private void button1_Click(object sender, EventArgs e) 
{ 
  using (WebClient client = new WebClient()) 
  { 
    client.DownloadFileAsync(new Uri(this.textBox1.Text.Trim()),Path.GetFileName(this.textBox1.Text.Trim())); 
    client.DownloadProgressChanged += client_DownloadProgressChanged; 
    client.DownloadFileCompleted += client_DownloadFileCompleted; 
  } 
} 
 
void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e) 
{ 
  this.label1.Text = string.Format("当前接收到{0}字节，文件大小总共{1}字节", e.BytesReceived, e.TotalBytesToReceive); 
  this.progressBar1.Value = e.ProgressPercentage; 
} 
 
void client_DownloadFileCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) 
{ 
  if (e.Cancelled) 
  { 
    MessageBox.Show("文件下载被取消", "提示", MessageBoxButtons.OKCancel); 
  } 
  this.progressBar1.Value = 0; 
  MessageBox.Show("文件下载成功", "提示"); 
} 

/// <summary> 
/// 请求状态 
/// </summary> 
public class RequestState 
{ 
  /// <summary> 
  /// 缓冲区大小 
  /// </summary> 
  public int BUFFER_SIZE { get; set; } 
 
  /// <summary> 
  /// 缓冲区 
  /// </summary> 
  public byte[] BufferRead { get; set; } 
 
  /// <summary> 
  /// 保存路径 
  /// </summary> 
  public string SavePath { get; set; } 
 
  /// <summary> 
  /// 请求流 
  /// </summary> 
  public HttpWebRequest Request { get; set; } 
 
  /// <summary> 
  /// 响应流 
  /// </summary> 
  public HttpWebResponse Response { get; set; } 
 
  /// <summary> 
  /// 流对象 
  /// </summary> 
  public Stream ResponseStream { get; set; } 
 
  /// <summary> 
  /// 文件流 
  /// </summary> 
  public FileStream FileStream { get; set; } 
} 

//下载文件的url 
string url = this.textBox1.Text.Trim(); 
 
//创建一个初始化请求对象 
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(new Uri(url)); 
 
//设置下载相关参数 
RequestState requestState = new RequestState(); 
requestState.BUFFER_SIZE = 1024; 
requestState.BufferRead = new byte[requestState.BUFFER_SIZE]; 
requestState.Request = request; 
requestState.SavePath = Path.Combine("D:\\", Path.GetFileName(url)); 
requestState.FileStream = new FileStream(requestState.SavePath, FileMode.OpenOrCreate); 
 
//开始异步请求资源 
request.BeginGetResponse(new AsyncCallback(ResponseCallback), requestState); 

/// <summary> 
/// 请求资源方法的回调函数 
/// </summary> 
/// <param name="asyncResult">用于在回调函数当中传递操作状态</param> 
private void ResponseCallback(IAsyncResult asyncResult) 
{ 
  RequestState requestState = (RequestState)asyncResult.AsyncState; 
  requestState.Response = (HttpWebResponse)requestState.Request.EndGetResponse(asyncResult); 
 
  Stream responseStream = requestState.Response.GetResponseStream(); 
  requestState.ResponseStream = responseStream; 
 
  //开始异步读取流 
  responseStream.BeginRead(requestState.BufferRead, 0, requestState.BufferRead.Length, ReadCallback, requestState); 
} 

/// <summary> 
/// 异步读取流的回调函数 
/// </summary> 
/// <param name="asyncResult">用于在回调函数当中传递操作状态</param> 
private void ReadCallback(IAsyncResult asyncResult) 
{ 
  RequestState requestState = (RequestState)asyncResult.AsyncState; 
  int read = requestState.ResponseStream.EndRead(asyncResult); 
  if (read > 0) 
  { 
    //将缓冲区的数据写入该文件流 
    requestState.FileStream.Write(requestState.BufferRead, 0, read); 
 
    //开始异步读取流 
    requestState.ResponseStream.BeginRead(requestState.BufferRead, 0, requestState.BufferRead.Length, ReadCallback, requestState); 
  } 
  else 
  { 
    requestState.Response.Close(); 
    requestState.FileStream.Close(); 
  } 
}