ConnectToConnectionPoint offers low-level alternative wiring for VBA events

I really like when I chance across some low-level technical wizardy to help out when VBA hits its limits ( it is what this blog does best!) Today, on StackOverflow a reward bounty of 500 is being awarded to a cracking answer which uses the ConnectToConnectionPoint Win32 API function call to sink events without using the WithEvents keyword.

So, I came across this StackOverflow Q & A where the questioner is asking how to reduce WithEvent declarations and subs with VBA and ActiveX and the responder provides a solution which uses ConnectToConnectionPoint to acquire events without using WithEvents.

The responder says they found the original code on a Japanese website and indeed I believe they are referring to this from Keiichi Tsunoda: Implementation of the event handling by API : ConnectToConnectionPoint. ConnectToConnectionPoint is defined in shlwapi.h which is part of the Windows Shell API (so it's not part of the original COM runtime API).

Googling a little more and I found a VBFormus post, a Mr Excel post and a GitHub Gist which I have placed in the Links section below.

How significant is this for Excel VBA? I do believe it is already possible to reduce the number of WithEvent declarations by introducing a class and holding an array of instances of those classes. Each class instance would be instantiated with the reference to a ActiveX control acquired using OLEObjects() for a worksheet or Controls() for a UserForm. However, the fact that the implementation of ConnectToConnectionPoint is in the Windows Shell library which is what Windows Desktop and the Windows Explorer use suggests that its use for sinking events from other Windows processes may have a more dramatic potential.

However, Mathieu Guindon who runs the RubberDuck project thinks this is a key technology to solving a glitch that had been an obstacle in implementing MVVM for VBA, here is his blog post Making MVVM Work in VBA Part 2 - Event Propagation

Links

• Microsoft Docs - Win32 apps - ConnectToConnectionPoint function (shlwapi.h)
• Stack Overflow - Reduce WithEvent declarations and subs with VBA and ActiveX
• Keiichi Tsunoda in Japan - http://addinbox.sakura.ne.jp/ - Implementation of the event handling by API : ConnectToConnectionPoint
• GitHub Gist - kumatti1 - Use ConnectToConnectionPoint to fire DocumentComplete
• VBForums - ConnectToConnectionPoint not working for sinking Ribbon CommandBarButton Click Event
• MrExcel Message Board - Single Class and EventHandler for multiple Control types in Userform
• Rubberduck News - Making MVVM Work in VBA Part 2 - Event Propagation
• Google - ConnectToConnectionPoint

Soft Links assist with Hard Link Hell

If you build an Excel application of any size then you will probably use more than one workbook. To access information in another workbook the standard way is to link. However, having workbooks linked to one another often leads to problems managing the opening and closing of linked workbooks. In this post I offer a 'soft link' which aims to break the hard links which come as a default and let your code take control.

There is such a thing as dependency hell where it required to gather antecedent code or data. A specific instance on Windows is DLL Hell concerning the loading of correct executable libraries. In Excel, we have our own form which I am calling 'Hard Link Hell'.

I call Hard Link Hell the mess that VBA coders can encounter when we build a VBA application of size that spans multiple workbooks. In my opinion, a VBA coder ought to exercise as much control as possible over the opening and closing of workbooks. Have a cell in one workbook link to another raises the spectre of Excel opening linked workbooks when we were not expecting it.

Admittedly, there is some control over the behaviour. So from the Data ribbon if I select Edit Links then I get the following dialog box...

... where we can see in the bottom right corner the Startup Prompt button which if pressed raises the following dialog ...

Nevertheless, I have had Hard Link Hell in the past where I have had the break links and relink to a new workbook. I remember it being a nightmare. So, in this post I give some code called Soft Links which means VBA code can take control of when to open linked workbooks. The code ships two functions to be called from a worksheet, SoftLink(workbookName, sheetName, rangeName) which actually return an Excel.Range object but Excel is clever enough to call the Value property; but this works only for single cell references. So for multiple cells use SoftLinkValue(workbookName, sheetName, rangeName). Be aware that the source cell(s) must be named using a range name. Also in the listing are some test procedures

Note, you will have to write code to open the source workbooks or you will get a #VALUE!, but we wanted to take control and so comes the responsibility to ensure the source workbook is loaded when this function is calculated. Enjoy!

Option Explicit

'* Use this for a source comprising multiple cells
Public Function SoftLinkValue(ByVal sWorkbookName As String, ByVal sSheetName As String, ByVal sRangeName As String)
    Dim rng As Excel.Range
    Set rng = SoftLink(sWorkbookName, sSheetName, sRangeName)
    SoftLinkValue = rng.Value
End Function

'* Use this for a source comprising single cell, also useful in other VBA code
Public Function SoftLink(ByVal sWorkbookName As String, ByVal sSheetName As String, ByVal sRangeName As String) As Excel.Range
    Dim wb As Excel.Workbook
    Set wb = OernColItem(Application.Workbooks, sWorkbookName)

    If Not wb Is Nothing Then

        Dim ws As Excel.Worksheet
        Set ws = OernColItem(wb.Worksheets, sSheetName)
        
        If Not ws Is Nothing Then
            Set SoftLink = OernWorksheetRange(ws, sRangeName)
        End If
    End If
End Function

Private Function OernWorksheetRange(ByRef ws As Excel.Worksheet, ByVal sRangeName As String) As Excel.Range
    On Error Resume Next
    Set OernWorksheetRange = ws.Range(sRangeName)
End Function

Private Function OernColItem(ByRef col As Object, ByVal idx As Variant) As Object
    On Error Resume Next
    Set OernColItem = col.Item(idx)
End Function

'**** TEST ****

Sub TestVBACallingSoftLink_LocalSheet()

    Const csSHEET1 As String = "Sheet1"

    Dim rng As Excel.Range
    Set rng = SoftLink(ThisWorkbook.Name, csSHEET1, "A1")
    
    Debug.Assert Not rng Is Nothing
    If Not rng Is Nothing Then
        Debug.Assert rng.Address = "$A$1"
        Debug.Assert rng.Worksheet.Name = csSHEET1
    End If
End Sub


Sub TestVBACallingSoftLink_ExternalWorkbook()
    Const csSHEET1 As String = "Sheet1"

    '*** test setup: create new workbook, add a name
    Dim wbNew As Excel.Workbook
    Set wbNew = Application.Workbooks.Add
    
    Const csNAME_FOO As String = "Foo"
    Dim ws As Excel.Worksheet
    Set ws = wbNew.Worksheets.Item(1)
    ws.Names.Add Name:=csNAME_FOO, RefersToR1C1:="=Sheet1!R4C8"
    ws.Range(csNAME_FOO).Value2 = 42
    '*** end of test setup:

    '*** now we can call our function to get a link to an external workbook
    Dim rng As Excel.Range
    Set rng = SoftLink(wbNew.Name, csSHEET1, csNAME_FOO)
    
    Debug.Assert Not rng Is Nothing
    If Not rng Is Nothing Then
        Debug.Assert rng.Address = "$H$4"
        Debug.Assert rng.Worksheet.Name = csSHEET1
        Stop
    End If
    wbNew.Close False
    Stop
End Sub