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處理非同步應用程式中的重新進入 (C# 和 Visual Basic)

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當您在應用程式中包含非同步程式碼，您應該考慮和可防止重新進入，參考重新進入非同步作業，在完成之前 如果您無法識別並不處理重新進入的可能性，它可能會產生未預期的結果。

本主題內容

注意事項注意事項

在 檢閱及執行範例應用程式 中的指示說明如何執行程式碼是 Windows Presentation Foundation (WPF) 應用程式或做為 Windows 市集應用程式。

若要將此範例當做 WPF 應用程式，您必須將 Visual Studio 2012 中， Windows 桌面上的 Visual Studio Express 2012 for或者在您的電腦上安裝 .NET Framework 4.5。

若要將此範例當做 Windows 市集 應用程式，您必須在電腦上安裝 Windows 8。此外，因此，如果您要從 Visual Studio 執行這個範例，您也必須在 Visual Studio 2012 已安裝或 Visual Studio Express 2012 for Windows 8 。

辨識 Reentrancy

在本主題中的範例中，使用者選取 [啟動] 按鈕啟始下載一系列的網站和計算位元組總數下載的非同步應用程式。 這個範例的同步版本將回應方式不論有多少次使用者選取按鈕，因為，在第一次以後， UI 執行緒忽略這些事件，直到應用程式完成執行。 在非同步應用程式，不過， UI 執行緒會繼續回應，因此，您可以重新進入非同步作業，在完成之前

如果使用者只能使用一次，請選擇 [啟動] 按鈕。下列範例顯示預期的輸出。 下載的網站清單會顯示與大小，以位元組為單位)，每個網站。 位元組總數會以結尾。

1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
4. msdn.microsoft.com/en-us/library/hh290140.aspx               117152
5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
7. msdn.microsoft.com                                            42972
8. msdn.microsoft.com/en-us/library/ff730837.aspx               146159

TOTAL bytes returned:  890591

然而，在中，如果使用者一次選取按鈕時，事件處理常式重複叫用，因此，下載流程每次重新進入。 因此，多個非同步作業同時執行，輸出交錯結果，因此，位元組總數會造成混淆。

1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
4. msdn.microsoft.com/en-us/library/hh290140.aspx               117152
5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
7. msdn.microsoft.com                                            42972
4. msdn.microsoft.com/en-us/library/hh290140.aspx               117152
8. msdn.microsoft.com/en-us/library/ff730837.aspx               146159

TOTAL bytes returned:  890591

5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
4. msdn.microsoft.com/en-us/library/hh290140.aspx               117152
7. msdn.microsoft.com                                            42972
5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
8. msdn.microsoft.com/en-us/library/ff730837.aspx               146159

TOTAL bytes returned:  890591

6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
7. msdn.microsoft.com                                            42972
8. msdn.microsoft.com/en-us/library/ff730837.aspx               146159

TOTAL bytes returned:  890591

您可以檢閱捲動造成此輸出至本主題結尾的程式碼。 您可以實驗程式碼透過下載方案加入至您的本機電腦然後執行 WebsiteDownload 項目或使用程式碼在這個主題的結尾建立專案。如需詳細資訊和指示，請參閱 檢閱及執行範例應用程式。

處理 Reentrancy

您可以根據處理重新以各種方式，您希望應用程式。 本主題提供下列範例:

  • 停用切換

    請停用 [啟動] 按鈕，當作業執行時，讓使用者無法中斷。

  • 移除後再重新啟動作業

    取消仍然參與的任何作業，當使用者再次選擇 [啟動] 按鈕，然後讓最近要求的動作會繼續。

  • 加入多個動作並將輸出

    允許所有要求的作業以非同步方式執行，不過，協調輸出所示，以便從每個作業的結果會顯示以及順序。

JJ651641.collapse_all(zh-tw,VS.110).gif停用切換

您可以封鎖 [啟動] 按鈕，在作業時停用按鈕負責在 StartButton_Click 事件處理常式的頂端。 您可以重新啟用按鈕從 finally 區塊內，當作業完成時，讓使用者可以再次執行應用程式。

下列程式碼顯示這些變更，標示星號。 您可以加入程式碼。本主題結尾的，也可以下載完成的應用程式從 Async 範例:在 .NET Framework 傳統型應用程式的 Reentrancy 或 Async 範例:在 Windows 市集應用程式的 Reentrancy。 專案名稱是 DisableStartButton。

Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
    ' This line is commented out to make the results clearer in the output.
    'ResultsTextBox.Text = ""

    ' ***Disable the Start button until the downloads are complete. 
    StartButton.IsEnabled = False

    Try
        Await AccessTheWebAsync()

    Catch ex As Exception
        ResultsTextBox.Text &= vbCrLf & "Downloads failed."
    ' ***Enable the Start button in case you want to run the program again. 
    Finally
        StartButton.IsEnabled = True

    End Try
End Sub

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    // This line is commented out to make the results clearer in the output.
    //ResultsTextBox.Text = "";

