Using a passed string to access different class properties - vba

Question

Preface: I'm very much a dabbler at coding, and I'm not surprised my current code doesn't work, but I can't figure out whether I'm trying an approach that is fundamentally not possible or whether I just don't understand the correct syntax. Unlike the majority of the rest of my current project code, I haven't yet found the solution in the many other posts here.
Context: Playing Elite: Dangerous. I have a list of star systems in an Excel worksheet with each row containing one system (columns: name, x, y, z coordinates, and some properties such as Visited, RareGoodsSource). I've created a StarSystem class and read the worksheet into a Collection of StarSystems (named colSys). This works. For each property of the class I have a separate worksheet (columns: name, property) where I manually adjust property values (e.g. just visited Tau Ceti in game, on worksheet "csvVisited" manually add row "Tau Ceti", "TRUE"). In VBA I then compare those to the values in the Collection elements and update the latter if necessary. (Eventually I pump all this stuff to AutoCAD to visualise and plan travel routes.)
Issue: I currently have a separate Sub for each property, identical except for the name of the worksheet (e.g. "csvVisited" / "csvRareGoodsSource") and the references to access the property (e.g. colSys.Item(r.Value).Visited / colSys.Item(r.Value).RareGoodsSource). This works. But it seems Wrong from the perspectives of aesthetics, efficiency, and maintenance. Surely I should have only one Sub, which I pass Visited or RareGoodsSource as required?
My current code for this generic sub is at the end of the post, first I have an extremely abstracted version for clarity. My first attempt was to simply literally replace Visited with strProperty everywhere in the Sub, and pass Visited or RareGoodsSource to the Sub into that string variable.
This works fine for the worksheet reference, presumably because .Item() requires a string anyway. I am not entirely surprised it does not work for the property reference, because I'm passing a string variable in the hopes VBA understands this as an object property name, but I have been unable to find how one should do this. Hopefully it's just a result of my embarrassing lack of basic programming knowledge, and I just need some brackets or quotes or &'s somewhere.
Simplified example code, which works correctly (...except the bit that doesn't, obviously):
Sub TestVisited()
Call TestGeneric("Visited")
End Sub
Sub TestGeneric(strProperty As String)
Dim wsCSV As Worksheet
Set wsCSV = ActiveWorkbook.Worksheets.Item("csv" & strProperty)
'successfully sets wsCSV to Worksheets.Item("csvVisited"),
'presumably because .Item() expects a string anyway.
Dim r As Range
For Each r In wsCSV.Range(wsCSV.Cells(2, 1), wsCSV.Cells(4, 1))
Debug.Print "Explicitly coded: " & colSys.Item(r.Value).Visited
Debug.Print "Passed as string: " & colSys.Item(r.Value).strProperty
Next r
'The first Debug.Print works, the second does not:
'"Object doesn't support this property or method."
End Sub
The current real code for context:
(Note I've disabled the error trap on the .Contains replacement, because otherwise that would trap this problem instead.)
Sub UpdatePropertyFromWorksheetCSVProperty(strProperty As String)
'set the cell column/row positions in Worksheets.
Let celCSVDataColumn = 2
'prepare reference to Worksheet to read.
Dim wsCSV As Worksheet
Set wsCSV = ActiveWorkbook.Worksheets.Item("csv" & strProperty)
'prepare reference to Range to read.
Dim rngData As Range
Set rngData = wsCSV.Range(wsCSV.Cells(celFirstDataRow, celKeyColumn), wsCSV.Cells( _
wsCSV.Cells(wsCSV.Rows.Count, celKeyColumn).End(xlUp).Row _
, celKeyColumn)) ' middle segment finds the last occupied cell in column A and returns its row index.
'for each Worksheet row, compare the property value in the Worksheet to the value in the Collection Element,
'if different write the Worksheet value to the Collection Element, and flag the Element as ModifiedSinceRead.
Dim r As Range
For Each r In rngData
'check Sytem exists in the Collection.
'except VBA Collections don't have a .Contains method apparently.
'use error trapping instead.
'On Error GoTo ErrorHandler
'compare/copy Worksheet and Collection values.
If Not colSys.Item(r.Value).strProperty = r.Offset(0, celCSVDataColumn - 1).Value Then
On Error GoTo 0 'disables error trap again.
Let colSys.Item(r.Value).strProperty = r.Offset(0, celCSVDataColumn - 1).Value
Let colSys.Item(r.Value).xlsModifiedSinceRead = True
'DEBUG: test to immediate window
Debug.Print "System " & colSys.Item(r.Value).Name & " " & strProperty & " property changed to " & colSys.Item(r.Value).strProperty & "."
'
End If
ResumeNextSystem:
Next r
'DEBUG: test to immediate window
Debug.Print colSys(1).Name & vbTab & colSys(1).x & vbTab & colSys(1).RareGoodsSource & vbTab & colSys(1).RareGoodsChecked & vbTab & colSys(1).Visited & vbTab & colSys(1).xlsModifiedSinceRead
Debug.Print colSys(10160).Name & vbTab & colSys(10160).x & vbTab & colSys(10160).RareGoodsSource & vbTab & colSys(10160).RareGoodsChecked & vbTab & colSys(10160).Visited & vbTab & colSys(10160).xlsModifiedSinceRead
Debug.Print colSys("Lave").Name & vbTab & colSys("Lave").x & vbTab & colSys("Lave").RareGoodsSource & vbTab & colSys("Lave").RareGoodsChecked & vbTab & colSys("Lave").Visited & vbTab & colSys("Lave").xlsModifiedSinceRead
'
Exit Sub
ErrorHandler:
MsgBox ("Processing Worksheet " & wsCSV.Name & " error at system " & r.Value & ", skipping to next.")
'DEBUG: test to immediate window
Debug.Print "Processing Worksheet " & wsCSV.Name & " error at system " & r.Value & ", skipping to next."
'
Resume ResumeNextSystem
End Sub
Solution in real code:
'stays as-is:
Set wsCSV = ActiveWorkbook.Worksheets.Item("csv" & strProperty)
'Get old:
If Not colSys.Item(r.Value).strProperty = r.Offset(0, celCSVDataColumn - 1).Value Then
'new:
If Not CallByName(colSys.Item(r.Value), strProperty, VbGet) = r.Offset(0, celCSVDataColumn - 1).Value Then
'Let old:
Let colSys.Item(r.Value).strProperty = r.Offset(0, celCSVDataColumn - 1).Value
'new:
CallByName colSys.Item(r.Value), strProperty, VbLet, r.Offset(0, celCSVDataColumn - 1).Value

Answer 1

You can use CallByName built-in function to get the property.
v = CallByName(colSys.Item(r.Value), strProperty, vbGet)
This KB article explains it: https://support.microsoft.com/kb/186143