Q:

Trying to apply bold to a whole row but keep getting null reference - NPOI

The 1st time I was applying bold to columns, but I only wanted a row to be bold not the whole column.  So I decided to do the same thing but using row. RowStyle.  Compiled without errors, but what I did get is a runtime error where is says r.RowStyle. SetFont(font);.  I made a class that to do with everything related to excel and it's in this class I get this error(r. RowStyle. SetFont(font);):
NullReferenceException was handled
Object reference not set to an instance of an object. 
Debugged the whole process and nothings null.  I don't understand why I get this error when using RowStyle and when I use CellStyle I don't get that error. 
This is my class:
using System;
using System. Collections. Generic;
using System. Linq;
using System. Text;
using System. Threading. Tasks;   

using NPOI.HSSF.UserModel;
using NPOI.HPSF;
using NPOI.POIFS.FileSystem;
using NPOI.SS.UserModel;

using System. IO;

//works with everything related to excel
namespace takeMyTime_text
{
    class exelSheet
    {
            string exelPath; // where it's being saved

        HSSFWorkbook wb2 = new HSSFWorkbook();

        ISheet sheet;

        IRow r;

        IFont font;

        String[] headerTitles = { "Date", "In", "Out", "In", "Out", "Description" };

        // assing values to class variables
        public void getValues(string path, string worksheetName)
        {
            exelPath = path;
        }

        //excel header
        public void header()
        {
            #region set bold properties

            font = wb2.CreateFont();

            font. FontHeightInPoints = 11;

            font. FontName = "Arial";

            font. Boldweight = (short)FontBoldWeight. Bold;

            #endregion

            sheet = wb2.CreateSheet("test sheet");

            //se tiene que usar esto cada vez que vallas a escribir en el mismo row
            r = sheet. CreateRow(0);

            r.RowStyle. SetFont(font);

            for (int i = 0; i < headerTitles. Length; i++)
            {
                r.CreateCell(i).SetCellValue(headerTitles[i]);
            }
        }

        //excel footer
        public void footer(int row, int col, string totalHours, int row2, int col2)
        {
            //ws. Cells[row, col] = new Cell("Worked hours:");

            //ws. Cells[row2, col2] = new Cell(totalHours);

            //wb. Worksheets. Add(ws);

            //wb. Save(exelPath);            
        }

        // write the date on the excel file
        public void writeDate(DateTime dt, int col, int row)
        {
            r = sheet. CreateRow(row);

            r.CreateCell(col).SetCellValue(dt. Month + "/" + dt. Day + "/" + dt. Year);          
        }

        //write and value on a cel
        public void writeValues(string text, int col, int row)
        {
            //r = sheet. CreateRow(row);

            r.CreateCell(col).SetCellValue(text);           
        }

        //guarda la info en un excel
        public void writeToFile()
        {
            FileStream file = new FileStream(exelPath, FileMode. Create);

            wb2.Write(file);

            file. Close();
        }
    }
}

A:

Looks like your RowStyle is null.  Try the following:
var style = wb2.CreateCellStyle();
style. SetFont(font);

r = sheet. CreateRow(0);
r.RowStyle = style;

