从服务器下载图片

<appSettings>
<add key="AvatarDownlodUrl" value="http://175.143.101.30:4523/download.aspx"/>
</appSettings>

using System;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.ComponentModel;
using System.Windows.Threading;
using System.Threading;
using System.IO;
using System.Net;
using System.Configuration;

namespace Messenger
{
    /// <summary>
    /// 头像图片，负责下载、缓存头像。
    /// </summary>
    public sealed class AvatarImage : Yeylol.MVVM.BaseModel
    {
        static string defaultAvatarFile = AppDomain.CurrentDomain.BaseDirectory + "data\\Avatar\\0.jpg";
        Dispatcher Dispatcher;
        Thread ted;

        public AvatarImage(int? avatarID)
        {
            this.AvatarID = avatarID;
            this.Dispatcher = Dispatcher.CurrentDispatcher;
            this.FilePath = AppDomain.CurrentDomain.BaseDirectory + "data\\Avatar\\" + AvatarID + ".jpg";

            if (File.Exists(this.FilePath))
            {
                this.LoadAvatarInDisk(FilePath);
            }
            else
            {
                this.LoadAvatarInNetwork();
            }
        }

        /// <summary>
        /// 获取头像编号
        /// </summary>
        public int? AvatarID { get; private set; }

        /// <summary>
        /// 头象磁盘路径
        /// </summary>
        public string FilePath { get; private set; }

        /// <summary>
        /// 头像
        /// </summary>
        public ImageSource AvatarSource { get; private set; }


        /// <summary>
        /// 在磁盘中加载头像
        /// </summary>
        void LoadAvatarInDisk(string filename)
        {
            this.Dispatcher.BeginInvoke(new Action<string>((filePath) =>
            {
                try
                {
                    AvatarSource = new BitmapImage(new Uri(filePath));
                }
                catch
                {
                    return;
                }
                OnPropertyChanged("AvatarSource");
            }), filename);
        }

        /// <summary>
        /// 在网络中加载头像
        /// </summary>
        void LoadAvatarInNetwork()
        {
            ted = new Thread(LoadAvatarInNetworkThread);
            ted.Start();
        }

        void LoadAvatarInNetworkThread()
        {
            Exception error = null;
            WebClient wc = new WebClient();
            string fileDownloadUrl = ConfigurationManager.AppSettings["AvatarDownlodUrl"] + "?FileID=" + AvatarID;

            lock (Dispatcher)
            {
                if (!Directory.Exists(Path.GetDirectoryName(FilePath)))
                {
                    Directory.CreateDirectory(Path.GetDirectoryName(FilePath));
                }
            }

            try
            {
                wc.DownloadFile(fileDownloadUrl, this.FilePath);
            }
            catch (Exception ex)
            {
                error = ex;
            }
            finally
            {
                wc.Dispose();
            }

            if (error == null)
            {
                LoadAvatarInDisk(FilePath);
            }
            else
            {
                try
                {
                    if (File.Exists(FilePath))
                    {
                        File.Delete(FilePath);
                    }
                }
                catch { }
                LoadAvatarInDisk(defaultAvatarFile);
            }
        }
    }
}

  <appSettings>
    <add key="FilePathPrefix" value="Avatars\" />
  </appSettings>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Text;
using System.Threading;

namespace Chat.Comm.WCF
{
    public partial class FileDownloaderServer : System.Web.UI.Page
    {
        int FileID
        {
            get
            {
                if (this.Request["FileID"] == null)
                    return -1;
                try
                {
                    return int.Parse(this.Request["FileID"].ToString().Trim());
                }
                catch
                {
                    return -1;
                }
            }
        }


        bool IsGray
        {
            get
            {
                return this.Request["isgray"] == "1";
            }
        }

        string GetFilePath()
        {
            return Common.FilePathPrefix + DAL.ChatDal.GetFilePath(FileID);
        }

        /// <summary>
        /// 生成灰度图像
        /// </summary>
        /// <param name="img"></param>
        /// <returns></returns>
        private System.Drawing.Image TransformImage(System.Drawing.Image img)
        {
            int widht = (int)img.Width;
            int heigth = (int)img.Height;

