How to read an Excel cell value as string from a Visio add-in

Question

I am trying to read a single cell value from a Vision add-in from an Excel file the user selects with a File Dialog, then selects an Excel cell in Excel.

The user can select a range of more than one cell. however I will read only the first cell.

The below is VBA code from Visio Guy adapted for VB.NET:

Original code: http://visguy.com/vgforum/index.php?topic=7450.msg31415#msg31415

Sub GetExcelCellValue()

    Dim XlApp As Object
    Dim XlWrkbook As Excel.Workbook
    Dim XlSheet As Excel.Worksheet
    Dim rng As Excel.Range
    Dim docPath As String

    docPath = Application.ActiveDocument.Path
    XlApp = CreateObject("Excel.Application")

    ' msoFileDialogFilePicker = 3
    With XlApp.FileDialog(3)
        .Filters.Clear
        .Filters.Add("Excel Files", "*.xls, *.xlsx, *.xlsm")
        .InitialFileName = docPath
        .Show
        XlApp.Workbooks.Open(FileName:= .SelectedItems(1))
    End With

    XlWrkbook = XlApp.Workbooks(1)
    XlSheet = XlWrkbook.Worksheets("Sheet1")
    XlApp.Visible = True

    rng = XlApp.InputBox("Select a single cell", "Obtain Range Object", Type:=8)

    Dim FirstRow As String
    Dim FirstCol As String
    Dim FirstValue2 As String


    FirstRow = rng.Row
    FirstCol = rng.Column

    rng.Copy()

    'FirstValue2 = Convert.ToString(rng.Cells(FirstRow, FirstCol))
    FirstValue2 = Convert.ToString(rng.Worksheet.Cells(FirstRow, FirstCol).Value2)

    'Transfer Excel contents to Visio shapes on active page
    MsgBox("This is the first cell: " & FirstValue2)


    XlApp.Quit

End Sub

I tried several solutions that are given in the answers here on Stackoverflow: https://stackoverflow.com/questions/5646145/how-can-i-extract-a-string-from-an-excel-cell/41127000

,unfortunately those solutions get me these kind of errors:

Exception thrown: 'System.Reflection.TargetInvocationException' in mscorlib.dll Exception thrown: 'System.Runtime.InteropServices.COMException' in mscorlib.dll Exception thrown: 'System.Reflection.TargetInvocationException' in Microsoft.Office.Tools.Common.Implementation.dll

To test I did add a Range.Copy() which clearly shows the right Range is selected and copied, if I do a paste the cell content(s) show up as selected via the code as shown here.

How can I reliably read Excel cell values from a selected Excel cells into Visio? Thank you for sharing your insights and experience!