In continuation from here

We already covered here how to create the form, set references and place the required buttons. Now we will open an existing workbook.

If you want to open an existing Excel file then we will not use .Add method. Instead we will use the .Open method. Double click on the Open button and paste the code from Button2_Click() below. This is how your code will look when you want to open an Excel file.

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
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        '~~> Opens an existing Workbook. Change path and filename as applicable
        xlWorkBook = xlApp.Workbooks.Open("C:\TutorialSample.xlsx")

        '~~> Display Excel
        xlApp.Visible = True
    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

Now when you run the project, your form will launch and when you click on “Open” button, Excel will launch and the relevant workbook will open.

On a separate note, if you do not want to hardcode the file name then you can also use the openFileDialog to choose the file as shown below.

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim openFileDialog1 As New OpenFileDialog()

        '~~> Change the parameters for openFileDialog here
        openFileDialog1.InitialDirectory = "c:\"
        openFileDialog1.Filter = "Excel files (*.xls*)|*.xls*"
        openFileDialog1.Title = "Select Site Master to use"
        openFileDialog1.RestoreDirectory = True

        If openFileDialog1.ShowDialog() = System.Windows.Forms.DialogResult.OK Then
            Dim xlApp As New Excel.Application
            Dim xlWorkBook As Excel.Workbook

            With xlApp
                .Visible = True

                '~~> Open workbook
                xlWorkBook = .Workbooks.Open(openFileDialog1.FileName)
            End With
        End If
    End Sub