This worked for me using a windows form application
Imports Microsoft.Office.Interop.Excel
Public Class Form1
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim exl As New Microsoft.Office.Interop.Excel.Application()
Dim wb As Microsoft.Office.Interop.Excel.Workbook = exl.Workbooks.Open("C:\Test\ExcelTest.xlsx")
Dim ws As Microsoft.Office.Interop.Excel.Worksheet = TryCast(exl.ActiveSheet, Microsoft.Office.Interop.Excel.Worksheet)
exl.Visible = True
Dim rang As Range = ws.Cells(1, 1)
rang.Borders.Weight = Microsoft.Office.Interop.Excel.XlBorderWeight.xlThick
rang.Borders(Microsoft.Office.Interop.Excel.XlBordersIndex.xlEdgeBottom).LineStyle = Microsoft.Office.Interop.Excel.XlLineStyle.xlDashDotDot
'''''wb.Close(True, Type.Missing, Type.Missing)
''''exl.Quit()
End Sub
End Class