TPL 和傳統 .NET 非同步程式設計

.NET 提供下列兩個標準模式來執行 i/o 系結和計算系結的非同步作業:

工作平行程式庫 (TPL) 可以用各種方式搭配非同步模式的其中一種。 您可以將 APM 和 EAP 作業公開為連結 Task 庫取用者的物件,也可以公開 apm 模式,但使用 Task 物件在內部執行。 在這兩種情況下,藉由使用 Task 物件,您可以簡化程式碼,並利用下列有用的功能:

  • 在啟動工作的任何時間之後,註冊工作接續形式的回呼。

  • 使用和方法來協調執行的多個作業,以回應 Begin_ 方法 ContinueWhenAll ContinueWhenAny ,或是 WaitAllWaitAny 方法。

  • 封裝相同物件中的非同步 i/o 系結和計算系結作業 Task

  • 監視物件的狀態 Task

  • 使用將作業的狀態封送處理至 Task 物件 TaskCompletionSource<TResult>

將 APM 作業包裝在工作中

System.Threading.Tasks.TaskFactorySystem.Threading.Tasks.TaskFactory<TResult> 類別提供數個多載的 TaskFactory.FromAsyncTaskFactory<TResult>.FromAsync 方法,可讓您在一個或多個實例中封裝 APM begin/end 方法組 Task Task<TResult> 。 不同的多載會容納具有零至三個輸入參數的任何 begin/end 方法組。

對於具有傳回 End 值 (Visual Basic) 中之方法的配對 Function ,請使用中的方法 TaskFactory<TResult> 來建立 Task<TResult> 。 針對傳回 End void (Sub 中 Visual Basic) 的方法,請使用中的方法 TaskFactory 來建立 Task

Begin 方法有三個以上的參數,或是包含 refout 參數的少數情況下,會提供其他僅封裝 End 方法的 FromAsync 多載。

下列範例示範 FromAsync 多載的簽章,該多載會比對 FileStream.BeginReadFileStream.EndRead 方法。

public Task<TResult> FromAsync<TArg1, TArg2, TArg3>(
    Func<TArg1, TArg2, TArg3, AsyncCallback, object, IAsyncResult> beginMethod, //BeginRead
     Func<IAsyncResult, TResult> endMethod, //EndRead
     TArg1 arg1, // the byte[] buffer
     TArg2 arg2, // the offset in arg1 at which to start writing data
     TArg3 arg3, // the maximum number of bytes to read
     object state // optional state information
    )
Public Function FromAsync(Of TArg1, TArg2, TArg3)(
                ByVal beginMethod As Func(Of TArg1, TArg2, TArg3, AsyncCallback, Object, IAsyncResult),
                ByVal endMethod As Func(Of IAsyncResult, TResult),
                ByVal dataBuffer As TArg1,
                ByVal byteOffsetToStartAt As TArg2,
                ByVal maxBytesToRead As TArg3,
                ByVal stateInfo As Object)

這個多載會採用三個輸入參數,如下所示。 第一個參數是 Func<T1,T2,T3,T4,T5,TResult> 委派,這與 FileStream.BeginRead 方法的簽章相符。 第二個參數是 Func<T,TResult> 委派,會取得 IAsyncResult 並傳回 TResult。 因為 EndRead 會傳回整數,所以編譯器會推斷 TResult 的類型為 Int32,並且推斷工作的類型為 Task。 最後四個參數和在 FileStream.BeginRead 方法中的相同:

  • 用來儲存檔案資料的緩衝區。

  • 開始寫入資料的緩衝區之位移。

  • 從檔案讀取的資料數量上限。

  • 選擇性的物件,其中儲存要傳遞至回呼的使用者定義狀態資料。

使用 System.threading.tasks.task.continuewith 作為回呼功能

如果您需要在檔案中資料的存取權,而不只是需要位元組數,則 FromAsync 方法仍不足夠。 請改用 Task,其 Result 屬性包含檔案資料。 您可將接續加入原始工作來達到此目的。 接續會執行通常由 AsyncCallback 委派所執行的工作。 當前項完成,而且資料緩衝區已滿時,則會叫用它。 (FileStream 物件應該在傳回之前關閉。)

下列範例顯示如何傳回 Task 封裝 BeginRead / EndRead 類別配對的 FileStream

const int MAX_FILE_SIZE = 14000000;
public static Task<string> GetFileStringAsync(string path)
{
    FileInfo fi = new FileInfo(path);
    byte[] data = null;
    data = new byte[fi.Length];

    FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);

    //Task<int> returns the number of bytes read
    Task<int> task = Task<int>.Factory.FromAsync(
            fs.BeginRead, fs.EndRead, data, 0, data.Length, null);

    // It is possible to do other work here while waiting
    // for the antecedent task to complete.
    // ...

