C# 使用HTTP下载文件，支持续传

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;

namespace Flames.Utils.Net
{
    public class Http
    {

        /// <summary>
        /// 下载进度
        /// </summary>
        public event Action<int> ShowDownloadPercent;

        /// <summary>
        /// 一次下载量
        /// </summary>
        private int ByteSize = 1000000;

        /// <summary>
        /// 下载中的后缀，下载完成去掉
        /// </summary>
        private const string Suffix = ".downloading";

        /// <summary>
        /// Http方式下载文件
        /// </summary>
        /// <param name="url">http地址</param>
        /// <param name="localfile">本地文件</param>
        /// <returns></returns>
        public int DownloadFile(string url, string localfile)
        {
            int ret = 0;
            string localfileReal = localfile;
            string localfileWithSuffix = localfileReal + Suffix;

            try
            {
                long startPosition = 0;
                FileStream writeStream = null;

                if (string.IsNullOrEmpty(url) || string.IsNullOrEmpty(localfileReal)) return 1;

                if (File.Exists(localfileReal)) return 0;

                //取得远程文件长度
                long remoteFileLength = GetHttpLength(url);

                if (remoteFileLength == 0) return 2;

                //判断要下载的文件是否存在
                if (File.Exists(localfileWithSuffix))
                {
                    writeStream = File.OpenWrite(localfileWithSuffix);
                    startPosition = writeStream.Length;
                    if (startPosition > remoteFileLength)
                    {
                        writeStream.Close();
                        File.Delete(localfileWithSuffix);
                        writeStream = new FileStream(localfileWithSuffix, FileMode.Create);
                    }
                    else if (startPosition == remoteFileLength)
                    {
                        DownloadFileOk(localfileReal, localfileWithSuffix);
                        writeStream.Close();
                        return 0;
                    }
                    else
                    {
                        writeStream.Seek(startPosition, SeekOrigin.Begin);
                    }
                }
                else
                {
                    writeStream = new FileStream(localfileWithSuffix, FileMode.Create);
                }

                HttpWebRequest req = null;
                HttpWebResponse rsp = null;
                try
                {
                    req = (HttpWebRequest)HttpWebRequest.Create(url);
                    if (startPosition > 0) req.AddRange((int)startPosition);
                    
                    rsp = (HttpWebResponse)req.GetResponse();
                    using (Stream readStream = rsp.GetResponseStream())
                    {
                        byte[] btArray = new byte[ByteSize];
                        long currPostion = startPosition;
                        int contentSize = 0;
                        while ((contentSize = readStream.Read(btArray, 0, btArray.Length)) > 0)
                        {
                            writeStream.Write(btArray, 0, contentSize);
                            currPostion += contentSize;

                            ShowDownloadPercent?.Invoke((int)(currPostion * 100 / remoteFileLength));
                        }
                    }
                }
                catch (Exception ex)
                {
                    ret = 3;
                }
                finally
                {
                    if (writeStream != null)
                        writeStream.Close();
                    if (rsp != null)
                        rsp.Close();
                    if (req != null)
                        req.Abort();

                    if (ret == 0)
                        DownloadFileOk(localfileReal, localfileWithSuffix);
                }
            }
            catch (Exception ex)
            {
                //Log.Info("获取远程文件失败！exception：\n" + ex.ToString());
                ret = 4;
            }

            return ret;
        }

        /// <summary>
        /// 下载完成
        /// </summary>
        private void DownloadFileOk(string localfileReal, string localfileWithSuffix)
        {
            try
            {
                //去掉.downloading后缀
                FileInfo fi = new FileInfo(localfileWithSuffix);
                fi.MoveTo(localfileReal);
            }
            catch (Exception ex)
            {

            }
            finally
            {
                //通知完成
                if (ShowDownloadPercent != null)
                    ShowDownloadPercent(100);
            }
        }

        // 从文件头得到远程文件的长度
        private long GetHttpLength(string url)
        {
            long length = 0;
            HttpWebRequest req = null;
            HttpWebResponse rsp = null;
            try
            {
                req = (HttpWebRequest)HttpWebRequest.Create(url);
                rsp = (HttpWebResponse)req.GetResponse();
                if (rsp.StatusCode == HttpStatusCode.OK)
                    length = rsp.ContentLength;
            }
            catch (Exception ex)
            {

            }
            finally
            {
                if (rsp != null)
                    rsp.Close();
                if (req != null)
                    req.Abort();
            }

            return length;
        }
    }
}