    // ***Disable the Start button until the downloads are complete. 
    StartButton.IsEnabled = false; 

    try
    {
        await AccessTheWebAsync();
    }
    catch (Exception)
    {
        ResultsTextBox.Text += "\r\nDownloads failed.";
    }
    // ***Enable the Start button in case you want to run the program again. 
    finally
    {
        StartButton.IsEnabled = true;
    }
}

由於變更，按鈕沒有回應，當 AccessTheWebAsync 下載網站時，處理序，所以無法重新進入。

JJ651641.collapse_all(zh-tw,VS.110).gif移除後再重新啟動作業

而不會停用 [啟動] 按鈕，您可以將按鈕現用，但是，如果使用者，請選取該按鈕，取消已經參與並讓最近起始動作繼續作業。

如需取消作業的詳細資訊，請參閱 最佳化您的非同步應用程式。

若要設定這個案例中，對 檢閱及執行範例應用程式中提供的基本程式碼進行下列變更。 您也可以下載完成的應用程式從 Async 範例:在 .NET Framework 傳統型應用程式的 Reentrancy 或 Async 範例:在 Windows 市集應用程式的 Reentrancy。 這個專案名稱是 CancelAndRestart。

  1. 宣告 CancellationTokenSource 變數，則為 cts，在所有方法的範圍。

    Class MainWindow // Or Class MainPage

    ' *** Declare a System.Threading.CancellationTokenSource.
    Dim cts As CancellationTokenSource

    public partial class MainWindow : Window   // Or class MainPage
{
    // *** Declare a System.Threading.CancellationTokenSource.
    CancellationTokenSource cts;

  2. 在 StartButton_Click中，判斷作業是否已在執行中。 如果 cts 的值為 null (在 Visual Basic 中為Nothing )，任何作業已在使用中。 如果值不是 null，正在執行的作業已取消。

    ' *** If a download process is already underway, cancel it.
If cts IsNot Nothing Then
    cts.Cancel()
End If

    // *** If a download process is already underway, cancel it.
if (cts != null)
{
    cts.Cancel();
}

  3. 將 cts 設為代表目前處理序的不同的值。

    ' *** Now set cts to cancel the current process if the button is chosen again.
Dim newCTS As CancellationTokenSource = New CancellationTokenSource()
cts = newCTS

    // *** Now set cts to a new value that you can use to cancel the current process
// if the button is chosen again.
CancellationTokenSource newCTS = new CancellationTokenSource();
cts = newCTS;

  4. 在 StartButton_Click時，目前的處理序完成，因此，將 cts 的值為 null。

    ' *** When the process completes, signal that another process can proceed.
If cts Is newCTS Then
    cts = Nothing
End If

    // *** When the process is complete, signal that another process can begin.
if (cts == newCTS)
    cts = null;

下列程式碼顯示在 StartButton_Click上的所有變更。 加入標記以星號。

Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)

    ' This line is commented out to make the results clearer. 
    'ResultsTextBox.Text = ""


    ' *** If a download process is underway, cancel it.
    If cts IsNot Nothing Then
        cts.Cancel()
    End If

    ' *** Now set cts to cancel the current process if the button is chosen again.
    Dim newCTS As CancellationTokenSource = New CancellationTokenSource()
    cts = newCTS

    Try
        ' *** Send a token to carry the message if the operation is canceled.
        Await AccessTheWebAsync(cts.Token)
        

    Catch ex As OperationCanceledException
        ResultsTextBox.Text &= vbCrLf & "Download canceled." & vbCrLf

    Catch ex As Exception
        ResultsTextBox.Text &= vbCrLf & "Downloads failed."
    End Try

    ' *** When the process is complete, signal that another process can proceed.
    If cts Is newCTS Then
        cts = Nothing
    End If
End Sub

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    // This line is commented out to make the results clearer in the output.
    //ResultsTextBox.Clear();

    // *** If a download process is already underway, cancel it.
    if (cts != null)
    {
        cts.Cancel();
    }

    // *** Now set cts to cancel the current process if the button is chosen again.
    CancellationTokenSource newCTS = new CancellationTokenSource();
    cts = newCTS;

    try
    {
        // ***Send cts.Token to carry the message if there is a cancellation request.
        await AccessTheWebAsync(cts.Token);
        
    }
    // *** Catch cancellations separately.
    catch (OperationCanceledException)
    {
        ResultsTextBox.Text += "\r\nDownloads canceled.\r\n";
    }
    catch (Exception)
    {
        ResultsTextBox.Text += "\r\nDownloads failed.\r\n";
    }
    // *** When the process is complete, signal that another process can proceed.
    if (cts == newCTS)
        cts = null;
}

在 AccessTheWebAsync中，進行下列變更。

  • 將參數接受來自 StartButton_Click的取消語彙基元。

  • 因為 GetAsync 接受 CancellationToken 引數，請使用 GetAsync 方法下載網站。

  • 在呼叫 DisplayResults 之前顯示每個下載網站的結果，檢查 ct 驗證目前作業已取消。

下列程式碼顯示這些變更，標示星號。

' *** Provide a parameter for the CancellationToken from StartButton_Click.
Private Async Function AccessTheWebAsync(ct As CancellationToken) As Task