    // Add the continuation, which returns a Task<string>.
    return task.ContinueWith((antecedent) =>
    {
        fs.Close();

        // Result = "number of bytes read" (if we need it.)
        if (antecedent.Result < 100)
        {
            return "Data is too small to bother with.";
        }
        else
        {
            // If we did not receive the entire file, the end of the
            // data buffer will contain garbage.
            if (antecedent.Result < data.Length)
                Array.Resize(ref data, antecedent.Result);

            // Will be returned in the Result property of the Task<string>
            // at some future point after the asynchronous file I/O operation completes.
            return new UTF8Encoding().GetString(data);
        }
    });
}
Const MAX_FILE_SIZE As Integer = 14000000
Shared Function GetFileStringAsync(ByVal path As String) As Task(Of String)
    Dim fi As New FileInfo(path)
    Dim data(fi.Length - 1) As Byte

    Dim fs As FileStream = New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)

    ' Task(Of Integer) returns the number of bytes read
    Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
        AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)

    ' It is possible to do other work here while waiting
    ' for the antecedent task to complete.
    ' ...

    ' Add the continuation, which returns a Task<string>. 
    Return myTask.ContinueWith(Function(antecedent)
                                   fs.Close()
                                   If (antecedent.Result < 100) Then
                                       Return "Data is too small to bother with."
                                   End If
                                   ' If we did not receive the entire file, the end of the
                                   ' data buffer will contain garbage.
                                   If (antecedent.Result < data.Length) Then
                                       Array.Resize(data, antecedent.Result)
                                   End If

                                   ' Will be returned in the Result property of the Task<string>
                                   ' at some future point after the asynchronous file I/O operation completes.
                                   Return New UTF8Encoding().GetString(data)
                               End Function)

End Function

然後可以呼叫此方法,如下所示。


Task<string> t = GetFileStringAsync(path);

// Do some other work:
// ...

try
{
     Console.WriteLine(t.Result.Substring(0, 500));
}
catch (AggregateException ae)
{
    Console.WriteLine(ae.InnerException.Message);
}
Dim myTask As Task(Of String) = GetFileStringAsync(path)

' Do some other work
' ...

Try
    Console.WriteLine(myTask.Result.Substring(0, 500))
Catch ex As AggregateException
    Console.WriteLine(ex.InnerException.Message)
End Try

提供自訂狀態資料

在一般 IAsyncResult 作業中,如果您的 AsyncCallback 委派需要一些自訂狀態資料,則您必須透過 Begin 方法中的最後一個參數傳遞它,以便資料可以封裝到 IAsyncResult 物件,而該物件最終會傳遞至回呼方法。 當使用 FromAsync 方法時,這通常不必要。 如果自訂資料對接續為已知,則可以直接在接續委派中擷取。 下列範例類似上述範例,但不檢查前項的 Result 屬性,接續會檢查自訂狀態資料,此為接續的使用者委派所直接存取。

public Task<string> GetFileStringAsync2(string path)
{
    FileInfo fi = new FileInfo(path);
    byte[] data = new byte[fi.Length];
    MyCustomState state = GetCustomState();
    FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, true);
    // We still pass null for the last parameter because
    // the state variable is visible to the continuation delegate.
    Task<int> task = Task<int>.Factory.FromAsync(
            fs.BeginRead, fs.EndRead, data, 0, data.Length, null);

    return task.ContinueWith((antecedent) =>
    {
        // It is safe to close the filestream now.
        fs.Close();

        // Capture custom state data directly in the user delegate.
        // No need to pass it through the FromAsync method.
        if (state.StateData.Contains("New York, New York"))
        {
            return "Start spreading the news!";
        }
        else
        {
            // If we did not receive the entire file, the end of the
            // data buffer will contain garbage.
            if (antecedent.Result < data.Length)
                Array.Resize(ref data, antecedent.Result);

