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运行效果：

项目结构图：

DownLoad.cs文件代码：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace DawnLoad
{
    public partial class DownLoad : Form
    {
        public DownLoad()
        {
            InitializeComponent();
        }

        //下载路径,文件保存路径
        public static string DownLoadPath;
        public static string StoragePath;

        /// <summary>
        /// 加载事件
        /// </summary>
        private void DownLoad_Load(object sender, EventArgs e)
        {

        }

        /// <summary>
        /// 下载事件
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button1_Click_1(object sender, EventArgs e)
        {
            //给定下载路径
            DownLoadPath = this.txt_DowanLoadPath.Text.Trim();

            //判断所输入的路径是不是为空
            Command.IsNotEmpty.txtIsNotEmpty(DownLoadPath, StoragePath);

            //格式化文件保存路径，使的保存的路径后面所带的文件名，跟下载时候所带有的文件名称一致
            string newStoragePath = Command.FomatPath.FomatStoragePath(DownLoadPath, StoragePath);

            //下载开始下载
            Command.DownLoad.DownloadFile(DownLoadPath, newStoragePath, progressBar1, lbl_JD);
        }

        /// <summary>
        /// 浏览事件，选择保存路径
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button2_Click(object sender, EventArgs e)
        {
            FolderBrowserDialog folderDlg = new FolderBrowserDialog();
            folderDlg.ShowDialog();
            folderDlg.Description = "请选择保存文件的文件夹！";
            if (folderDlg.SelectedPath == "")
            {
                return;
            }
            else
            {
                StoragePath = folderDlg.SelectedPath;
                this.txt_StoragePathPath.Text = StoragePath;
                return;
            }
        }
    }
}

Command文件夹下的DownLoad.cs代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace DawnLoad.Command
{
    public static class DownLoad
    {
        /// <summary>        
        /// c#,.net 下载文件        
        /// </summary>        
        /// <param name="URL">下载文件地址</param>       
        /// <param name="Filename">下载后的存放地址</param>        
        /// <param name="Prog">用于显示的进度条</param>
        /// <param name="label1">显示下载完成度</param>
        public static void DownloadFile(string URL, string filename, System.Windows.Forms.ProgressBar prog, System.Windows.Forms.Label label1)
        {
            float percent = 0;
            try
            {
                System.Net.HttpWebRequest Myrq = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(URL);
                System.Net.HttpWebResponse myrp = (System.Net.HttpWebResponse)Myrq.GetResponse();
                long totalBytes = myrp.ContentLength;
                if (prog != null)
                {
                    prog.Maximum = (int)totalBytes;
                }
                System.IO.Stream st = myrp.GetResponseStream();
                System.IO.Stream so = new System.IO.FileStream(filename, System.IO.FileMode.Create);
                long totalDownloadedByte = 0;
                byte[] by = new byte[1024 * 1024 * 1];
                int osize = st.Read(by, 0, (int)by.Length);
                while (osize > 0)
                {
                    totalDownloadedByte = osize + totalDownloadedByte;
                    System.Windows.Forms.Application.DoEvents();
                    so.Write(by, 0, osize);
                    if (prog != null)
                    {
                        prog.Value = (int)totalDownloadedByte;
                    }
                    osize = st.Read(by, 0, (int)by.Length);

                    percent = (float)totalDownloadedByte / (float)totalBytes * 100;
                    label1.Text = " 当前下载进度" + percent.ToString() + "%";
                    System.Windows.Forms.Application.DoEvents(); //必须加注这句代码，否则label1将因为循环执行太快而来不及显示信息
                }
                MessageBox.Show("下载完成！", "提示");
                so.Close();
                st.Close();
            }
            catch (System.Exception ex)
            {
                MessageBox.Show(ex.Message, "提示");
            }
        }
    }
}

Command文件夹下的FomatPath.cs代码：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DawnLoad.Command
{
    public static class FomatPath
    {
        /// <summary>
        /// 格式化保存文件的路径
        /// </summary>
        /// <param name="DownLoadPath">下载文件的URL，用来截取文件名</param>
        /// <param name="StoragePath">保存文件的原始路径</param>
        /// <returns>格式化后的路径</returns>
        public static string FomatStoragePath(string DownLoadPath, string StoragePath)
        {
            string newStoragePath = string.Empty;

            string newDownLoadPath = Path.GetFileName(DownLoadPath);

            newDownLoadPath = newDownLoadPath.Replace('&', ' ');

            newStoragePath = StoragePath + "\" +  newDownLoadPath;

            return newStoragePath;
        }
    }
}

Command文件夹下的IsNotEmpty.cs代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace DawnLoad.Command
{
    public static class IsNotEmpty
    {
        public static void txtIsNotEmpty(string DownLoadPath, string StoragePath)
        {
            if (DownLoadPath.Trim().Length == 0)
            {
                MessageBox.Show("下载路径文本框不能为空！", "提示");
                return;
            }

            if (StoragePath.Trim().Length == 0)
            {
                MessageBox.Show("保存路径文本框不能为空！", "提示");
                return;
            }
        }
    }
}

源码：百度云，[下载工具]文件夹。