Application not quitting after calling quit

Just Calling .Quit() will not remove the Application from memory. It is very important to close the objects after you are done with your coding. This ensures that all objects are released properly and nothing remains in the memory.

See this example

Imports Excel = Microsoft.Office.Interop.Excel
 
Public Class Form1
    '~~> Define your Excel Objects
    Dim xlApp As New Excel.Application
    Dim xlWorkBook As Excel.Workbook
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        '~~> Add a New Workbook
        xlWorkBook = xlApp.Workbooks.Add
 
        '~~> Display Excel
        xlApp.Visible = True
 
        '~~> Do some stuff Here
 
        '~~> Save the file
        xlWorkBook.SaveAs(Filename:="C:\Tutorial\SampleNew.xlsx", FileFormat:=51)
 
        '~~> Close the File
        xlWorkBook.Close()
 
        '~~> Quit the Excel Application
        xlApp.Quit()
 
        '~~> Clean Up
        releaseObject (xlApp)
        releaseObject (xlWorkBook)
    End Sub
 
    '~~> Release the objects
    Private Sub releaseObject(ByVal obj As Object)
        Try
            System.Runtime.InteropServices.Marshal.ReleaseComObject (obj)
            obj = Nothing
        Catch ex As Exception
            obj = Nothing
        Finally
            GC.Collect()
        End Try
    End Sub
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Me.Close()
    End Sub
End Class

Also worth mentioning is the 2 DOT Rule.

If you love automating Excel from VB.Net then you might also want to have a look at this link.

FOLLOWUP

The problem is the 2 DOT Rule as I mentioned above. When you use the 2 DOT Rule (Ex: Excel.XlBordersIndex.xlDiagonalDown) then you have to do the Garbage Collection by using GC.Collect(). So All you need to do is add this part

    Finally
        GC.Collect()

in the Private Sub ReleaseObject(ByVal obj As Object)

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

FINAL CODE

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim xlApp As New excel.Application
    Dim xlWorkBook As excel.Workbook
    Dim xlWorkSheet As excel.Worksheet
    Dim xlRange As excel.Range
    'Dim misValue As Object = System.Reflection.Missing.Value

    xlWorkBook = xlApp.Workbooks.Add
    xlWorkSheet = DirectCast(xlWorkBook.Sheets("sheet1"), excel.Worksheet)

    xlApp.Visible = True

    Dim headers = (From ch In DataGridView1.Columns _
                  Let header = DirectCast(DirectCast(ch, DataGridViewColumn).HeaderCell, DataGridViewColumnHeaderCell) _
                  Select header.Value).ToArray()
    Dim headerText() As String = Array.ConvertAll(headers, Function(v) v.ToString)

    Dim items() = (From r In DataGridView1.Rows _
          Let row = DirectCast(r, DataGridViewRow) _
          Where Not row.IsNewRow _
          Select (From cell In row.Cells _
              Let c = DirectCast(cell, DataGridViewCell) _
              Select c.Value).ToArray()).ToArray()

    Dim table As String = String.Join(vbTab, headerText) & Environment.NewLine
    For Each a In items
        Dim t() As String = Array.ConvertAll(a, Function(v) v.ToString)
        table &= String.Join(vbTab, t) & Environment.NewLine
    Next
    table = table.TrimEnd(CChar(Environment.NewLine))
    Clipboard.SetText(table)

    Dim alphabet() As Char = "abcdefghijklmnopqrstuvwxyz".ToUpper.ToCharArray

    xlRange = xlWorkSheet.Range("B2:" & alphabet(headerText.Length) & (items.Length + 2).ToString)

    xlRange.Select()
    xlWorkSheet.Paste()

    xlRange.Borders(excel.XlBordersIndex.xlDiagonalDown).LineStyle = excel.XlLineStyle.xlLineStyleNone
    xlRange.Borders(excel.XlBordersIndex.xlDiagonalUp).LineStyle = excel.XlLineStyle.xlLineStyleNone

    With xlRange.Borders(excel.XlBordersIndex.xlEdgeLeft)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlMedium
    End With
    With xlRange.Borders(excel.XlBordersIndex.xlEdgeTop)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlMedium
    End With
    With xlRange.Borders(excel.XlBordersIndex.xlEdgeBottom)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlMedium
    End With
    With xlRange.Borders(excel.XlBordersIndex.xlEdgeRight)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlMedium
    End With
    With xlRange.Borders(excel.XlBordersIndex.xlInsideVertical)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlThin
    End With
    With xlRange.Borders(excel.XlBordersIndex.xlInsideHorizontal)
        .LineStyle = excel.XlLineStyle.xlContinuous
        .ColorIndex = 1 'black
        .TintAndShade = 0
        .Weight = excel.XlBorderWeight.xlThin
    End With

    xlWorkBook.SaveAs(Filename:="C:\Users\Siddharth Rout\Desktop\Word1.xls", FileFormat:=56)
    xlWorkBook.Close()
    xlApp.Quit()

    ReleaseObject(xlRange)
    ReleaseObject(xlWorkSheet)
    ReleaseObject(xlWorkBook)
    ReleaseObject(xlApp)
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