WebClient.DownloadProgressChanged 事件

定义

命名空间:

System.Net

程序集:

System.Net.WebClient.dll

程序集:

System.dll

程序集:

netstandard.dll

在异步下载操作成功传输部分或全部数据后发生。

public:
 event System::Net::DownloadProgressChangedEventHandler ^ DownloadProgressChanged;

public event System.Net.DownloadProgressChangedEventHandler? DownloadProgressChanged;

public event System.Net.DownloadProgressChangedEventHandler DownloadProgressChanged;

member this.DownloadProgressChanged : System.Net.DownloadProgressChangedEventHandler 

Public Custom Event DownloadProgressChanged As DownloadProgressChangedEventHandler 

Public Event DownloadProgressChanged As DownloadProgressChangedEventHandler 

事件类型

DownloadProgressChangedEventHandler

示例

下面的代码示例演示如何设置事件的 DownloadProgressChanged 事件处理程序。

// Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
void DownLoadFileInBackground4( String^ address )
{
   WebClient^ client = gcnew WebClient;
   Uri ^uri = gcnew Uri(address);

   // Specify a DownloadFileCompleted handler here...

   // Specify a progress notification handler.
   client->DownloadProgressChanged += gcnew DownloadProgressChangedEventHandler( DownloadProgressCallback4 );
   
   client->DownloadFileAsync( uri, "serverdata.txt" );
}

static void DownloadProgressCallback4(Object^ sender, DownloadProgressChangedEventArgs^ e)
{
   // Displays the operation identifier, and the transfer progress.
   Console::WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
      (String ^)e->UserState,
      e->BytesReceived,
      e->TotalBytesToReceive,
      e->ProgressPercentage);
}

// Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
public static void DownLoadFileInBackground4(string address)
{
    WebClient client = new WebClient();
    Uri uri = new Uri(address);

    // Specify a DownloadFileCompleted handler here...

    // Specify a progress notification handler.
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback4);

    client.DownloadFileAsync(uri, "serverdata.txt");
}

private static void DownloadProgressCallback4(object sender, DownloadProgressChangedEventArgs e)
{
    // Displays the operation identifier, and the transfer progress.
    Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
        (string)e.UserState,
        e.BytesReceived,
        e.TotalBytesToReceive,
        e.ProgressPercentage);
}

' Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
Public Shared Sub DownLoadFileInBackground4(ByVal address As String)

    Dim client As WebClient = New WebClient()

    ' Specify a DownloadFileCompleted handler here...

    '  Specify a progress notification handler.
    AddHandler client.DownloadProgressChanged, AddressOf DownloadProgressCallback4

    Dim uri as Uri = New Uri(address)
    client.DownloadFileAsync(uri, "serverdata.txt")

End Sub

Private Shared Sub DownloadProgressCallback4(ByVal sender As Object, ByVal e As DownloadProgressChangedEventArgs)
    ' Displays the operation identifier, and the transfer progress.
    Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...", _
    CStr(e.UserState), e.BytesReceived, e.TotalBytesToReceive, e.ProgressPercentage)
End Sub

注解

每次异步下载进行时都会引发此事件。 当使用以下任一方法开始下载时，将引发此事件。

方法	说明
DownloadDataAsync	从资源下载数据并返回 Byte 数组，而不会阻止调用线程。
DownloadFileAsync	将数据从资源下载到本地文件，而不会阻止调用线程。
OpenReadAsync	从资源返回数据，而不阻止调用线程。

此 DownloadProgressChangedEventHandler 事件的委托。 该 DownloadProgressChangedEventArgs 类为事件处理程序提供事件数据。

有关如何处理事件的详细信息，请参阅 处理和引发事件。

备注

被动 FTP 文件传输将始终显示进度百分比为零，因为服务器未发送文件大小。 若要显示进度，可以通过重写 GetWebRequest(Uri) 虚拟方法将 FTP 连接更改为活动：

internal class MyWebClient : WebClientProtocol
{
    protected override WebRequest GetWebRequest(Uri address)
    {
        FtpWebRequest req = (FtpWebRequest)base.GetWebRequest(address);
        req.UsePassive = false;
        return req;
    }
}