Programmatically add workbook event and save it - vba

Question

I have a list of excel files in a spreadsheet. I'd like to loop through them and add a worksheet event to each. Save it, close it and move on to the next. The problem is that when I reopen (manually) the workbook the code is gone.
Inside the for each loop:
Set xl = Workbooks.Open(filepath)
addCode xl 'subroutine to add code
xl.Save
xl.Close SaveChanges:=False
The addCode subroutine is:
Sub addCode(book As Excel.Workbook)
acsh = book.ActiveSheet.CodeName
startline = book.VBProject.VBComponents(acsh).CodeModule.CreateEventProc("SelectionChange", "Worksheet") + 1
book.VBProject.VBComponents(acsh).CodeModule.InsertLines startline, codetoadd
End Sub
If I comment out xl.Close the code is in the workbook and works. I can manually save and close the file and the code remains. I've added a break point between xl.save and xl.close and made a copy of the file. After the code is done neither has the changes. I've tried using xl.saveas and xl.close SaveChanges:=True. All have identical results.
I'm using Excel 2013, I've told excel to trust access to the VBA object model. I've tried using XLS files and XLSM files. Obviously XLSX won't work.

Answer 1

Here is some sample code which is working for me on Excel 2010. The changes I made to your example code are:
use a .xlsm for the target workbook - I know you said you already did that.
reference a specific worksheet in the AddCode sub rather than pick up the sheet name from ActiveSheet.
set the workbook dirty status per Ralph's comment
Don't set the SaveChanges flag when closing the target workbook
Other than that, my version is pretty similar to yours. I think it is the wb.Saved = False line that does the trick i.e. the dirty flag. I tried to use the SaveAs method on the VBProject itself thinking it would be the same as hitting the save button when you are in the VBA Editor itself. However, this just gives unhelpful errors.
Here's the sample code:
Option Explicit
Sub Test()
Dim wbTarget As Workbook
Dim strCode As String
' get target workbook
Set wbTarget = Workbooks.Open("\\server\path\Book3.xlsm")
' test setting code to worksheet change
strCode = "Debug.Print ""Sheet selection changed to: "" & Target.Address"
AddWorksheetChangeCode wbTarget, "Sheet1", strCode
' test saving the target workbook
With wbTarget
' set book to dirty to force the save
.Saved = False
.Save
.Close
End With
End Sub
Sub AddWorksheetChangeCode(ByRef wb As Workbook, strWorksheetName As String, strCode As String)
Dim intInsertLine As Integer
' create stub for event and get line to insert
intInsertLine = wb.VBProject.VBComponents(strWorksheetName).CodeModule.CreateEventProc("SelectionChange", "Worksheet") + 1
' add event logic
wb.VBProject.VBComponents(strWorksheetName).CodeModule.InsertLines intInsertLine, strCode
End Sub