How to protect certain columns in an Excel file from modifications?

I'm creating an Excel file using C#, but I want some columns to be protected from modification. The problem is that I don't know how many rows I'm going to have in that file. I used the following code,I know there is a Range, but I need a universal solution when I don't know how many rows an Excel file has:

xlWorkSheet.Cells.Locked = false;
xlWorkSheet.Range["A1:A3"].Locked = true;
xlWorkSheet.Range["B1"].Locked = true;
xlWorkSheet.Range["C1"].Locked = true;
xlWorkSheet.Range["D1:D3"].Locked = true;
xlWorkSheet.Range["E1:E3"].Locked = true;
xlWorkSheet.Range["F1:F3"].Locked = true;
xlWorkSheet.Protect(misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue, misValue);