            // Will be returned in the Result property of the Task<string>
            // at some future point after the asynchronous file I/O operation completes.
            return new UTF8Encoding().GetString(data);
        }
    });
}
Public Function GetFileStringAsync2(ByVal path As String) As Task(Of String)
    Dim fi = New FileInfo(path)
    Dim data(fi.Length - 1) As Byte
    Dim state As New MyCustomState()

    Dim fs As New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, data.Length, True)
    ' We still pass null for the last parameter because
    ' the state variable is visible to the continuation delegate.
    Dim myTask As Task(Of Integer) = Task(Of Integer).Factory.FromAsync(
            AddressOf fs.BeginRead, AddressOf fs.EndRead, data, 0, data.Length, Nothing)

    Return myTask.ContinueWith(Function(antecedent)
                                   fs.Close()
                                   ' Capture custom state data directly in the user delegate.
                                   ' No need to pass it through the FromAsync method.
                                   If (state.StateData.Contains("New York, New York")) Then
                                       Return "Start spreading the news!"
                                   End If

                                   ' If we did not receive the entire file, the end of the
                                   ' data buffer will contain garbage.
                                   If (antecedent.Result < data.Length) Then
                                       Array.Resize(data, antecedent.Result)
                                   End If
                                   '/ Will be returned in the Result property of the Task<string>
                                   '/ at some future point after the asynchronous file I/O operation completes.
                                   Return New UTF8Encoding().GetString(data)
                               End Function)

End Function

同步處理多個 System.threading.tasks.taskfactory.fromasync 工作

當搭配 FromAsync 方法使用時,靜態 ContinueWhenAllContinueWhenAny 方法會提供額外的彈性。 下列範例示範如何初始化多個非同步 I/O 作業,然後等候它們全部完成,才執行接續。

public Task<string> GetMultiFileData(string[] filesToRead)
{
    FileStream fs;
    Task<string>[] tasks = new Task<string>[filesToRead.Length];
    byte[] fileData = null;
    for (int i = 0; i < filesToRead.Length; i++)
    {
        fileData = new byte[0x1000];
        fs = new FileStream(filesToRead[i], FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, true);

        // By adding the continuation here, the
        // Result of each task will be a string.
        tasks[i] = Task<int>.Factory.FromAsync(
                 fs.BeginRead, fs.EndRead, fileData, 0, fileData.Length, null)
                 .ContinueWith((antecedent) =>
                     {
                         fs.Close();

                         // If we did not receive the entire file, the end of the
                         // data buffer will contain garbage.
                         if (antecedent.Result < fileData.Length)
                             Array.Resize(ref fileData, antecedent.Result);

                         // Will be returned in the Result property of the Task<string>
                         // at some future point after the asynchronous file I/O operation completes.
                         return new UTF8Encoding().GetString(fileData);
                     });
    }

    // Wait for all tasks to complete.
    return Task<string>.Factory.ContinueWhenAll(tasks, (data) =>
    {
        // Propagate all exceptions and mark all faulted tasks as observed.
        Task.WaitAll(data);

        // Combine the results from all tasks.
        StringBuilder sb = new StringBuilder();
        foreach (var t in data)
        {
            sb.Append(t.Result);
        }
        // Final result to be returned eventually on the calling thread.
        return sb.ToString();
    });
}
Public Function GetMultiFileData(ByVal filesToRead As String()) As Task(Of String)
    Dim fs As FileStream
    Dim tasks(filesToRead.Length - 1) As Task(Of String)
    Dim fileData() As Byte = Nothing
    For i As Integer = 0 To filesToRead.Length
        fileData(&H1000) = New Byte()
        fs = New FileStream(filesToRead(i), FileMode.Open, FileAccess.Read, FileShare.Read, fileData.Length, True)