    ' Declare an HttpClient object.
    Dim client = New HttpClient()

    ' Make a list of web addresses.
    Dim urlList As List(Of String) = SetUpURLList()

    Dim total = 0
    Dim position = 0

    For Each url In urlList
        ' *** Use the HttpClient.GetAsync method because it accepts a 
        ' cancellation token.
        Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

        ' *** Retrieve the website contents from the HttpResponseMessage.
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        ' *** Check for cancellations before displaying information about the 
        ' latest site. 
        ct.ThrowIfCancellationRequested()

        position += 1
        DisplayResults(url, urlContents, position)

        ' Update the total.
        total += urlContents.Length
    Next

    ' Display the total count for all of the websites.
    ResultsTextBox.Text &=
        String.Format(vbCrLf & vbCrLf & "TOTAL bytes returned:  " & total & vbCrLf)
End Function

// *** Provide a parameter for the CancellationToken from StartButton_Click.
async Task AccessTheWebAsync(CancellationToken ct)
{
    // Declare an HttpClient object.
    HttpClient client = new HttpClient();

    // Make a list of web addresses.
    List<string> urlList = SetUpURLList();

    var total = 0;
    var position = 0;

    foreach (var url in urlList)
    {
        // *** Use the HttpClient.GetAsync method because it accepts a 
        // cancellation token.
        HttpResponseMessage response = await client.GetAsync(url, ct);

        // *** Retrieve the website contents from the HttpResponseMessage.
        byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

        // *** Check for cancellations before displaying information about the 
        // latest site. 
        ct.ThrowIfCancellationRequested();

        DisplayResults(url, urlContents, ++position);

        // Update the total.
        total += urlContents.Length;
    }

    // Display the total count for all of the websites.
    ResultsTextBox.Text +=
        string.Format("\r\n\r\nTOTAL bytes returned:  {0}\r\n", total);
}

如果您選取 [啟動] 按鈕幾次，該應用程式執行時，它應該會產生類似下列的輸出結果。

1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
4. msdn.microsoft.com/en-us/library/hh290140.aspx               122505
5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
Download canceled.

1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
Download canceled.

1. msdn.microsoft.com/en-us/library/hh191443.aspx                83732
2. msdn.microsoft.com/en-us/library/aa578028.aspx               205273
3. msdn.microsoft.com/en-us/library/jj155761.aspx                29019
4. msdn.microsoft.com/en-us/library/hh290140.aspx               117152
5. msdn.microsoft.com/en-us/library/hh524395.aspx                68959
6. msdn.microsoft.com/en-us/library/ms404677.aspx               197325
7. msdn.microsoft.com                                            42972
8. msdn.microsoft.com/en-us/library/ff730837.aspx               146159

TOTAL bytes returned:  890591

排除部分清單，請在 StartButton_Click 中的第一個程式碼會清除文字方塊，每次使用者重新啟動作業。

JJ651641.collapse_all(zh-tw,VS.110).gif加入多個動作並將輸出

這個第三個範例是最複雜的因為應用程式每次啟動另一個非同步作業使用者選取 [啟動] 按鈕和所有動作加入至完成。 所有要求的作業下載從清單的網站非同步，不過，依序顯示從作業的輸出。 即實際下載活動交錯，，因為在 辨識 Reentrancy 的輸出所示，不過，分別顯示每個群組結果清單。

作業共用全域 Task， pendingWork，做為閘道管理員做為顯示處理序。

您可以將變更遇到此範例程式碼可以在 建置應用程式上，也可以依照 下載應用程式 中的指示下載這個範例會執行 QueueResults 項目。

如果使用者只能使用一次，然後選取 [啟動] 按鈕下列輸出顯示結果。 字母標籤， A，表示這個結果與 [啟動] 按鈕第一次被選取。 數字在下載目標清單顯示 URL 的順序。

#Starting group A.
#Task assigned for group A.

A-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87389
A-2. msdn.microsoft.com/en-us/library/aa578028.aspx               209858
A-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870
A-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119027
A-5. msdn.microsoft.com/en-us/library/hh524395.aspx                71260
A-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199186
A-7. msdn.microsoft.com                                            53266
A-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148020

TOTAL bytes returned:  918876


#Group A is complete.

如果使用者選取 [啟動] 按鈕三次，應用程式會產生類似下列的輸出。 開始使用井字號的資訊行 (#) 追蹤應用程式的進度。

#Starting group A.
#Task assigned for group A.

A-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87389
A-2. msdn.microsoft.com/en-us/library/aa578028.aspx               207089
A-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870
A-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119027
A-5. msdn.microsoft.com/en-us/library/hh524395.aspx                71259
A-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199185

#Starting group B.
#Task assigned for group B.

A-7. msdn.microsoft.com                                            53266

#Starting group C.
#Task assigned for group C.

