excel.Application xlApp = new excel.Application();
excel.Workbook xlWorkbook = xlApp.Workbooks.Add(Type.Missing);
excel.Worksheet xlWorksheet = xlWorkbook.ActiveSheet;
excel.Range xlRange = xlWorksheet.UsedRange;
foreach (excel.Worksheet sheet in xlWorkbook.Worksheets)
{
    string path = filepath;
    excel.Range pathrange = (excel.Range)sheet.Cells[1, 1];

    pathrange.Formula = "=HYPERLINK("" + path + "", "" + path +
        "")";
}