C#异步下载文件

using UnityEngine;
using System.Collections;
using System.Net;
using System.ComponentModel;

public class NewBehaviourScript : MonoBehaviour
{
private WebClient client;

/// <summary>
/// 初始化
/// </summary>
private void Start()
{
client = new WebClient();
client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressChanged);
client.DownloadFileCompleted += new AsyncCompletedEventHandler(DownLoadCompleted);
}

/// <summary>
/// 按钮响应函数
/// </summary>
public void OnClickLoad()
{
StartToLoad();
}

/// <summary>
/// 开始下载函数
/// </summary>

private void StartToLoad()
{
string url = "D:\\a.txt";           //换成远程服务器上的文件路径
string saveFileName = "D:\\b.txt";  //本地存放文件名
client.DownloadFileAsync(new System.Uri(url), saveFileName);
}

/// <summary>
/// 下载完成的回调函数
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void DownLoadCompleted(object sender, AsyncCompletedEventArgs e)
{
Debug.Log("load finished!");
}

/// <summary>
/// 进度更新回调函数
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
Debug.Log("progress is "+e.ProgressPercentage);
}
}