How Can I Build A Contiguous Range Like Union Without Using Excel

I know the function to only get blank cells/constant cells/formula cell/etc. using the UsedRange.SpecialCells. But here my concern is different, The following code demonstrate the making union of a range, it is just a sample in actual i open two excel files and compare the values, i will get the changed cell address and making a union range which contains all the changed cells.

I just want to replace the excel union with some UDF (User Defined Function) which will union the range and then return the address same as excel.

Sub UnionSample()

Dim objApp As Excel.Application
Dim objWB As Excel.Workbook
Dim objWS As Excel.Worksheet
Dim objRng As Excel.Range
Dim objUnionRng As Excel.Range
Dim strRange As String

Set objApp = New Excel.Application
'Any excel file containing more data.
Set objWB = objApp.Workbooks.Open("C:\1.xlsx", 0, True)
objApp.DisplayAlerts = False
objApp.ScreenUpdating = False
objApp.EnableEvents = False

For Each objWS In objWB.Worksheets
For Each objRng In objWS.UsedRange
If objUnionRng Is Nothing Then
Set objUnionRng = objRng
Else
'Want to replace this code with some customized function, which just return the address same as excel.
Set objUnionRng = objApp.Union(objUnionRng, objRng)
End If
Next
If Not objUnionRng Is Nothing Then
strRange = strRange & objWS.Name & ": " & objUnionRng.Address & vbNewLine
Set objUnionRng = Nothing
End If
Next

If Trim(strRange) <> "" Then
MsgBox strRange
End If

On Error Resume Next
objApp.DisplayAlerts = True
objApp.ScreenUpdating = True
objApp.EnableEvents = True
Set objRng = Nothing
Set objUnionRng = Nothing
Set objWS = Nothing
objWB.Close False
Set objWB = Nothing
objApp.Quit
Set objApp = Nothing

End Sub