        ' By adding the continuation here, the 
        ' Result of each task will be a string.
        tasks(i) = Task(Of Integer).Factory.FromAsync(AddressOf fs.BeginRead,
                                                      AddressOf fs.EndRead,
                                                      fileData,
                                                      0,
                                                      fileData.Length,
                                                      Nothing).
                                                  ContinueWith(Function(antecedent)
                                                                   fs.Close()
                                                                   'If we did not receive the entire file, the end of the
                                                                   ' data buffer will contain garbage.
                                                                   If (antecedent.Result < fileData.Length) Then
                                                                       ReDim Preserve fileData(antecedent.Result)
                                                                   End If

                                                                   'Will be returned in the Result property of the Task<string>
                                                                   ' at some future point after the asynchronous file I/O operation completes.
                                                                   Return New UTF8Encoding().GetString(fileData)
                                                               End Function)
    Next

    Return Task(Of String).Factory.ContinueWhenAll(tasks, Function(data)

                                                              ' Propagate all exceptions and mark all faulted tasks as observed.
                                                              Task.WaitAll(data)

                                                              ' Combine the results from all tasks.
                                                              Dim sb As New StringBuilder()
                                                              For Each t As Task(Of String) In data
                                                                  sb.Append(t.Result)
                                                              Next
                                                              ' Final result to be returned eventually on the calling thread.
                                                              Return sb.ToString()
                                                          End Function)
End Function

僅 System.threading.tasks.taskfactory.fromasync End 方法的工作

Begin 方法需要三個以上的輸入參數或具有或參數的少數情況下, ref out 您可以使用多載 FromAsync ,例如, TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult,TResult>) 只代表 End 方法。 這些方法也可以在您傳遞的任何案例中使用 IAsyncResult ,而且想要將它封裝在工作中。

static Task<String> ReturnTaskFromAsyncResult()
{
    IAsyncResult ar = DoSomethingAsynchronously();
    Task<String> t = Task<string>.Factory.FromAsync(ar, _ =>
        {
            return (string)ar.AsyncState;
        });

    return t;
}
Shared Function ReturnTaskFromAsyncResult() As Task(Of String)
    Dim ar As IAsyncResult = DoSomethingAsynchronously()
    Dim t As Task(Of String) = Task(Of String).Factory.FromAsync(ar, Function(res) CStr(res.AsyncState))
    Return t
End Function

啟動和取消 System.threading.tasks.taskfactory.fromasync 工作

方法所傳回的工作 FromAsync 具有的狀態 WaitingForActivation ,並且會在工作建立之後的某個時間點啟動系統。 如果您嘗試呼叫這類工作上的 Start,就會引發例外狀況。

FromAsync因為基礎 .Net api 目前不支援檔案或網路 i/o 的進行中取消,所以無法取消工作。 您可以加入取消功能到封裝 FromAsync 呼叫的方法,但是您只可以在 FromAsync 呼叫之前或完成之後回應取消 (例如在接續工作中)。

有些類別支援 EAP,例如 WebClient 支援取消,您可以使用取消語彙基元整合該原生的取消功能。

將複雜的 EAP 作業公開為工作

TPL 不提供任何專門用來封裝事件架構非同步作業的方法,和 FromAsync 系列方法包裝 IAsyncResult 模式的方式相同。 但是,TPL 並不提供 System.Threading.Tasks.TaskCompletionSource<TResult> 類別,這可用來代表任意一組作業為 Task<TResult>。 此作業可能為同步或非同步,而且可能為 I/O 繫結或計算繫結,或兩者兼具。

下列範例示範如何使用 TaskCompletionSource<TResult>來公開一組非同步 WebClient 作業給用戶端程式碼做為基本的 Task<TResult>。 此方法可讓您輸入 Web URL 和要搜尋的詞彙或名稱之陣列,然後傳回在每個網站出現搜尋詞彙的次數。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

public class SimpleWebExample
{
  public Task<string[]> GetWordCountsSimplified(string[] urls, string name,
                                                CancellationToken token)
  {
      TaskCompletionSource<string[]> tcs = new TaskCompletionSource<string[]>();
      WebClient[] webClients = new WebClient[urls.Length];
      object m_lock = new object();
      int count = 0;
      List<string> results = new List<string>();

      // If the user cancels the CancellationToken, then we can use the
      // WebClient's ability to cancel its own async operations.
      token.Register(() =>
      {
          foreach (var wc in webClients)
          {
              if (wc != null)
                  wc.CancelAsync();
          }
      });

      for (int i = 0; i < urls.Length; i++)
      {
          webClients[i] = new WebClient();

          #region callback
          // Specify the callback for the DownloadStringCompleted
          // event that will be raised by this WebClient instance.
          webClients[i].DownloadStringCompleted += (obj, args) =>
          {

              // Argument validation and exception handling omitted for brevity.

