how to put a line break in excel cell in NPOI in C#?

Try following:

 IWorkbook wb = new HSSFWorkbook();
 ICellStyle style = wb.CreateCellStyle();
 style.BorderBottom = BorderStyle.Thin;
 style.BorderTop = BorderStyle.Thin;
 style.BorderLeft = BorderStyle.Thin;
 style.BorderRight = BorderStyle.Thin;
 style.WrapText = true;

 ISheet sheet = wb.CreateSheet("Orders");
 IRow row = sheet.CreateRow(0);

 AddStyledCell(row, 0, hstyle).SetCellValue("#");
 AddStyledCell(row, 1, hstyle).SetCellValue("NAME");
 AddStyledCell(row, 2, hstyle).SetCellValue("ADDRESS");

row = sheet.CreateRow(i++);
AddStyledCell(row, 0, style).SetCellValue("1");
AddStyledCell(row, 1, style).SetCellValue("name");
AddStyledCell(row, 2, style).SetCellValue("line1" + Environment.NewLine + "line2");

In this WrapText = true; is reqired.

Yes, Finally I found the solution. Just remember two things

set style:

style.WrapText = true;

and second

AddStyledCell(row, 2, style).SetCellValue("line1" + Environment.NewLine + "line2");

Thanks...

Also, we can use 'HSSFRichTextString' as:

using NPOI.HSSF.UserModel;

HSSFRichTextString richString = new HSSFRichTextString("line1\nline2");
AddStyledCell(row, 2, style).SetCellValue(richString);