C#文件操作(上传，下载，读取，写入)

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1. 通过byte[]数据下载文件（这种方法可用于以开放Api的形式传递文件内容）

public void FileDownLoadByByte(byte[] fileData, string fileName)
{
　　//添加这个编码可以防止在IE下文件名乱码的问题
　　fileName = HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8);
    
    Response.Clear();
    Response.ClearHeaders();
    Response.Buffer = true;
    Response.ContentType = "application/octet-stream";
    Response.AppendHeader("Content-Disposition", "attachment;filename=" + fileName);
    Response.AppendHeader("Content-Length", fileData.Length.ToString());
    Response.BinaryWrite(fileData);
    Response.Flush();
    Response.End();
}

2. 根据路劲，从服务器下载文件

①

/// <summary>
/// 从服务器下载文件
/// </summary>
/// <param name="fileName">客户端保存的文件名</param>
/// <param name="serverFilePath">服务器端要下载的文件路径</param>
protected void LoadFileFromServer(string fileName, string serverFilePath)
{
    FileInfo fileInfo = new FileInfo(serverFilePath);
    Response.Clear();
    Response.ClearContent();
    Response.ClearHeaders();
    Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
    Response.AddHeader("Content-Length", fileInfo.Length.ToString());
    Response.AddHeader("Content-Transfer-Encoding", "binary");
    Response.ContentType = "application/octet-stream";
    Response.ContentEncoding = System.Text.Encoding.GetEncoding("gb2312");
    Response.WriteFile(fileInfo.FullName);
    Response.Flush();
    Response.End();
}

②

/// <summary>
/// 微软为Response对象提供了一个新的方法TransmitFile来解决使用Response.BinaryWrite
///     下载超过400mb的文件时导致Aspnet_wp.exe进程回收而无法成功下载的问题。
///     将后台文件写入 HTTP 响应输出流（不在内存中进行缓冲）
/// </summary>
public void TransmitFile()
{
    Response.ContentType = "application/x-zip-compressed";
    Response.AddHeader("Content-Disposition", "attachment;filename=download.zip");
    string filename = Server.MapPath("~/Download/xx.zip");
    Response.TransmitFile(filename);
}

3. 通过流的形式下载文件

public void LoadFileByStream()
{
    string fileName = "aaa.txt";//客户端保存的文件名
    string filePath = Server.MapPath("~/Download/xx.txt");

    //以字符流的形式下载文件
    FileStream fs = new FileStream(filePath, FileMode.Open);
    byte[] bytes = new byte[(int)fs.Length];
    fs.Read(bytes, 0, bytes.Length);
    fs.Close();
    Response.ContentType = "application/octet-stream";
    //通知浏览器下载文件而不是打开
    Response.AddHeader("Content-Disposition", "attachment;  filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
    Response.BinaryWrite(bytes);
    Response.Flush();
    Response.End();
}

4. Excel下载

参考另一篇博文

http://www.cnblogs.com/TiestoRay/archive/2013/02/02/2576134.html

5. 如果是用的.NET MVC3(及以上) 一切变得更加简单

例如

public ActionResult LoadFile2()
{
    string filePath = Server.MapPath("~/Download/xx.jpg");
    return File(filePath, "application/x-jpg", "demo.jpg");
}

它共有六种可用形式

FileContentResult File(byte[] fileContents, string contentType);
FileStreamResult File(Stream fileStream, string contentType);
FilePathResult File(string fileName, string contentType);
virtual FileContentResult File(byte[] fileContents, string contentType, string fileDownloadName);
virtual FileStreamResult File(Stream fileStream, string contentType, string fileDownloadName);
virtual FilePathResult File(string fileName, string contentType, string fileDownloadName);

6. Ftp文件处理

参考 C#Ftp文件处理

7. 文件上传

将这种稍作修改即可解决跨域文件上传的问题，具体内容参考跨域文件上传解决方案

后台代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace SmsWebSystem.Upload
{
    /// <summary>
    /// FileHandler 的摘要说明
    /// </summary>
    public class FileHandler : IHttpHandler
    {
        private const int MAX_UPLOAD_SIZE = 2;

        /// <summary>
        /// fileSelector是前台代码中文件控件的name
        /// </summary>
        /// <param name="req"></param>
        /// <param name="res"></param>
        /// <returns></returns>
        public string UploadFile(HttpRequest req ,HttpResponse res)
        {
            if (req.Files["fileSelector"].ContentLength > MAX_UPLOAD_SIZE * 1024 * 1024)
            {
                return String.Format("请上传{0}M以内的文件。", MAX_UPLOAD_SIZE);
            }

            string uploadFileName = req.Files["fileSelector"].FileName;
            string path = HttpContext.Current.Server.MapPath(uploadFileName);
            req.Files["fileSelector"].SaveAs(path);
            return "";
        }
        
        /// <summary>
        /// 需要指定上传结束后的回调函数   这里是uploadCallBack
        /// </summary>
        /// <param name="context"></param>
        public void ProcessRequest(HttpContext context)
        {
            string result = UploadFile(context.Request, context.Response);
            if (String.IsNullOrEmpty(result))
            {
                result = "上传成功";
            }
            context.Response.Write(String.Format("<script>top.uploadCallBack('{0}');</script>", result));
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}

前台代码

<html xmlns="http://www.w3.org/1999/xhtml">
<body>
    <form id="fileform" name = "fileform" enctype="multipart/form-data"
            action="FileHandler.ashx" target="hideFrame" method="post">
        <input type="file" id="fileSelector" name="fileSelector" />
    </form>
    <iframe id="hideFrame" name="hideFrame" style="display:none;"></iframe>
    <input type="button" id="btnUpload" value="上传" />
</body>
<script>
    document.querySelector("#btnUpload").onclick = function () {
        this.style.backgroundColor = "blue";
        document.querySelector("#fileform").submit();
    }

    function uploadCallBack(rst) {
        document.querySelector("#btnUpload").style.backgroundColor = "white";
        alert(rst);
    }
</script>
</html>

8. 使用文件流写入文件数据

FileStream fs = new FileStream(FILE_PATH,FileMode.OpenOrCreate);

string content = "";//要写入的内容
//获得字节数组
byte [] data =new UTF8Encoding().GetBytes(content);

//开始写入，第二个参数用文件流的长度 表示向文件末尾追加内容
fs.Write(data,fs,data.Length);

//清空缓冲区
fs.Flush();

//关闭流
fs.Close();

要注意的一点是，如果上传的文件过大，可能会报异常

【HTTP错误404.13 - Not Found 请求筛选模块被配置为拒绝超过请求内容长度的请求，原因是Web服务器上的请求筛选被配置为拒绝该请求，因为内容长度超过配置的值】

需要配置IIS配置文件（%windir%/system32/inetsrv/config/applicationhost.config）及项目配置文件 web.config

具体内容参考《IIS请求筛选模块被配置为拒绝超过请求内容长度的请》

9.使用文件流读取文件数据

byte[] byData = new byte[100];

//读取文件
FileStream fs = new FileStream(FILE_PATH,FileMode.Open);
byte[] tmpData = new byte[fs.Length];
char[] charData = new char[fs.Length];
fs.Read(tmpData, 0, 100); 
//解码
Decoder d = Encoding.UTF8.GetDecoder();
d.GetChars(tmpData, 0, tmpData.Length, charData, 0);

Console.WriteLine(charData);