C#-Code

using System;
using System.Data;
using System.Windows.Forms;
using System.IO;

using ExcelDataReader;

namespace read_an_excel_file__xls_or_xlsx
{
    public partial class Form1 : Form
    {
        public Form1()        {
            InitializeComponent();
        }

        DataSet result;
        private void btOpen_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog ofd = new OpenFileDialog()    {Filter = "Excel1|*.xls|Exel2|*.xlsx", ValidateNames = true })
            {
                if (ofd.ShowDialog() == DialogResult.OK)
                {
                    FileStream fs = File.Open(ofd.FileName, FileMode.Open, FileAccess.Read);
                    FileInfo fi = new FileInfo(ofd.FileName);
                    this.tb_filename.Text = fi.Name;
                    using (var reader = ExcelReaderFactory.CreateReader(fs))
                    {
                        result = reader.AsDataSet();
                        comboBox1.Items.Clear();
                        foreach (DataTable dt in result.Tables)
                            comboBox1.Items.Add(dt.TableName);
                        reader.Close();
                    }                
                }
            }
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)        {
            dataGridView1.DataSource = result.Tables[comboBox1.SelectedIndex];
        }

        private void dataGridView1_MouseClick(object sender, MouseEventArgs e)
        {
            this.tb_selected_cell.Text = 
            dataGridView1.SelectedCells[0].RowIndex.ToString()
+ "  " +
            dataGridView1.SelectedCells[0].ColumnIndex.ToString();

            string s = 
                dataGridView1[dataGridView1.SelectedCells[0].ColumnIndex, dataGridView1.SelectedCells[0].RowIndex].Value.ToString();
            this.tb_content.Text = s;
        }
    }
}

/*
Lösung basiert im Wesentlichen auf 

https://www.youtube.com/watch?v=JlzzdB3K-1M

https://stackoverflow.com/questions/27634477/using-exceldatareader-to-read-excel-data-starting-from-a-particular-cell
(If you are using ExcelDataReader 3+ you will find that there isn't any method for AsDataSet() for your reader object, 
You need to also install another package for ExcelDataReader.DataSet, then you can use the AsDataSet() method.)

und

https://www.youtube.com/watch?v=_h_4-HxrMMc

wichtig ist, dass ExcelDataReader und ExcelDataReader.DataSet über NuGet eingebunden werden.
*/