HOW TO：同時發出多個 Web 要求 (C# 和 Visual Basic)

發行項
05/01/2013

在非同步方法，，會在建立時，工作開始。等候 (Visual Basic) 或等候 (C#) 運算子套用至中的工作會在處理無法繼續的方法，直到工作完成為止。常用的工作等候，而建立，，如下列範例所示。

Dim result = Await someWebAccessMethodAsync(url)

var result = await someWebAccessMethodAsync(url);

不過，您可以從等候工作分離的工作，如果程式具有的內容不相依於這項工作的完成的其他工作。

' The following line creates and starts the task.
Dim myTask = someWebAccessMethodAsync(url)

' While the task is running, you can do other work that does not depend
' on the results of the task.
' . . . . .

' The application of Await suspends the rest of this method until the task is 
' complete.
Dim result = Await myTask

// The following line creates and starts the task.
var myTask = someWebAccessMethodAsync(url);

// While the task is running, you can do other work that doesn't depend
// on the results of the task.
// . . . . .

// The application of await suspends the rest of this method until the task is complete.
var result = await myTask;

在開始工作和等待之間，您可以啟動其他工作。其他工作平行隱含地執行，不過，其他執行緒就不會建立物件。

下列程式啟動三個非同步 Web 下載依它們被呼叫的順序然後等候它們。請注意，在中，當您執行程式時，工作不按順序一定是完成其建立並等待。它們開始執行時，會在建立時，，以及一或多項工作可能會完成，在方法到達等待運算式之前。

如需同時啟動多個工作的其他範例，請參閱 HOW TO：使用 Task.WhenAll 擴充逐步解說的內容 (C# 和 Visual Basic)。

若要完成這個專案，您必須在電腦上安裝 Visual Studio 2012 。如需詳細資訊，請參閱 Microsoft 網站 (英文)。

您可以下載這個範例的程式碼。開發人員程式碼範例

若要設定專案

若要設定 WPF 應用程式，請完成下列步驟。您可以在中找到這些步驟的詳細說明逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)。
- 建立包含文字方塊和按鈕的 WPF 應用程式。將按鈕 startButton，並將文字方塊 resultsTextBox。
- 將 System.Net.Http的參考。
- 在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中，將 Imports 陳述式或 using 指示詞 System.Net.Http的。

加入程式碼

在 [設計] 視窗，這個檔案是 MainWindow.xaml，在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中按兩下按鈕建立 startButton_Click 事件處理常式。如果選取這個選項，選項按鈕，選取 [內容] 視窗的 [所選取項目的事件處理常式] 圖示，然後輸入在 [按一下] 文字方塊的 startButton_Click 。

複製下列程式碼，並將它貼到 startButton_Click 主體在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中的。

resultsTextBox.Clear()
Await CreateMultipleTasksAsync()
resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click."

resultsTextBox.Clear();
await CreateMultipleTasksAsync();
resultsTextBox.Text += "\r\n\r\nControl returned to startButton_Click.\r\n";

程式碼會呼叫非同步方法， CreateMultipleTasksAsync，驅動應用程式。

將下列方法支援加入至專案:

ProcessURLAsync 使用一個 HttpClient 方法下載網站的內容做為位元組陣列。支援方法， ProcessURLAsync 然後顯示並傳回陣列的長度。
DisplayResults 顯示位元組數目將位元組陣列中的每個 URL 的。這會顯示每項工作何時完成下載。

複製下列方法，並在 MainWindow.xaml.vb 或 MainWindow.xaml.cs 中的 startButton_Click 事件處理常式之後貼上項目。

Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)

    Dim byteArray = Await client.GetByteArrayAsync(url)
    DisplayResults(url, byteArray)
    Return byteArray.Length
End Function


Private Sub DisplayResults(url As String, content As Byte())

    ' 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.
    Dim bytes = content.Length
    ' Strip off the "http://".
    Dim displayURL = url.Replace("http://", "")
    resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
End Sub

async Task<int> ProcessURLAsync(string url, HttpClient client)
{
    var byteArray = await client.GetByteArrayAsync(url);
    DisplayResults(url, byteArray);
    return byteArray.Length;
}


private void DisplayResults(string url, byte[] content)
{
    // 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.
    var bytes = content.Length;
    // Strip off the "http://".
    var displayURL = url.Replace("http://", "");
    resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);
}

最後，定義方法 CreateMultipleTasksAsync，執行下列步驟。

這個方法會宣告 HttpClient 物件，您必須在 ProcessURLAsync的存取方法 GetByteArrayAsync 。
方法會建立並啟動型別 Task<TResult>三項工作， TResult 是整數。在每項工作完成， DisplayResults 顯示下載內容之工作的 URL 和長度。因為工作以非同步方式執行，因此出現的順序可能會與其宣告的順序不同。
這個方法等候每項工作的完成。每個 Await 或 await 運算子暫止 CreateMultipleTasksAsync 的執行，直到這項等候的工作完成。運算子會從呼叫有時擷取傳回值指派給每一工作完成的 ProcessURLAsync 。
當工作完成時，以及整數值所擷取的，這個方法會網站的長度並顯示結果。

