WebClient下载图片

参考：
1. 用VS2008建立一个Windows应用程序：DownloadFileDemo
源代码如下：
1) FileTransformation.cs(该类包含了一个下载文件的方法)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net;
using System.IO;

namespace DownloadFileDemo
{
    class FileTransformation
    {
        //被下载的文件地址，绝对路径
        private string webServerUri = "";

        //本地端用于保存下载的数据的文件全路径
        private string localFilePath = "";

        public FileTransformation(string webServerUri, string localFilePath)
        {
            this.webServerUri = webServerUri;
            this.localFilePath = localFilePath;
        }

        public int DownloadFile()
        {
            try
            {
                WebRequest wr = WebRequest.Create(webServerUri);
            }
            catch (WebException exp)
            {
                return 1;
            }

            try
            {
                //create a new WebClient instance
                WebClient client = new WebClient();
                client.DownloadFile(webServerUri, localFilePath);

                return 0;
            }
            catch (WebException exp)
            {
                return 2;
            }
        }
    }
}

Form1.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.IO;
using System.Threading;

namespace DownloadFileDemo
{
    public partial class Form1 : Form
    {
        //define delegate
        private delegate void SetTextCallBack();

        OpenFileDialog ofDialog = new OpenFileDialog();

        public Form1()
        {
            InitializeComponent();
        }

        private void InvokeStartDownload()
        {
            //initiate delegate
            SetTextCallBack stcb = new SetTextCallBack(StartDownload);

            Invoke(stcb);
        }

        private void StartDownload()
        {
            //Server URL
            string fullServerUrl = txtSrcAddress.Text;

            //Server Path
            //string webServerUrl = fullServerUrl.Substring(0, fullServerUrl.LastIndexOf('/'));
           
            //Local file name
            string localFileName = fullServerUrl.Substring(fullServerUrl.LastIndexOf('/') + 1, fullServerUrl.Length - fullServerUrl.LastIndexOf('/') - 1);
           
            //Local file full path
            string localFullFilePath = txtTarAddress.Text + '\\' + localFileName;

            FileTransformation ft = new FileTransformation(fullServerUrl, localFullFilePath);

            int result = ft.DownloadFile();
            if (result == 1)
            {
                lblMsg.Text = "WebRequest实例化失败！";
                lblMsg.Visible = true;
            }
            else if(result == 2)
            {
                lblMsg.Text = "文件下载失败！";
                lblMsg.Visible = true;
            }
            else if (result == 0)
            {
                lblMsg.Text = "文件下载成功！";
                lblMsg.Visible = true;
            }

        }

        private void BtnStart_Click(object sender, EventArgs e)
        {
            Thread th = new Thread(new ThreadStart(InvokeStartDownload));
            th.Start();
        }

        private void BtnBrowse_Click(object sender, EventArgs e)
        {
            ofDialog.Title = "选择文件";
            ofDialog.Filter = "All files(*.*)|*.*";
            if (ofDialog.ShowDialog() == DialogResult.OK)
            {
                txtTarAddress.Text = ofDialog.FileName;
            }
        }
    }
}

UI界面如下：

运行结果如下：