I Have two Excel files having some contact numbers,

void CompareExcel()
        {

            string filePath = @"F:\Book2.xlsx";
 
            Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook xlWorkBook;

            try
            {
                xlWorkBook = xlApp.Workbooks.Open(filePath, 0, true, 5, "", "", false, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "\t", true, false, 0, true, 0);

                Microsoft.Office.Interop.Excel.Worksheet worksheet1 = (Microsoft.Office.Interop.Excel.Worksheet)xlApp.Worksheets["Sheet1"];
                Microsoft.Office.Interop.Excel.Worksheet worksheet2 = (Microsoft.Office.Interop.Excel.Worksheet)xlApp.Worksheets["Sheet2"];

                int sheet1LastRowCount = worksheet1.UsedRange.Rows.Count;
                int sheet2LastRowCount = worksheet2.UsedRange.Rows.Count;

                for (int i = 2; i < sheet1LastRowCount; i++)
                {
                    for (int j = 2; j < sheet2LastRowCount; j++)
                    {
                        if (worksheet1.Range["B" + i, "B" + i].Value2 == worksheet2.Range["B" + j, "B" + j].Value2)
                        {
                            Console.WriteLine(worksheet1.Range["A" + i, "A" + i].Value2);
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            finally
            {
                xlApp.Quit();
            }

            
        }