WebClient.DownloadFile(线程机制，异步下载文件)

      线程机制(避免卡屏)，异步下载文件。

　　我做网站的监控，WebClient.DownloadFile这个方法是我经常用到的，必要的时候肯定是要从网上下载些什么（WebRequest 也可以下载网络文件，不妨搜下，不过WebClient.DownloadFile使用更简单）。

　　今天简单的演示下WebClient.DownloadFileAsync的使用，刚刚写好的实例，有问题你拍我。

　　解释什么的都不需要了吧。这里面应用了线程机制，异步调用。wo先展示代码，在把一些关键点放置到后面，并给一些修改建议(设计器生成代码在下，可以不用看，只需要复制粘贴使用)。

测试中用到的网络文件（随机抽取.exe,.rar,.zip），都是无害的，请放心测试：

http://dl1sw.baidu.com/client/9h162/fzjqd.exe
http://sq.onlinedown.net/down/sgbs.rar
http://zj.down.chinaz.com/201408/bdswxt_v1.0.zip
http://zj.down.chinaz.com/201408/zyzfwxt_v1.1.zip

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.Threading;
using System.IO;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            AppDomain.CurrentDomain.UnhandledException += new UnhandledExceptionEventHandler(CurrentDomain_UnhandledException);
        }

        static void CurrentDomain_UnhandledException(object sender, UnhandledExceptionEventArgs e)
        {
            File.AppendAllText(AppDomain.CurrentDomain.BaseDirectory + "error.log", DateTime.Now.ToString() + "	" + e.ExceptionObject.ToString() + Environment.NewLine);
        }

        /// <summary>
        /// 文件总数
        /// </summary>
        int _totalFile = 0;
        /// <summary>
        /// 已下载文件
        /// </summary>
        int _loadFile = 0;

        private void button1_Click(object sender, EventArgs e)
        {//Run
            string richText = richTextBox1.Text;
            if (string.IsNullOrEmpty(richText))
            {
                MessageBox.Show("请在文本框中输入需要下载的网络文件");
            }

            Thread thread = new Thread(new ThreadStart(() => createWebClient(richText)));
            thread.Start();
        }

        //WebClient webClient = null;

        private void createWebClient(string richText)
        {
            string[] array = richText.Split(new string[] { "
", "
" }, StringSplitOptions.RemoveEmptyEntries);

            _totalFile = array.Length;

            foreach (string item in array)
            {
                WebClient webClient = new WebClient();

                webClient.DownloadProgressChanged += new DownloadProgressChangedEventHandler(webClient_DownloadProgressChanged);
                webClient.DownloadFileCompleted += new AsyncCompletedEventHandler(webClient_DownloadFileCompleted);

                string fileName = AppDomain.CurrentDomain.BaseDirectory + DateTime.Now.ToFileTimeUtc() + item.Substring(item.LastIndexOf('/') + 1);

                webClient.DownloadFileAsync(new Uri(item), fileName);

            }
        }

        private void webClient_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            this.Invoke(new MethodInvoker(delegate {
                this.progressBar2.Value = e.ProgressPercentage;
                this.label2.Text = string.Format("正在下载文件，完成进度{0}%  {1}/{2}(字节)"
                                    ,e.ProgressPercentage
                                    , e.BytesReceived
                                    , e.TotalBytesToReceive);
             }));
        }

        private void webClient_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
        {
            _loadFile++;

            int percent = (int)(100.0 * _loadFile / _totalFile);

            this.Invoke(new MethodInvoker(delegate {
                this.progressBar1.Value = percent;
                this.label1.Text = string.Format("已完成文件下载{0}%  {1}/{2}(文件个数)"
                    , percent
                    , _loadFile
                    , _totalFile);
            }));

            if (sender is WebClient)
            {
                ((WebClient)sender).CancelAsync();
                ((WebClient)sender).Dispose();
            }
        }