              // Split the string into an array of words,
              // then count the number of elements that match
              // the search term.
              string[] words = args.Result.Split(' ');
              string NAME = name.ToUpper();
              int nameCount = (from word in words.AsParallel()
                               where word.ToUpper().Contains(NAME)
                               select word)
                              .Count();

              // Associate the results with the url, and add new string to the array that
              // the underlying Task object will return in its Result property.
              lock (m_lock)
              {
                 results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, name));

                 // If this is the last async operation to complete,
                 // then set the Result property on the underlying Task.
                 count++;
                 if (count == urls.Length)
                 {
                    tcs.TrySetResult(results.ToArray());
                 }
              }
          };
          #endregion

          // Call DownloadStringAsync for each URL.
          Uri address = null;
          address = new Uri(urls[i]);
          webClients[i].DownloadStringAsync(address, address);
      } // end for

      // Return the underlying Task. The client code
      // waits on the Result property, and handles exceptions
      // in the try-catch block there.
      return tcs.Task;
   }
}
Imports System.Collections.Generic
Imports System.Net
Imports System.Threading
Imports System.Threading.Tasks

Public Class SimpleWebExample
    Dim tcs As New TaskCompletionSource(Of String())
    Dim token As CancellationToken
    Dim results As New List(Of String)
    Dim m_lock As New Object()
    Dim count As Integer
    Dim addresses() As String
    Dim nameToSearch As String

    Public Function GetWordCountsSimplified(ByVal urls() As String, ByVal str As String,
                                            ByVal token As CancellationToken) As Task(Of String())
        addresses = urls
        nameToSearch = str

        Dim webClients(urls.Length - 1) As WebClient

        ' If the user cancels the CancellationToken, then we can use the
        ' WebClient's ability to cancel its own async operations.
        token.Register(Sub()
                           For Each wc As WebClient In webClients
                               If wc IsNot Nothing Then
                                   wc.CancelAsync()
                               End If
                           Next
                       End Sub)

        For i As Integer = 0 To urls.Length - 1
            webClients(i) = New WebClient()

            ' Specify the callback for the DownloadStringCompleted
            ' event that will be raised by this WebClient instance.
            AddHandler webClients(i).DownloadStringCompleted, AddressOf WebEventHandler

            Dim address As New Uri(urls(i))
            ' Pass the address, and also use it for the userToken
            ' to identify the page when the delegate is invoked.
            webClients(i).DownloadStringAsync(address, address)
        Next

        ' Return the underlying Task. The client code
        ' waits on the Result property, and handles exceptions
        ' in the try-catch block there.
        Return tcs.Task
    End Function

    Public Sub WebEventHandler(ByVal sender As Object, ByVal args As DownloadStringCompletedEventArgs)

        If args.Cancelled = True Then
            tcs.TrySetCanceled()
            Return
        ElseIf args.Error IsNot Nothing Then
            tcs.TrySetException(args.Error)
            Return
        Else
            ' Split the string into an array of words,
            ' then count the number of elements that match
            ' the search term.
            Dim words() As String = args.Result.Split(" "c)

            Dim name As String = nameToSearch.ToUpper()
            Dim nameCount = (From word In words.AsParallel()
                             Where word.ToUpper().Contains(name)
                             Select word).Count()

            ' Associate the results with the url, and add new string to the array that
            ' the underlying Task object will return in its Result property.
            SyncLock (m_lock)
                results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, nameToSearch))
                count = count + 1
                If (count = addresses.Length) Then
                    tcs.TrySetResult(results.ToArray())
                End If
            End SyncLock
        End If
    End Sub
End Class

如需更完整的範例,其中包含其他例外狀況處理,並示範如何從用戶端程式碼呼叫方法,請參閱如何:將 EAP 模式包裝在工作中

請記住,由所建立的任何工作 TaskCompletionSource<TResult> 都會由該啟動, TaskCompletionSource 因此,使用者程式碼不應該 Start 在該工作上呼叫方法。