A-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148010

TOTAL bytes returned:  916095

B-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87389
B-2. msdn.microsoft.com/en-us/library/aa578028.aspx               207089
B-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870
B-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119027
B-5. msdn.microsoft.com/en-us/library/hh524395.aspx                71260
B-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199186

#Group A is complete.

B-7. msdn.microsoft.com                                            53266
B-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148010

TOTAL bytes returned:  916097

C-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87389
C-2. msdn.microsoft.com/en-us/library/aa578028.aspx               207089

#Group B is complete.

C-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870
C-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119027
C-5. msdn.microsoft.com/en-us/library/hh524395.aspx                72765
C-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199186
C-7. msdn.microsoft.com                                            56190
C-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148010

TOTAL bytes returned:  920526

#Group C is complete.

群組 B 和 C 開頭，在群組 A 完成前，不過，每個群組的輸出單獨出現。 群組中的所有輸出出現，後面接著群組 B 的所有輸出，然後群組的 C# 所有輸出。 應用程式一定會顯示群組順序，此外，每個群組，永遠顯示有關個別網站的資訊順序 URL 會出現在 URL 清單。

不過，您無法預測下載實際發生的命令。 在多個群組啟動後，會產生下載工作是所有作用中。 您不能假設， A-1 在 B-1 前將下載，因此，您無法假設， A-1 在 A-2 前將下載。

JJ651641.collapse_all(zh-tw,VS.110).gif全域定義

範例程式碼包含從任何方法可以看見下列兩個全域宣告。

Class MainWindow    ' Class MainPage in Windows Store app.

    ' ***Declare the following variables where all methods can access them. 
    Private pendingWork As Task = Nothing
    Private group As Char = ChrW(AscW("A") - 1)

public partial class MainWindow : Window  // Class MainPage in Windows Store app.
{
    // ***Declare the following variables where all methods can access them. 
    private Task pendingWork = null;   
    private char group = (char)('A' - 1);

Task 變數，則為 pendingWork，監控顯示處理序並防止任何群組中斷另一個群組顯示作業。 字元變數，則為 group，標籤從驗證不同群組的輸出結果按預期的順序顯示。

JJ651641.collapse_all(zh-tw,VS.110).gif按一下事件處理常式

事件處理常式，則為 StartButton_Click，將群組字母，每次使用者選擇 [啟動] 按鈕。 然後會呼叫處理常式 AccessTheWebAsync 參與下載動作。

Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
    ' ***Verify that each group's results are displayed together, and that
    ' the groups display in order, by marking each group with a letter.
    group = ChrW(AscW(group) + 1)
    ResultsTextBox.Text &= String.Format(vbCrLf & vbCrLf & "#Starting group {0}.", group)

    Try
        ' *** Pass the group value to AccessTheWebAsync.
        Dim finishedGroup As Char = Await AccessTheWebAsync(group)

        ' The following line verifies a successful return from the download and 
        ' display procedures. 
        ResultsTextBox.Text &= String.Format(vbCrLf & vbCrLf & "#Group {0} is complete." & vbCrLf, finishedGroup)

    Catch ex As Exception
        ResultsTextBox.Text &= vbCrLf & "Downloads failed."

    End Try
End Sub

private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    // ***Verify that each group's results are displayed together, and that
    // the groups display in order, by marking each group with a letter.
    group = (char)(group + 1);
    ResultsTextBox.Text += string.Format("\r\n\r\n#Starting group {0}.", group);

    try
    {
        // *** Pass the group value to AccessTheWebAsync.
        char finishedGroup = await AccessTheWebAsync(group);

        // The following line verifies a successful return from the download and
        // display procedures. 
        ResultsTextBox.Text += string.Format("\r\n\r\n#Group {0} is complete.\r\n", finishedGroup);
    }
    catch (Exception)
    {
        ResultsTextBox.Text += "\r\nDownloads failed.";
    }
}

JJ651641.collapse_all(zh-tw,VS.110).gifAccessTheWebAsync 方法

這個範例會將 AccessTheWebAsync 轉換成方法。 第一個方法，則為 AccessTheWebAsync，開始群組的所有下載工作並將 pendingWork 控制項會顯示處理序。 方法會使用語言整合查詢 (LINQ 查詢)，並啟動所有下載的 ToArray<TSource> 同時指派。

AccessTheWebAsync 接著會呼叫 FinishOneGroupAsync 等候每下載完成並顯示其長度。

FinishOneGroupAsync 會傳回指派給 AccessTheWebAsync的 pendingWork 的工作。 值是由其他作業防止中斷，在工作完成之前。

Private Async Function AccessTheWebAsync(grp As Char) As Task(Of Char)

    Dim client = New HttpClient()

    ' Make a list of the web addresses to download.
    Dim urlList As List(Of String) = SetUpURLList()

    ' ***Kick off the downloads. The application of ToArray activates all the download tasks.
    Dim getContentTasks As Task(Of Byte())() =
        urlList.Select(Function(addr) client.GetByteArrayAsync(addr)).ToArray()

