Цикл по столбцам Excel Файла

using System;
using Exel = Microsoft.Office.Interop.Excel;
 
class Program
{
    static void Main(string[] args)
    {
        Exel.Application app = new Microsoft.Office.Interop.Excel.Application();
        app.Workbooks.Add(Type.Missing);
        Exel.Worksheet sheet = (Exel.Worksheet)app.ActiveSheet;
        for (int i = 1; i < 100; i++)
            sheet.Cells[1, i] = i;
        app.Visible = true;
    }
}

a=sheet.cells[5,10];

                excelapp = new Excel.Application();
                excelapp.Visible = false;
                excelapp.DisplayAlerts = true;
                excelappworkbooks = excelapp.Workbooks;
                excelappworkbook = excelapp.Workbooks.Open(@"C:\Fact" + textBox3.Text + "Data.xls",
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing);
                excelsheets = excelappworkbook.Worksheets;
 
                //Excel.Application app = new Microsoft.Office.Interop.Excel.Application();
                //app.Workbooks.Add(Type.Missing);
                Excel.Worksheet sheet = (Excel.Worksheet)excelappworkbook.ActiveSheet;
                label24.Text = sheet.Cells[2, 2].ToString();

sheet.Cells[2, 2].Value.ToString();