Can VBA Reach Across Instances of Excel?

Cornelius’ answer is partially correct. His code gets the current instance and then makes a new instance. GetObject only ever gets the first instance, no matter how many instances are available. The question I believe is how can you get a specific instance from among many instances.

For a VBA project, make two modules, one code module, and the other as a form with one command button named Command1. You might need to add a reference to Microsoft.Excel.

This code displays all the name of each workbook for each running instance of Excel in the Immediate window.

'------------- Code Module --------------

Option Explicit

Declare Function FindWindowEx Lib "User32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long
Declare Function GetClassName Lib "User32" Alias "GetClassNameA" (ByVal hWnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
Declare Function IIDFromString Lib "ole32" (ByVal lpsz As Long, ByRef lpiid As UUID) As Long
Declare Function AccessibleObjectFromWindow Lib "oleacc" (ByVal hWnd As Long, ByVal dwId As Long, ByRef riid As UUID, ByRef ppvObject As Object) As Long

Type UUID 'GUID
  Data1 As Long
  Data2 As Integer
  Data3 As Integer
  Data4(7) As Byte
End Type

'------------- Form Module --------------

Option Explicit

Const IID_IDispatch As String = "{00020400-0000-0000-C000-000000000046}"
Const OBJID_NATIVEOM As Long = &HFFFFFFF0

'Sub GetAllWorkbookWindowNames()
Sub Command1_Click()
    On Error GoTo MyErrorHandler

    Dim hWndMain As Long
    hWndMain = FindWindowEx(0&, 0&, "XLMAIN", vbNullString)

    Do While hWndMain <> 0
        GetWbkWindows hWndMain
        hWndMain = FindWindowEx(0&, hWndMain, "XLMAIN", vbNullString)
    Loop

    Exit Sub

MyErrorHandler:
    MsgBox "GetAllWorkbookWindowNames" & vbCrLf & vbCrLf & "Err = " & Err.Number & vbCrLf & "Description: " & Err.Description
End Sub

Private Sub GetWbkWindows(ByVal hWndMain As Long)
    On Error GoTo MyErrorHandler

    Dim hWndDesk As Long
    hWndDesk = FindWindowEx(hWndMain, 0&, "XLDESK", vbNullString)

    If hWndDesk <> 0 Then
        Dim hWnd As Long
        hWnd = FindWindowEx(hWndDesk, 0, vbNullString, vbNullString)

        Dim strText As String
        Dim lngRet As Long
        Do While hWnd <> 0
            strText = String$(100, Chr$(0))
            lngRet = GetClassName(hWnd, strText, 100)

            If Left$(strText, lngRet) = "EXCEL7" Then
                GetExcelObjectFromHwnd hWnd
                Exit Sub
            End If

            hWnd = FindWindowEx(hWndDesk, hWnd, vbNullString, vbNullString)
            Loop

        On Error Resume Next
    End If

    Exit Sub

MyErrorHandler:
    MsgBox "GetWbkWindows" & vbCrLf & vbCrLf & "Err = " & Err.Number & vbCrLf & "Description: " & Err.Description
End Sub

Public Function GetExcelObjectFromHwnd(ByVal hWnd As Long) As Boolean
    On Error GoTo MyErrorHandler

    Dim fOk As Boolean
    fOk = False

    Dim iid As UUID
    Call IIDFromString(StrPtr(IID_IDispatch), iid)

    Dim obj As Object
    If AccessibleObjectFromWindow(hWnd, OBJID_NATIVEOM, iid, obj) = 0 Then 'S_OK
        Dim objApp As Excel.Application
        Set objApp = obj.Application
        Debug.Print objApp.Workbooks(1).Name

        Dim myWorksheet As Worksheet
        For Each myWorksheet In objApp.Workbooks(1).Worksheets
            Debug.Print "     " & myWorksheet.Name
            DoEvents
        Next

        fOk = True
    End If

    GetExcelObjectFromHwnd = fOk

    Exit Function

MyErrorHandler:
    MsgBox "GetExcelObjectFromHwnd" & vbCrLf & vbCrLf & "Err = " & Err.Number & vbCrLf & "Description: " & Err.Description
End Function