Excel.Application instance = null;
try
{
      instance = (Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
}
catch (Exception ex)
{
      MessageBox.Show(@"Ошибка выполнения" + ex.Message);
}

Excel.Workbook instanceExcel = null;
Excel.Worksheet MyWorkSheet = null;
try
{          
     instanceExcel = (Excel.Workbook)System.Runtime.InteropServices.Marshal.BindToMoniker(@"D:\111.xlsx");
     MyWorkSheet = (Excel.Worksheet)instanceExcel.ActiveSheet;

...