        private System.ComponentModel.IContainer components = null;
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }
        private void InitializeComponent()
        {
            this.progressBar1 = new System.Windows.Forms.ProgressBar();
            this.progressBar2 = new System.Windows.Forms.ProgressBar();
            this.label1 = new System.Windows.Forms.Label();
            this.label2 = new System.Windows.Forms.Label();
            this.label3 = new System.Windows.Forms.Label();
            this.label4 = new System.Windows.Forms.Label();
            this.richTextBox1 = new System.Windows.Forms.RichTextBox();
            this.button1 = new System.Windows.Forms.Button();
            this.SuspendLayout();
            // 
            // progressBar1
            // 
            this.progressBar1.Location = new System.Drawing.Point(81, 160);
            this.progressBar1.Name = "progressBar1";
            this.progressBar1.Size = new System.Drawing.Size(330, 12);
            this.progressBar1.TabIndex = 0;
            // 
            // progressBar2
            // 
            this.progressBar2.Location = new System.Drawing.Point(81, 224);
            this.progressBar2.Name = "progressBar2";
            this.progressBar2.Size = new System.Drawing.Size(330, 12);
            this.progressBar2.TabIndex = 1;
            // 
            // label1
            // 
            this.label1.AutoSize = true;
            this.label1.Location = new System.Drawing.Point(59, 187);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(41, 12);
            this.label1.TabIndex = 2;
            this.label1.Text = "label1";
            // 
            // label2
            // 
            this.label2.AutoSize = true;
            this.label2.Location = new System.Drawing.Point(59, 252);
            this.label2.Name = "label2";
            this.label2.Size = new System.Drawing.Size(41, 12);
            this.label2.TabIndex = 3;
            this.label2.Text = "label2";
            // 
            // label3
            // 
            this.label3.AutoSize = true;
            this.label3.Location = new System.Drawing.Point(12, 160);
            this.label3.Name = "label3";
            this.label3.Size = new System.Drawing.Size(65, 12);
            this.label3.TabIndex = 4;
            this.label3.Text = "总 进 度：";
            // 
            // label4
            // 
            this.label4.AutoSize = true;
            this.label4.Location = new System.Drawing.Point(12, 224);
            this.label4.Name = "label4";
            this.label4.Size = new System.Drawing.Size(65, 12);
            this.label4.TabIndex = 5;
            this.label4.Text = "当前进度：";
            // 
            // richTextBox1
            // 
            this.richTextBox1.Location = new System.Drawing.Point(14, 12);
            this.richTextBox1.Name = "richTextBox1";
            this.richTextBox1.Size = new System.Drawing.Size(397, 131);
            this.richTextBox1.TabIndex = 6;
            this.richTextBox1.Text = "";
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(150, 280);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 7;
            this.button1.Text = "Run";
            this.button1.UseVisualStyleBackColor = true;
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(423, 315);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.richTextBox1);
            this.Controls.Add(this.label4);
            this.Controls.Add(this.label3);
            this.Controls.Add(this.label2);
            this.Controls.Add(this.label1);
            this.Controls.Add(this.progressBar2);
            this.Controls.Add(this.progressBar1);
            this.MaximumSize = new System.Drawing.Size(439, 353);
            this.MinimumSize = new System.Drawing.Size(439, 353);
            this.Name = "Form1";
            this.Text = "WebClient下载实现实例";
            this.ResumeLayout(false);
            this.PerformLayout();

        }
        private System.Windows.Forms.ProgressBar progressBar1;
        private System.Windows.Forms.ProgressBar progressBar2;
        private System.Windows.Forms.Label label1;
        private System.Windows.Forms.Label label2;
        private System.Windows.Forms.Label label3;
        private System.Windows.Forms.Label label4;
        private System.Windows.Forms.RichTextBox richTextBox1;
        private System.Windows.Forms.Button button1;
    }
}

大致看了一下，貌似就以下几句需要新手斟酌，大神请绕行。

第一句： this.Invoke(new MethodInvoker(delegate {

第二句：Thread thread = new Thread(new ThreadStart(() => createWebClient(richText)));

第三句：if (sender is WebClient)

第一句是关键的一句，不再创建控件的线程中调用/修改控件的属性值（Text），会报异常，为什么这么用就OK了？

第二句，第三句就简单了，不提示了，不会了找度度。

本程序可以添加的功能，添加一个Stop按钮，终断下载（请不要嗤之以鼻，在创建窗体的线程里操作new Thread让其立即终断下载，还是有点深度）

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原文地址：https://www.cnblogs.com/preacher/p/3929709.html