            System.Drawing.Bitmap map = new System.Drawing.Bitmap(widht, heigth);
            System.Drawing.Bitmap mp = img.Clone() as System.Drawing.Bitmap;

            try
            {
                for (int x = 0; x < widht; x++)
                {
                    for (int y = 0; y < heigth; y++)
                    {
                        System.Drawing.Color px = mp.GetPixel(x, y);
                        int r, b, g, Result = 0;
                        r = px.R;
                        b = px.B;
                        g = px.G;
                        Result = ((int)(0.7 * r) + (int)(0.2 * g) + (int)(0.1 * b));
                        map.SetPixel(x, y, System.Drawing.Color.FromArgb(Result, Result, Result));
                    }
                }
                mp.Dispose();
            }
            catch (Exception ex)
            {
                Logger.Write(ex.Message);
                return null;
            }

            return map;
        }

        protected void Page_Load(object sender, EventArgs e)
        {
            string filePath = GetFilePath();

            #region 生成灰度图片

            if (IsGray)
            {
                string grayfile = filePath + "_gray.jpg";

                if (!File.Exists(grayfile))
                {
                    System.Drawing.Image i = System.Drawing.Image.FromFile(filePath);
                    System.Drawing.Image igray = TransformImage(i);
                    if (igray != null)
                        igray.Save(grayfile);
                }
                filePath = grayfile;
            }

            #endregion

            long speed = 100000;
            DownloadFile(this.Context, filePath, speed);
        }

        /**/
        /// <summary>
        /// 下载文件，支持大文件、续传、速度限制。支持续传的响应头Accept-Ranges、ETag，请求头Range 。
        /// Accept-Ranges：响应头，向客户端指明，此进程支持可恢复下载.实现后台智能传输服务（BITS），值为：bytes；
        /// ETag：响应头，用于对客户端的初始（200）响应，以及来自客户端的恢复请求，
        /// 必须为每个文件提供一个唯一的ETag值（可由文件名和文件最后被修改的日期组成），这使客户端软件能够验证它们已经下载的字节块是否仍然是最新的。
        /// Range：续传的起始位置，即已经下载到客户端的字节数，值如：bytes=1474560- 。
        /// 另外：UrlEncode编码后会把文件名中的空格转换中+（+转换为%2b），但是浏览器是不能理解加号为空格的，所以在浏览器下载得到的文件，空格就变成了加号；
        /// 解决办法：UrlEncode 之后, 将 "+" 替换成 "%20"，因为浏览器将%20转换为空格
        /// </summary>
        /// <param name="httpContext">当前请求的HttpContext</param>
        /// <param name="filePath">下载文件的物理路径，含路径、文件名</param>
        /// <param name="speed">下载速度：每秒允许下载的字节数</param>
        /// <returns>true下载成功，false下载失败</returns>
        public static bool DownloadFile(HttpContext httpContext, string filePath, long speed)
        {
            httpContext.Response.Clear();
            bool ret = true;
            try
            {
                #region --验证：HttpMethod，请求的文件是否存在#region
                switch (httpContext.Request.HttpMethod.ToUpper())
                { //目前只支持GET和HEAD方法
                    case "GET":
                    case "HEAD":
                        break;
                    default:
                        httpContext.Response.StatusCode = 501;
                        return false;
                }
                if (!File.Exists(filePath))
                {
                    httpContext.Response.StatusCode = 404;
                    return false;
                }
                #endregion

                #region 定义局部变量#region 定义局部变量

                long startBytes = 0;
                long stopBytes = 0;
                int packSize = 1024 * 10; //分块读取，每块10K bytes
                string fileName = Path.GetFileName(filePath);
                FileStream myFile = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
                BinaryReader br = new BinaryReader(myFile);
                long fileLength = myFile.Length;

                int sleep = (int)Math.Ceiling(1000.0 * packSize / speed);//毫秒数：读取下一数据块的时间间隔
                string lastUpdateTiemStr = File.GetLastWriteTimeUtc(filePath).ToString("r");
                string eTag = HttpUtility.UrlEncode(fileName, Encoding.UTF8) + lastUpdateTiemStr;//便于恢复下载时提取请求头;
                #endregion

