Try changing NumberFormat
to text:
workSheet.Cells[i + 1, 7].NumberFormat = "@";
workSheet.Cells[i + 1, 7] = list[i].Code;
Actually I suggest you to do it for entire range:
workSheet.Range["G1","G100"].NumberFormat = "@";
and then in loop:
workSheet.Cells[i + 1, 7] = list[i].Code;