    ' ***Call the method that awaits the downloads and displays the results.
    ' Assign the Task that FinishOneGroupAsync returns to the gatekeeper task, pendingWork.
    pendingWork = FinishOneGroupAsync(urlList, getContentTasks, grp)

    ResultsTextBox.Text &=
        String.Format(vbCrLf & "#Task assigned for group {0}. Download tasks are active." & vbCrLf, grp)

    ' ***This task is complete when a group has finished downloading and displaying.
    Await pendingWork

    ' You can do other work here or just return.
    Return grp
End Function

private async Task<char> AccessTheWebAsync(char grp)
{
    HttpClient client = new HttpClient();

    // Make a list of the web addresses to download.
    List<string> urlList = SetUpURLList();

    // ***Kick off the downloads. The application of ToArray activates all the download tasks.
    Task<byte[]>[] getContentTasks = urlList.Select(url => client.GetByteArrayAsync(url)).ToArray();

    // ***Call the method that awaits the downloads and displays the results.
    // Assign the Task that FinishOneGroupAsync returns to the gatekeeper task, pendingWork.
    pendingWork = FinishOneGroupAsync(urlList, getContentTasks, grp);

    ResultsTextBox.Text += string.Format("\r\n#Task assigned for group {0}. Download tasks are active.\r\n", grp);

    // ***This task is complete when a group has finished downloading and displaying.
    await pendingWork;

    // You can do other work here or just return.
    return grp;
}

JJ651641.collapse_all(zh-tw,VS.110).gifFinishOneGroupAsync 方法

下載在群組中配置的這個方法循環，等候每一個，顯示下載網站的長度和將長度到總計。

FinishOneGroupAsync 中的第一個陳述式使用 pendingWork ，確保輸入方法不會妨礙已經在顯示流程或已暫止的作業。 如果這類作業正在進行中，編碼作業需要等待關閉。

Private Async Function FinishOneGroupAsync(urls As List(Of String), contentTasks As Task(Of Byte())(), grp As Char) As Task

    ' Wait for the previous group to finish displaying results.
    If pendingWork IsNot Nothing Then
        Await pendingWork
    End If

    Dim total = 0

    ' contentTasks is the array of Tasks that was created in AccessTheWebAsync.
    For i As Integer = 0 To contentTasks.Length - 1
        ' Await the download of a particular URL, and then display the URL and
        ' its length.
        Dim content As Byte() = Await contentTasks(i)
        DisplayResults(urls(i), content, i, grp)
        total += content.Length
    Next

    ' Display the total count for all of the websites.
    ResultsTextBox.Text &=
        String.Format(vbCrLf & vbCrLf & "TOTAL bytes returned:  " & total & vbCrLf)
End Function

private async Task FinishOneGroupAsync(List<string> urls, Task<byte[]>[] contentTasks, char grp)
{
    // ***Wait for the previous group to finish displaying results.
    if (pendingWork != null) await pendingWork;

    int total = 0;

    // contentTasks is the array of Tasks that was created in AccessTheWebAsync.
    for (int i = 0; i < contentTasks.Length; i++)
    {
        // Await the download of a particular URL, and then display the URL and
        // its length.
        byte[] content = await contentTasks[i];
        DisplayResults(urls[i], content, i, grp);
        total += content.Length;
    }

    // Display the total count for all of the websites.
    ResultsTextBox.Text +=
        string.Format("\r\n\r\nTOTAL bytes returned:  {0}\r\n", total);
}

您可以將變更遇到此範例程式碼可以在 建置應用程式上，也可以依照 下載應用程式 中的指示下載範例，然後執行 QueueResults 項目。

JJ651641.collapse_all(zh-tw,VS.110).gif問題值得注意

開始一個井字號的資訊行 (#) 在輸出澄清這個範例的運作方式。

輸出顯示下列樣式。

  • 群組可以啟動，當先前群組顯示其輸出時，，但不會破壞先前群組的輸出中顯示。

    #Starting group A.
#Task assigned for group A. Download tasks are active.

A-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87389
A-2. msdn.microsoft.com/en-us/library/aa578028.aspx               207089
A-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870
A-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119037
A-5. msdn.microsoft.com/en-us/library/hh524395.aspx                71260

#Starting group B.
#Task assigned for group B. Download tasks are active.

A-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199186
A-7. msdn.microsoft.com                                            53078
A-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148010

TOTAL bytes returned:  915919

B-1. msdn.microsoft.com/en-us/library/hh191443.aspx                87388
B-2. msdn.microsoft.com/en-us/library/aa578028.aspx               207089
B-3. msdn.microsoft.com/en-us/library/jj155761.aspx                30870

#Group A is complete.