                #region --验证：文件是否太大，是否是续传，且在上次被请求的日期之后是否被修改过
                if (myFile.Length > long.MaxValue)
                {//-------文件太大了-------
                    httpContext.Response.StatusCode = 413;//请求实体太大
                    return false;
                }

                if (httpContext.Request.Headers["If-Range"] != null)//对应响应头ETag：文件名+文件最后修改时间
                {
                    //----------上次被请求的日期之后被修改过--------------
                    if (httpContext.Request.Headers["If-Range"].Replace("\"", "") != eTag)
                    {//文件修改过
                        httpContext.Response.StatusCode = 412;//预处理失败
                        return false;
                    }
                }

                #endregion

                try
                {
                    #region -------添加重要响应头、解析请求头、相关验证

                    httpContext.Response.Clear();

                    if (httpContext.Request.Headers["Range"] != null)
                    {
                        //------如果是续传请求，则获取续传的起始位置，即已经下载到客户端的字节数------
                        httpContext.Response.StatusCode = 206;//重要：续传必须，表示局部范围响应。初始下载时默认为200
                        string[] range = httpContext.Request.Headers["Range"].Split(new char[] { ‘=‘, ‘-‘ });//"bytes=1474560-"
                        startBytes = Convert.ToInt64(range[1]);//已经下载的字节数，即本次下载的开始位置  
                        if (startBytes < 0 || startBytes >= fileLength)
                        {
                            //无效的起始位置
                            return false;
                        }
                        if (range.Length == 3)
                        {
                            stopBytes = Convert.ToInt64(range[2]);//结束下载的字节数，即本次下载的结束位置  
                            if (startBytes < 0 || startBytes >= fileLength)
                            {
                                return false;
                            }
                        }
                    }

                    httpContext.Response.Buffer = false;

                    httpContext.Response.AddHeader("Accept-Ranges", "bytes");//重要：续传必须
                    httpContext.Response.AppendHeader("ETag", "\"" + eTag + "\"");//重要：续传必须
                    httpContext.Response.AppendHeader("Last-Modified", lastUpdateTiemStr);//把最后修改日期写入响应                
                    httpContext.Response.ContentType = "application/octet-stream";//MIME类型：匹配任意文件类型
                    httpContext.Response.AddHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(fileName, Encoding.UTF8).Replace("+", "%20"));
                    httpContext.Response.AddHeader("Content-Length", (fileLength - startBytes).ToString());
                    httpContext.Response.AddHeader("Connection", "Keep-Alive");
                    httpContext.Response.ContentEncoding = Encoding.UTF8;
                    if (startBytes > 0)
                    {
                        //------如果是续传请求，告诉客户端本次的开始字节数，总长度，以便客户端将续传数据追加到startBytes位置后----------
                        httpContext.Response.AddHeader("Content-Range", string.Format("bytes {0}-{1}/{2}", startBytes, fileLength - 1, fileLength));
                    }
                    #endregion

                    #region -------向客户端发送数据块-------------------
                    br.BaseStream.Seek(startBytes, SeekOrigin.Begin);
                    int maxCount = (int)Math.Ceiling((fileLength - startBytes + 0.0) / packSize);//分块下载，剩余部分可分成的块数
                    for (int i = 0; i < maxCount && httpContext.Response.IsClientConnected; i++)
                    {
                        //客户端中断连接，则暂停
                        httpContext.Response.BinaryWrite(br.ReadBytes(packSize));
                        httpContext.Response.Flush();
                        if (sleep > 1) Thread.Sleep(sleep);
                    }
                    #endregion
                }
                catch
                {
                    ret = false;
                }
                finally
                {
                    br.Close();
                    myFile.Close();
                }
            }
            catch
            {
                ret = false;
            }
            return ret;
        }

    }
}