使用工作來執行 APM 模式

在某些情況下,您可能會想要 IAsyncResult 在 API 中使用 begin/end 方法組來直接公開模式。 例如,您可能想要維護現有 API 的一致性,或者您想要有需要此模式的自動化工具。 在這種情況下,您可以使用 Task 物件來簡化 APM 模式在內部的內部執行方式。

下列範例示範如何使用工作,針對長時間執行的計算系結方法,執行 APM begin/end 方法組。

class Calculator
{
    public IAsyncResult BeginCalculate(int decimalPlaces, AsyncCallback ac, object state)
    {
        Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
        Task<string> f = Task<string>.Factory.StartNew(_ => Compute(decimalPlaces), state);
        if (ac != null) f.ContinueWith((res) => ac(f));
        return f;
    }

    public string Compute(int numPlaces)
    {
        Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId);

        // Simulating some heavy work.
        Thread.SpinWait(500000000);

        // Actual implemenation left as exercise for the reader.
        // Several examples are available on the Web.
        return "3.14159265358979323846264338327950288";
    }

    public string EndCalculate(IAsyncResult ar)
    {
        Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId);
        return ((Task<string>)ar).Result;
    }
}

public class CalculatorClient
{
    static int decimalPlaces = 12;
    public static void Main()
    {
        Calculator calc = new Calculator();
        int places = 35;

        AsyncCallback callBack = new AsyncCallback(PrintResult);
        IAsyncResult ar = calc.BeginCalculate(places, callBack, calc);

        // Do some work on this thread while the calulator is busy.
        Console.WriteLine("Working...");
        Thread.SpinWait(500000);
        Console.ReadLine();
    }

    public static void PrintResult(IAsyncResult result)
    {
        Calculator c = (Calculator)result.AsyncState;
        string piString = c.EndCalculate(result);
        Console.WriteLine("Calling PrintResult on thread {0}; result = {1}",
                    Thread.CurrentThread.ManagedThreadId, piString);
    }
}
Class Calculator
    Public Function BeginCalculate(ByVal decimalPlaces As Integer, ByVal ac As AsyncCallback, ByVal state As Object) As IAsyncResult
        Console.WriteLine("Calling BeginCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
        Dim myTask = Task(Of String).Factory.StartNew(Function(obj) Compute(decimalPlaces), state)
        myTask.ContinueWith(Sub(antedecent) ac(myTask))

    End Function
    Private Function Compute(ByVal decimalPlaces As Integer)
        Console.WriteLine("Calling compute on thread {0}", Thread.CurrentThread.ManagedThreadId)

        ' Simulating some heavy work.
        Thread.SpinWait(500000000)

        ' Actual implemenation left as exercise for the reader.
        ' Several examples are available on the Web.
        Return "3.14159265358979323846264338327950288"
    End Function

    Public Function EndCalculate(ByVal ar As IAsyncResult) As String
        Console.WriteLine("Calling EndCalculate on thread {0}", Thread.CurrentThread.ManagedThreadId)
        Return CType(ar, Task(Of String)).Result
    End Function
End Class

Class CalculatorClient
    Shared decimalPlaces As Integer
    Shared Sub Main()
        Dim calc As New Calculator
        Dim places As Integer = 35
        Dim callback As New AsyncCallback(AddressOf PrintResult)
        Dim ar As IAsyncResult = calc.BeginCalculate(places, callback, calc)

        ' Do some work on this thread while the calulator is busy.
        Console.WriteLine("Working...")
        Thread.SpinWait(500000)
        Console.ReadLine()
    End Sub

    Public Shared Sub PrintResult(ByVal result As IAsyncResult)
        Dim c As Calculator = CType(result.AsyncState, Calculator)
        Dim piString As String = c.EndCalculate(result)
        Console.WriteLine("Calling PrintResult on thread {0}; result = {1}",
                   Thread.CurrentThread.ManagedThreadId, piString)
    End Sub

End Class

使用 StreamExtensions 範例程式碼

StreamExtensions.cs 檔案在 .NET Standard 平行擴充功能額外存放庫中,包含數個使用 Task 物件進行非同步檔案和網路 i/o 的參考。

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