C# .NET 5.0 上传、下载文件

using Microsoft.AspNetCore.Http;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;

namespace MEAS.Common
{
    public class FileHelper
    {
        //定义一个用于保存静态变量的实例
        private static FileHelper instance = null;
        //定义一个保证线程同步的标识
        private static readonly object locker = new object();
        //构造函数为私有，使外界不能创建该类的实例
        public static FileHelper Instance
        {
            get
            {
                if (instance == null)
                {
                    lock (locker)
                    {
                        if (instance == null) instance = new FileHelper();
                    }
                }
                return instance;
            }
        }

        /// 服务器文件下载
        /// </summary>
        /// <param name="url">服务器下载文件地址</param>
        /// <param name="filepath">文件存放地址，包含文件名</param>
        /// <returns></returns>
        public string DownLoadFile(string fileUrl, string filepath)
        {
            try
            {
                using (WebClient client = new WebClient())
                {
                    client.DownloadFile(fileUrl, filepath);
                }
            }
            catch (Exception) { throw; }

            return filepath;
        }

        /// <summary>
        /// 文件上传
        /// </summary>
        /// <param name="files">文件集合</param>
        /// <returns></returns>
        public string UploadFile(List<IFormFile> files)
        {
            string dir = string.Empty;
            string fileName = string.Empty;
            string targetPath = string.Empty;

            long size = files.Sum(f => f.Length);
            foreach (IFormFile formFile in files)
            {
                if (formFile.Length > 0)
                {
                    var filePath = Path.GetTempFileName();
                    dir = AppDomain.CurrentDomain.BaseDirectory + "\\ExcelTemplate\\FileUpload\\";
                    fileName = DateTime.Now.ToString("yyyy-MM-dd HH.mm.ss") +" "+ formFile.FileName;
                    targetPath = dir + "\\" + fileName; //文件全路径(e:\abc.txt)
                    using (var stream = System.IO.File.Create(targetPath))
                    {
                        formFile.CopyToAsync(stream);
                    }
                }
            }
            return targetPath;
        }

        /// http下载文件
        /// </summary>
        /// <param name="url">服务器下载文件地址</param>
        /// <param name="path">文件存放地址，包含文件名</param>
        /// <returns></returns>
        private string HttpDownloadFile(string url, string path)
        {
            try
            {
                // 设置参数
                HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
                //发送请求并获取相应回应数据
                HttpWebResponse response = request.GetResponse() as HttpWebResponse;
                //直到request.GetResponse()程序才开始向目标网页发送Post请求
                Stream responseStream = response.GetResponseStream();
                //创建本地文件写入流
                Stream stream = new FileStream(path, FileMode.Create);
                byte[] bArr = new byte[1024];
                int size = responseStream.Read(bArr, 0, (int)bArr.Length);

                while (size > 0)
                {
                    stream.Write(bArr, 0, size);
                    size = responseStream.Read(bArr, 0, (int)bArr.Length);
                }

                stream.Close();
                responseStream.Close();
                return path;
            }
            catch (Exception) { throw; }
        }


    }
}

参考：https://docs.microsoft.com/zh-cn/aspnet/core/mvc/models/file-uploads?view=aspnetcore-6.0