自己驗證收藏的程式碼, 用來備份程式碼, 內容全部來自網上, 時間有些久遠, 因此沒有標註出處.

using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Linq;
using System.Text;

class ClassExcelExport
{
    
 
/// <summary>
    /// Excel匯入成Datable
    /// </summary>
    /// <param name="file">匯入路徑(包含檔名與副檔名)</param>
    /// <returns></returns>
    public static DataTable ExcelToTable(string file)
    {
        DataTable dt = new DataTable();
        IWorkbook workbook;
        string fileExt = Path.GetExtension(file).ToLower();
        
 
using (FileStream fs = new FileStream(file, FileMode.Open, FileAccess.Read))
        {
            //XSSFWorkbook 適用XLSX格式，HSSFWorkbook 適用XLS格式
            if (fileExt == ".xlsx") { workbook = new XSSFWorkbook(fs); } else if (fileExt == ".xls") { workbook = new HSSFWorkbook(fs); } else { workbook = null
 
; }
            if (workbook == null) { return null; }
            ISheet sheet = workbook.GetSheetAt(0);

            //表頭  
            IRow header = sheet.GetRow(sheet.FirstRowNum);
            List<int> columns = new List<int>();
            for (int i = 0; i < header.LastCellNum; i++)
            {
                object obj = GetValueType(header.GetCell(i));
                if (obj == null || obj.ToString() == string.Empty)
                {
                    dt.Columns.Add(new DataColumn("Columns" + i.ToString()));
                }
                else
                    dt.Columns.Add(new DataColumn(obj.ToString()));
                columns.Add(i);
            }
            //資料  
            for (int i = sheet.FirstRowNum + 1; i <= sheet.LastRowNum; i++)
            {
                DataRow dr = dt.NewRow();
                bool hasValue = false;
                foreach (int j in columns)
                {
                    dr[j] = GetValueType(sheet.GetRow(i).GetCell(j));
                    if (dr[j] != null && dr[j].ToString() != string.Empty)
                    {
                        hasValue = true;
                    }
                }
                if (hasValue)
                {
                    dt.Rows.Add(dr);
                }
            }
        }
        return dt;
    }

    /// <summary>
    /// Datable匯出成Excel
    /// </summary>
    /// <param name="dt"></param>
    /// <param name="file">匯出路徑(包括檔名與副檔名)</param>
    public static void TableToExcel(DataTable dt, string file)
    {
        IWorkbook workbook;
        string fileExt = Path.GetExtension(file).ToLower();
        if (fileExt == ".xlsx") { workbook = new XSSFWorkbook(); } else if (fileExt == ".xls") { workbook = new HSSFWorkbook(); } else { workbook = null; }
        if (workbook == null) { return; }
        ISheet sheet = string.IsNullOrEmpty(dt.TableName) ? workbook.CreateSheet("Sheet1") : workbook.CreateSheet(dt.TableName);

        //表頭  
        IRow row = sheet.CreateRow(0);
        for (int i = 0; i < dt.Columns.Count; i++)
        {
            ICell cell = row.CreateCell(i);
            cell.SetCellValue(dt.Columns[i].ColumnName);
        }

        //資料  
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            IRow row1 = sheet.CreateRow(i + 1);
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                ICell cell = row1.CreateCell(j);
                cell.SetCellValue(dt.Rows[i][j].ToString());
            }
        }

        //轉為位元組陣列  
        MemoryStream stream = new MemoryStream();
        workbook.Write(stream);
        var buf = stream.ToArray();

        //儲存為Excel檔案  
        using (FileStream fs = new FileStream(file, FileMode.Create, FileAccess.Write))
        {
            fs.Write(buf, 0, buf.Length);
            fs.Flush();
        }
    }

    /// <summary>
    /// 獲取單元格型別
    /// </summary>
    /// <param name="cell"></param>
    /// <returns></returns>
    private static object GetValueType(ICell cell)
    {
        if (cell == null)
            return null;
        switch (cell.CellType)
        {
            case CellType.Blank: //BLANK:  
                return null;
            case CellType.Boolean: //BOOLEAN:  
                return cell.BooleanCellValue;
            case CellType.Numeric: //NUMERIC:  
                return cell.NumericCellValue;
            case CellType.String: //STRING:  
                return cell.StringCellValue;
            case CellType.Error: //ERROR:  
                return cell.ErrorCellValue;
            case CellType.Formula: //FORMULA:  
            default:
                return "=" + cell.CellFormula;
        }
    }
}

 private void btnExport_Click(object sender, EventArgs e)
        {
            string fileName=System.Threading.Thread.GetDomain().BaseDirectory + "匯出Excel檔案/" + _fileName + ".xls";
            ClassExcelExport.TableToExcel(qrydt, fileName);
            MessageBox.Show("匯出完成.");
            System.Diagnostics.Process.Start(fileName);
        }