B-4. msdn.microsoft.com/en-us/library/hh290140.aspx               119027
B-5. msdn.microsoft.com/en-us/library/hh524395.aspx                71260
B-6. msdn.microsoft.com/en-us/library/ms404677.aspx               199186
B-7. msdn.microsoft.com                                            53078
B-8. msdn.microsoft.com/en-us/library/ff730837.aspx               148010

TOTAL bytes returned:  915908

  • pendingWork 工作只能在 FinishOneGroupAsync 為 null (在 Visual Basic 中為Nothing ) 群組的 A，第一次啟動。 在進入 FinishOneGroupAsync時，西甲尚未完成等候運算式。 因此，控制項不會傳回至，則為 AccessTheWebAsync，且 pendingWork 的第一項工作尚未執行。

  • 下列兩行一起永遠出現在輸出中。 程式碼絕對不會中斷開始 StartButton_Click 群組的作業和指派群組之工作之間的 pendingWork。

    #Starting group B.
#Task assigned for group B. Download tasks are active.

    在群組進入 StartButton_Click之後，作業才會完成運算式等候，直到作業進入 FinishOneGroupAsync。 因此，在程式碼中的該區段期間，其他作業無法取得控制項。

檢閱及執行範例應用程式

進一步了解範例應用程式，您可以下載，建置自己或檢閱程式碼在這個主題的結尾，而不需要實作應用程式。

注意事項注意事項

若要執行這個範例是 Windows Presentation Foundation (WPF) 傳統型應用程式，您必須將 Visual Studio 2012 中， Windows 桌面上的 Visual Studio Express 2012 for或者在您的電腦上安裝 .NET Framework 4.5。

若要將此範例當做 Windows 市集 應用程式，您必須在電腦上安裝 Windows 8。此外，因此，如果您要從 Visual Studio 執行這個範例，您也必須在 Visual Studio 2012 已安裝或 Visual Studio Express 2012 for Windows 8 。Visual Studio 2010 無法載入為 .NET Framework 4.5 為目標的專案。

JJ651641.collapse_all(zh-tw,VS.110).gif下載應用程式

  1. 下載壓縮檔從 Async 範例:在 .NET Framework 傳統型應用程式的 Reentrancy 或 Async 範例:在 Windows 市集應用程式的 Reentrancy。

  2. 解壓縮您下載的檔案，然後啟動 Visual Studio。

  3. 在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。

  4. 巡覽至保存解壓縮的範例程式碼的資料夾，然後開啟方案 (.sln) 檔。

  5. 在 [方案總管] 中，開啟您要執行的項目的捷徑功能表，然後選擇 [做為 StartUpProject 的集合]。

  6. 選取 CTRL+F5 鍵建置及執行專案。

JJ651641.collapse_all(zh-tw,VS.110).gif建置應用程式

下列各節提供程式碼建立範例做為 WPF 應用程式或做為 Windows 市集 應用程式。

建置 WPF 應用程式

  1. 啟動 Visual Studio 2012。

  2. 在功能表列上，選擇 [檔案]、[新增]、[專案]。

    [新增專案] 對話方塊隨即開啟。

  3. 在 [已安裝的範本] 窗格中，展開 [Visual Basic] 或 [Visual C#]，然後展開 [視窗]。

  4. 在專案類型清單中，選取 [WPF 應用程式]。

  5. 將專案命名為 WebsiteDownloadWPF，然後選擇 [OK] 按鈕。

    新專案即會出現於 [方案總管] 中。

  6. 在 Visual Studio 程式碼編輯器中，選取 [MainWindow.xaml] 索引標籤。

    如果這個選項未顯示，請開啟 MainWindow.xaml 的捷徑功能表。 [方案總管]，然後選取 [檢視程式碼]。

  7. 在 MainWindow.xaml [XAML] 檢視中，以下列程式碼取代此程式碼。

    <Window x:Class="MainWindow"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WebsiteDownloadWPF"
    xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Width="517" Height="360">
        <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="-1,0,0,0" VerticalAlignment="Top" Click="StartButton_Click" Height="53" Background="#FFA89B9B" FontSize="36" Width="518"  />
        <TextBox x:Name="ResultsTextBox" HorizontalAlignment="Left" Margin="-1,53,0,-36" TextWrapping="Wrap" VerticalAlignment="Top" Height="343" FontSize="10" ScrollViewer.VerticalScrollBarVisibility="Visible" Width="518" FontFamily="Lucida Console" />
    </Grid>
</Window>

    <Window x:Class="WebsiteDownloadWPF.MainWindow"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WebsiteDownloadWPF"
    xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Width="517" Height="360">
        <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="-1,0,0,0" VerticalAlignment="Top" Click="StartButton_Click" Height="53" Background="#FFA89B9B" FontSize="36" Width="518"  />
        <TextBox x:Name="ResultsTextBox" HorizontalAlignment="Left" Margin="-1,53,0,-36" TextWrapping="Wrap" VerticalAlignment="Top" Height="343" FontSize="10" ScrollViewer.VerticalScrollBarVisibility="Visible" Width="518" FontFamily="Lucida Console" />
    </Grid>
</Window>

    包含文字方塊和按鈕的簡單視窗會出現在 MainWindow.xaml [設計] 檢視中。

  8. 加入 System.Net.Http的參考。

  9. 在 [方案總管]，開啟 MainWindow.xaml.vb 或 MainWindow.xaml.cs 的捷徑功能表，然後選取 [檢視程式碼]。

  10. 在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中，以下列程式碼取代中的程式碼。

    ' Add the following Imports statements, and add a reference for System.Net.Http.
Imports System.Net.Http
Imports System.Threading

Class MainWindow

    Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
        ' This line is commented out to make the results clearer in the output.
        'ResultsTextBox.Text = ""

        Try
            Await AccessTheWebAsync()

        Catch ex As Exception
            ResultsTextBox.Text &= vbCrLf & "Downloads failed."

