Can't properly close Excel application - excel

Question

i am very new to VB.net and i'm trying to proceed step by step with my application.
The application i'm trying to build will collect a series of macros i've written in Excel VBA environment.
Now, the following code pasted below, is the initial part, where basically i try to load a workbook (to be used as Active workbook) and to "unload it".
The issue comes when, after "unloading" the workbook, i try to open the very same workbook in excel.
Excel application return an error that is "Open in read-only". This cannot be accepted, and i need to understand how to unload the workbook and release it from the myAPP.
Imports Excel = Microsoft.Office.Interop.Excel
Public Class Form1
Dim workbook As Excel.Workbook
Dim worksheet As Excel.Worksheet
Dim APP As New Excel.Application
Private Sub opn_btn_Click(sender As Object, e As EventArgs) Handles opn_btn.Click
Dim strname As String
Dim cellname As String
With OpenFileDialog1
.InitialDirectory = "E:\Vs_Excel"
.Title = "Open xlsx file"
.ShowDialog()
End With
workbook = APP.Workbooks.Open(OpenFileDialog1.FileName)
worksheet = workbook.Worksheets("sheet1")
cellname = worksheet.Range("A1").Value
strname = OpenFileDialog1.FileName
Me.TextBox1.Text = strname
Me.TextBox2.Text = cellname
Dim lvwReport As View = View.List
With Me.ListView1
.GridLines = True
.View = lvwReport
.CheckBoxes = True
End With
'LoadListView() 'thi sroutine is written but not used yet. Must solve first the problem wioth closing ExcelApplication
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
workbook.Close()
APP.Quit()
Me.TextBox1.Text = ""
Me.TextBox2.Text = ""
ReleaseObject(worksheet)
worksheet = Nothing
ReleaseObject(workbook)
workbook = Nothing
ReleaseObject(APP)
APP = Nothing
End Sub
Private Sub ReleaseObject(ByVal obj As Object)
Try
Dim intRel As Integer = 0
Do
intRel = System.Runtime.InteropServices.Marshal.ReleaseComObject(obj)
Loop While intRel > 0
MsgBox("Final Released obj # " & intRel)
Catch ex As Exception
MsgBox("Error releasing object" & ex.ToString)
obj = Nothing
Finally
GC.Collect()
End Try
End Sub
End class