building, its Table 1 contains "Units", and Field 1 of Table 1 is "Unitname". So what I did was...

Public Class Unit
  Private myRow As Row
  Public Sub New(ByRef R as Row)
    myRow = Row
  End Sub
  Public Function Unitname() as String
    Return myRow.Fields(1)
  End Function
  ... etc ...
End Class

That doesn't look so bad does it? But I don't have Unit objects loaded, I have Rows. So then I also have to make a another class...

Public Class Building
  Private myProj as Project
...
  Public Function Units() As List(Of Unit)
    Dim ans as New List(Of Unit)
    For Each R In myProj.Tables(1).Rows
      ans.Add(new Unit(R))
    Next
    Return ans
    ' or I could use myProj.Tables(1).Rows.ConvertTo(Of Unit)
 End Function
 ...etc...

And of course I have to make the list of Buildings so I can get to the list of Units, and the dozens of other lists and accessors, turning into thousands of lines of code who's only purpose is to make one set of objects that points to another.

It works, and I know shipping is a feature. But what I really want to do is make a Unit that looks like...

Public Class Unit
  Inherits Row
  ...
End Class

Then I would "reverse cast" the List(Of Row) into a List(Of Unit). This would not only eliminate a lot of code, it would eliminate dangling pointers, reduce memory a whole lot, and eliminate a set-up set that might take some time. Theoretically possible, there's no difference except code, but I don't see a way to do it in VB.Net.

In Obj-C this is called swizzling (and/or extensions), and I think Java has a similar concept. I suspect that ADO.Net has to do something like this? Is there some sort of "wrap this in that" functionality I'm missing?

Answer 1

Save yourself a lot of time and use a DataSet. For database apps, object-oriented programming doesn't work too well. DataSets can be in-memory only and don't need a database engine.

Dim dsProject As New DataSet("Project")
Dim dtUnits As New DataTable("Units")
dtUnits.Columns.Add("Unitname", GetType(String))
dtUnits.Columns.Add("Address", GetType(String))
dsProject.Tables.Add(dtUnits)
dtUnits.Rows.Add("Unit 1", "1 Test Street")
dtUnits.Rows.Add("Unit 2", "2 Sample Avenue")
dtUnits.Rows.Add("Unit 3", "3a Demo Road")

Dim drMatch() As DataRow = dtUnits.Select("Unitname='Unit 1'")
If drMatch.GetUpperBound(0) >= 0 Then
  MsgBox(drMatch(0).Item("Address"))
Else
  MsgBox("No matching record")
End If

Answer 2

If you don't want to use DataSets, you could try a shared converter function:

  Class unit
    Inherits row

    Public Property UnitName() As String
      Get
        Return fields(1)
      End Get
      Set(ByVal value As String)
        fields(1) = value
      End Set
    End Property

    Shared Function UnitToRow(u As unit) As row
      Return DirectCast(u, row)
    End Function

    Shared Function RowToUnit(r As row) As unit
      Dim u As New unit
      u.fields = r.fields
      Return u
    End Function

  End Class

  Sub usage()
    Dim lstR As New List(Of row) '<--- source data

    Dim lstU As List(Of unit) = lstR.ConvertAll(New Converter(Of row, unit)(AddressOf unit.RowToUnit))
    MsgBox(lstU(0).UnitName)

    lstU(0).UnitName = "xxx"

    Dim lstR2 As List(Of row) = lstU.ConvertAll(New Converter(Of unit, row)(AddressOf unit.UnitToRow))
    MsgBox(lstR2(0).fields(1))

  End Sub