Reading data from selection in Excel file via C#

I'm trying to get data from Excel cells.

I'm using:

using Microsoft.Office.Interop.Excel;
using Application = Microsoft.Office.Interop.Excel.Application;

'selectedRange' is null.

I have to use .Net 4.7.2

Code below

 private void exportButton_Click(object sender, EventArgs e)
        {
            Application excelApp = null;
            Range selectedRange = null;

            try
            {
                excelApp = (Application)Marshal.GetActiveObject("Excel.Application");
                selectedRange = excelApp.Selection as Range;

                if (selectedRange != null)
                {
                    var selectedValues = (object[,])selectedRange.Value;

                    var rowCount = selectedValues.GetLength(0);
                    var columnCount = selectedValues.GetLength(1);

                    for (var row = 2; row <= rowCount; row++)
                    for (var col = 1; col < columnCount; col++)
                    {
                        "work with data here"
                    }


                    excelApp.Quit();
                }
                else
                {
                    MessageBox.Show("Selection is Null");
                }
            }
            catch (COMException ex)
            {
                debugConsole.AppendText(ex + Environment.NewLine);
            }
            finally
            {
                if (selectedRange != null)
                    Marshal.ReleaseComObject(selectedRange);
                if (excelApp != null)
                    Marshal.ReleaseComObject(excelApp);
            }
        }