        End Try
    End Sub


    Private Async Function AccessTheWebAsync() As Task

        ' Declare an HttpClient object.
        Dim client = New HttpClient()

        ' Make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        Dim total = 0
        Dim position = 0

        For Each url In urlList
            ' GetByteArrayAsync returns a task. At completion, the task
            ' produces a byte array.
            Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)

            position += 1
            DisplayResults(url, urlContents, position)

            ' Update the total.
            total += urlContents.Length
        Next

        ' Display the total count for all of the websites.
        ResultsTextBox.Text &=
            String.Format(vbCrLf & vbCrLf & "TOTAL bytes returned:  " & total & vbCrLf)
    End Function


    Private Function SetUpURLList() As List(Of String)
        Dim urls = New List(Of String) From
        {
            "https://msdn.microsoft.com/en-us/library/hh191443.aspx",
            "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
            "https://msdn.microsoft.com/en-us/library/jj155761.aspx",
            "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
            "https://msdn.microsoft.com/en-us/library/hh524395.aspx",
            "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
            "https://msdn.microsoft.com",
            "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
        }
        Return urls
    End Function


    Private Sub DisplayResults(url As String, content As Byte(), pos As Integer)
        ' Display the length of each website. The string format is designed
        ' to be used with a monospaced font, such as Lucida Console or
        ' Global Monospace.

        ' Strip off the "http:'".
        Dim displayURL = url.Replace("http://", "")
        ' Display position in the URL list, the URL, and the number of bytes.
        ResultsTextBox.Text &= String.Format(vbCrLf & "{0}. {1,-58} {2,8}", pos, displayURL, content.Length)
    End Sub
End Class

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add the following using directives, and add a reference for System.Net.Http.
using System.Net.Http;
using System.Threading;

namespace WebsiteDownloadWPF
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }


        private async void StartButton_Click(object sender, RoutedEventArgs e)
        {
            // This line is commented out to make the results clearer in the output.
            //ResultsTextBox.Text = "";

            try
            {
                await AccessTheWebAsync();
            }
            catch (Exception)
            {
                ResultsTextBox.Text += "\r\nDownloads failed.";
            }
        }


        private async Task AccessTheWebAsync()
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            var total = 0;
            var position = 0;

            foreach (var url in urlList)
            {
                // GetByteArrayAsync returns a task. At completion, the task
                // produces a byte array.
                byte[] urlContents = await client.GetByteArrayAsync(url);

                DisplayResults(url, urlContents, ++position);

                // Update the total.
                total += urlContents.Length;
            }

            // Display the total count for all of the websites.
            ResultsTextBox.Text +=
                string.Format("\r\n\r\nTOTAL bytes returned:  {0}\r\n", total);
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com/en-us/library/hh191443.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/jj155761.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/hh524395.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }


        private void DisplayResults(string url, byte[] content, int pos)
        {
            // Display the length of each website. The string format is designed
            // to be used with a monospaced font, such as Lucida Console or 
            // Global Monospace.

            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            // Display position in the URL list, the URL, and the number of bytes.
            ResultsTextBox.Text += string.Format("\n{0}. {1,-58} {2,8}", pos, displayURL, content.Length);
        }
    }
}

  11. 選取 CTRL+F5 鍵執行程式，然後選擇 [啟動] 按鈕幾次。

  12. 將從 停用切換、 移除後再重新啟動作業或 加入多個動作並將輸出 的變更處理重新進入。

建置 Windows 市集應用程式

  1. 啟動 Visual Studio 2012。

  2. 在功能表列上，選擇 [檔案]、[新增]、[專案]。

    [新增專案] 對話方塊隨即開啟。

  3. 在 [安裝] 中， [ [範本] 分類中，展開 [Visual Basic] 或 [Visual C#]，然後展開 [Windows 市集]。

  4. 在專案類型清單中，選取 [空白的應用程式 (XAML)]。

  5. 將專案命名為 WebsiteDownloadWin，然後選擇 [OK] 按鈕。

    新專案即會出現於 [方案總管] 中。

  6. 在 [方案總管] 中，開啟 MainPage.xaml 的捷徑功能表，然後選擇 [開啟]。

  7. 在 [XAML] 視窗中 MainPage.xaml，以下列程式碼取代程式碼。

    <Page
    x:Class="WebsiteDownloadWin.MainPage"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WebsiteDownloadWin"
    xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" FontSize="12">

