C# 基于NPOI+Office COM组件 实现20行代码在线预览文档（word，excel，pdf，txt，png）

using Microsoft.Office.Interop.Excel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Mvc;
namespace WebOnlineWord.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }
        //C#在线预览文档（word，excel，pdf，txt，png）
        //1、预览方式：将文件转换成html文件然后预览html文件
        //2、预览word文件：需要引入Interop.Microsoft.Office.Interop.Word.dll（Office COM组件）
        //3、预览Excel文件：需要引入Interop.Microsoft.Office.Interop.Excel.dll（Office COM组件) 
        //4、PDF文件直接嵌入到浏览器中进行查看，无需转换（需安装pdf阅读器）
        //5、文本文件直接嵌入到浏览器进行查看，无需转换
        //6、图片文件直接嵌入到浏览器进行查看，无需转换
        #region Excel预览方法
        /// <summary>
        ///  excel 转换为html
        /// </summary>
        /// <param name="path">要转换的文档的路径</param>
        /// <param name="savePath">转换成的html的保存路径</param>
        /// <param name="wordFileName">转换后html文件的名字</param>
        public JsonResult ExcelToHtml()
        {
            ResultJson result = new ResultJson();
            string path = Server.MapPath("/Content/excel.xlsx");  
            string savePath = Server.MapPath("/Content/"); 
            string wordFileName = "ExcelToHtml";
            string str = string.Empty;
            Microsoft.Office.Interop.Excel.Application repExcel = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook workbook = null;
            Microsoft.Office.Interop.Excel.Worksheet worksheet = null;
            workbook = repExcel.Application.Workbooks.Open(path, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            worksheet = (Microsoft.Office.Interop.Excel.Worksheet)workbook.Worksheets[1];
            object htmlFile = savePath + wordFileName + ".html";
            string resultUrl = htmlFile.ToString();
            object ofmt = Microsoft.Office.Interop.Excel.XlFileFormat.xlHtml;
            workbook.SaveAs(htmlFile, ofmt, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            object osave = false;
            workbook.Close(osave, Type.Missing, Type.Missing);
            repExcel.Quit();
            result.str = "/Content/" + wordFileName + ".html"; ;
            return Json(result, JsonRequestBehavior.AllowGet);
        }
        #endregion
        #region Excel预览方法
        /// <summary>
        ///  word 转换为html
        /// </summary>
        /// <param name="path">要转换的文档的路径</param>
        /// <param name="savePath">转换成的html的保存路径</param>
        /// <param name="wordFileName">转换后html文件的名字</param>
        public JsonResult WordToHtml()
        {
            ResultJson result = new ResultJson();
            string path = Server.MapPath("/Content/word.docx");
            string savePath = Server.MapPath("/Content/");
            string wordFileName = "WordToHtml";
            Microsoft.Office.Interop.Word.Application word = new Microsoft.Office.Interop.Word.Application();
            Type wordType = word.GetType();
            Microsoft.Office.Interop.Word.Documents docs = word.Documents;
            Type docsType = docs.GetType();
            Microsoft.Office.Interop.Word.Document doc = (Microsoft.Office.Interop.Word.Document)docsType.InvokeMember("Open", System.Reflection.BindingFlags.InvokeMethod, null, docs, new Object[] { (object)path, true, true });
            Type docType = doc.GetType();
            string strSaveFileName = savePath + wordFileName + ".html";
            object saveFileName = (object)strSaveFileName;
            docType.InvokeMember("SaveAs", System.Reflection.BindingFlags.InvokeMethod, null, doc, new object[] { saveFileName, Microsoft.Office.Interop.Word.WdSaveFormat.wdFormatFilteredHTML });
            docType.InvokeMember("Close", System.Reflection.BindingFlags.InvokeMethod, null, doc, null);
            wordType.InvokeMember("Quit", System.Reflection.BindingFlags.InvokeMethod, null, word, null);
            result.str = "/Content/" + wordFileName + ".html"; ;
            return Json(result, JsonRequestBehavior.AllowGet);
        }
        #endregion
        public class ResultJson
        {
            public bool res { get; set; }
            public string info { get; set; }
            public string str { get; set; }
        }
    }
}

@{
    ViewBag.Title = "Home Page";
}
<script src="~/Scripts/jquery-3.3.1.min.js"></script>
<script type="text/javascript">
    //预览excel
    function ExcelToHtml() {
        $.ajax({
            url: "/Home/ExcelToHtml",
            data: "",
            type: "POST",
            async: false,
            dataType: "json",
            success: function (data) {
                //获得窗口的垂直位置
                var iWidth = 1400;
                var iHeight = 800;
                var iTop = (window.screen.availHeight - 30 - iHeight) / 2;
                //获得窗口的水平位置
                var iLeft = (window.screen.availWidth - 10 - iWidth) / 2;
                window.open(data.str, '_blank', 'height=' + iHeight + ',innerHeight=' + iHeight + ',width=' + iWidth + ',innerWidth=' + iWidth + ',top=' + iTop + ',left=' + iLeft + ',status=no,toolbar=no,menubar=no,location=no,resizable=no,scrollbars=0,titlebar=no');
            }
        });
    }
    //预览word
    function WordToHtml() {
        $.ajax({
            url: "/Home/WordToHtml",
            data: "",
            type: "POST",
            async: false,
            dataType: "json",
            success: function (data) {
                //获得窗口的垂直位置
                var iWidth = 1400;
                var iHeight = 800;
                var iTop = (window.screen.availHeight - 30 - iHeight) / 2;
                //获得窗口的水平位置
                var iLeft = (window.screen.availWidth - 10 - iWidth) / 2;
                window.open(data.str, '_blank', 'height=' + iHeight + ',innerHeight=' + iHeight + ',width=' + iWidth + ',innerWidth=' + iWidth + ',top=' + iTop + ',left=' + iLeft + ',status=no,toolbar=no,menubar=no,location=no,resizable=no,scrollbars=0,titlebar=no');
            }
        });
    }
</script>
<div style="margin-top:20px;height:800px">
      <input type="button" onclick="ExcelToHtml()" value="预览excel" />
       <input type="button" onclick="WordToHtml()" value="预览word" />
</div>