複製下列方法，並將其貼入方案。

Private Async Function CreateMultipleTasksAsync() As Task

    ' Declare an HttpClient object, and increase the buffer size. The
    ' default buffer size is 65,536.
    Dim client As HttpClient =
        New HttpClient() With {.MaxResponseContentBufferSize = 1000000}

    ' Create and start the tasks. As each task finishes, DisplayResults 
    ' displays its length.
    Dim download1 As Task(Of Integer) =
        ProcessURLAsync("https://msdn.microsoft.com", client)
    Dim download2 As Task(Of Integer) =
        ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client)
    Dim download3 As Task(Of Integer) =
        ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client)

    ' Await each task.
    Dim length1 As Integer = Await download1
    Dim length2 As Integer = Await download2
    Dim length3 As Integer = Await download3

    Dim total As Integer = length1 + length2 + length3

    ' Display the total count for all of the websites.
    resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                         "Total bytes returned:  {0}" & vbCrLf, total)
End Function

private async Task CreateMultipleTasksAsync()
{
    // Declare an HttpClient object, and increase the buffer size. The
    // default buffer size is 65,536.
    HttpClient client =
        new HttpClient() { MaxResponseContentBufferSize = 1000000 };

    // Create and start the tasks. As each task finishes, DisplayResults 
    // displays its length.
    Task<int> download1 = 
        ProcessURLAsync("https://msdn.microsoft.com", client);
    Task<int> download2 = 
        ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client);
    Task<int> download3 = 
        ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client);

    // Await each task.
    int length1 = await download1;
    int length2 = await download2;
    int length3 = await download3;

    int total = length1 + length2 + length3;

    // Display the total count for the downloaded websites.
    resultsTextBox.Text +=
        string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
}

選取按F5鍵執行程式，然後選取啟動按鈕。

執行程式幾次驗證三項工作不相同順序一定要完成，以及完成命令不一定是其建立並等待的命令。

範例

下列程式碼包含完整的範例。

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


Class MainWindow

    Async Sub startButton_Click(sender As Object, e As RoutedEventArgs) Handles startButton.Click
        resultsTextBox.Clear()
        Await CreateMultipleTasksAsync()
        resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click."
    End Sub


    Private Async Function CreateMultipleTasksAsync() As Task

        ' Declare an HttpClient object, and increase the buffer size. The
        ' default buffer size is 65,536.
        Dim client As HttpClient =
            New HttpClient() With {.MaxResponseContentBufferSize = 1000000}

        ' Create and start the tasks. As each task finishes, DisplayResults 
        ' displays its length.
        Dim download1 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com", client)
        Dim download2 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client)
        Dim download3 As Task(Of Integer) =
            ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client)

        ' Await each task.
        Dim length1 As Integer = Await download1
        Dim length2 As Integer = Await download2
        Dim length3 As Integer = Await download3

        Dim total As Integer = length1 + length2 + length3

        ' Display the total count for all of the websites.
        resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                             "Total bytes returned:  {0}" & vbCrLf, total)
    End Function


    Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)

        Dim byteArray = Await client.GetByteArrayAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function


    Private Sub DisplayResults(url As String, content As Byte())

        ' 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.
        Dim bytes = content.Length
        ' Strip off the "http://".
        Dim displayURL = url.Replace("http://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    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 directive, and add a reference for System.Net.Http.
using System.Net.Http;


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

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();
            await CreateMultipleTasksAsync();
            resultsTextBox.Text += "\r\n\r\nControl returned to startButton_Click.\r\n";
        }


        private async Task CreateMultipleTasksAsync()
        {
            // Declare an HttpClient object, and increase the buffer size. The
            // default buffer size is 65,536.
            HttpClient client =
                new HttpClient() { MaxResponseContentBufferSize = 1000000 };

            // Create and start the tasks. As each task finishes, DisplayResults 
            // displays its length.
            Task<int> download1 = 
                ProcessURLAsync("https://msdn.microsoft.com", client);
            Task<int> download2 = 
                ProcessURLAsync("https://msdn.microsoft.com/en-us/library/hh156528(VS.110).aspx", client);
            Task<int> download3 = 
                ProcessURLAsync("https://msdn.microsoft.com/en-us/library/67w7t67f.aspx", client);

            // Await each task.
            int length1 = await download1;
            int length2 = await download2;
            int length3 = await download3;

            int total = length1 + length2 + length3;

            // Display the total count for the downloaded websites.
            resultsTextBox.Text +=
                string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
        }


        async Task<int> ProcessURLAsync(string url, HttpClient client)
        {
            var byteArray = await client.GetByteArrayAsync(url);
            DisplayResults(url, byteArray);
            return byteArray.Length;
        }


        private void DisplayResults(string url, byte[] content)
        {
            // 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.
            var bytes = content.Length;
            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);
        }
    }
}

請參閱

工作

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

HOW TO：使用 Task.WhenAll 擴充逐步解說的內容 (C# 和 Visual Basic)

概念

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

共用方式為

HOW TO：同時發出多個 Web 要求 (C# 和 Visual Basic)

若要設定專案

加入程式碼

範例

請參閱

工作

概念

其他資源