    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="325,77,0,0" VerticalAlignment="Top" Click="StartButton_Click" Height="145" Background="#FFA89B9B" FontSize="36" Width="711"  />
        <TextBox x:Name="ResultsTextBox" HorizontalAlignment="Left" Margin="325,222,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Height="546" FontSize="10" ScrollViewer.VerticalScrollBarVisibility="Visible" Width="711" FontFamily="Lucida Console" />
    </Grid>
</Page>

    包含文字方塊和 [啟動] 按鈕的簡易視窗會出現在 [設計] 視窗中 MainPage.xaml。

  8. 在 [方案總管] 中，開啟或 MainPage.xaml.vb MainPage.xaml.cs 的捷徑功能表，然後選擇 [檢視程式碼]。

  9. 使用下列程式碼取代或 MainPage.xaml.vb MainPage.xaml.cs 的程式碼。

    ' Add the following Imports statements.
Imports System.Threading.Tasks
Imports System.Threading
Imports System.Net.Http

Public NotInheritable Class MainPage
    Inherits Page

    Protected Overrides Sub OnNavigatedTo(e As Navigation.NavigationEventArgs)
    End Sub


    Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
        ' This line is commented out to make the results clearer in the output.
        'ResultsTextBox.Text = ""

        Try
            Await AccessTheWebAsync()

        Catch ex As Exception
            ResultsTextBox.Text &= vbCrLf & "Downloads failed."

        End Try
    End Sub


    Private Async Function AccessTheWebAsync() As Task

        ' Declare an HttpClient object.
        Dim client = New HttpClient()

        ' Make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        Dim total = 0
        Dim position = 0

        For Each url In urlList
            ' GetByteArrayAsync returns a task. At completion, the task
            ' produces a byte array.
            Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)

            position += 1
            DisplayResults(url, urlContents, position)

            ' Update the total.
            total += urlContents.Length
        Next

        ' Display the total count for all of the websites.
        ResultsTextBox.Text &=
            String.Format(vbCrLf & vbCrLf & "TOTAL bytes returned:  " & total & vbCrLf)
    End Function


    Private Function SetUpURLList() As List(Of String)
        Dim urls = New List(Of String) From
        {
            "https://msdn.microsoft.com/en-us/library/hh191443.aspx",
            "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
            "https://msdn.microsoft.com/en-us/library/jj155761.aspx",
            "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
            "https://msdn.microsoft.com/en-us/library/hh524395.aspx",
            "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
            "https://msdn.microsoft.com",
            "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
        }
        Return urls
    End Function


    Private Sub DisplayResults(url As String, content As Byte(), pos As Integer)
        ' Display the length of each website. The string format is designed
        ' to be used with a monospaced font, such as Lucida Console or
        ' Global Monospace.

        ' Strip off the "http:'".
        Dim displayURL = url.Replace("http://", "")
        ' Display position in the URL list, the URL, and the number of bytes.
        ResultsTextBox.Text &= String.Format(vbCrLf & "{0}. {1,-58} {2,8}", pos, displayURL, content.Length)
    End Sub
End Class

    using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// Add the following using directives. 
using System.Threading.Tasks;
using System.Threading;
using System.Net.Http;


namespace WebsiteDownloadWin
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }

        private async void StartButton_Click(object sender, RoutedEventArgs e)
        {
            // This line is commented out to make the results clearer in the output.
            //ResultsTextBox.Text = "";

            try
            {
                await AccessTheWebAsync();
            }
            catch (Exception)
            {
                ResultsTextBox.Text += "\r\nDownloads failed.";
            }
        }


        private async Task AccessTheWebAsync()
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            var total = 0;
            var position = 0;

            foreach (var url in urlList)
            {
                // GetByteArrayAsync returns a task. At completion, the task
                // produces a byte array.
                byte[] urlContents = await client.GetByteArrayAsync(url);

                DisplayResults(url, urlContents, ++position);

                // Update the total.
                total += urlContents.Length;
            }

            // Display the total count for all of the websites.
            ResultsTextBox.Text +=
                string.Format("\r\n\r\nTOTAL bytes returned:  {0}\r\n", total);
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com/en-us/library/hh191443.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/jj155761.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/hh524395.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }


        private void DisplayResults(string url, byte[] content, int pos)
        {
            // Display the length of each website. The string format is designed
            // to be used with a monospaced font, such as Lucida Console or
            // Global Monospace.

            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            // Display position in the URL list, the URL, and the number of bytes.
            ResultsTextBox.Text += string.Format("\n{0}. {1,-58} {2,8}", pos, displayURL, content.Length);
        }
    }
}

  10. 選取 CTRL+F5 鍵執行程式，然後選擇 [啟動] 按鈕幾次。

  11. 將從 停用切換、 移除後再重新啟動作業或 加入多個動作並將輸出 的變更處理重新進入。

請參閱

工作

逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)

概念

使用 Async 和 Await 設計非同步程式 (C# 和 Visual Basic)

其他資源

非同步程式設計 (Windows 市集應用程式)

快速入門:呼叫以 C# 或 Visual Basic 的非同步 API