TPL 和傳統 .NET 異步程序設計
.NET 提供下列兩種標準模式來執行 I/O 受限和計算受限的異步作業：
異步程序設計模型 （APM），其中異步作是由一對開始/結束方法表示。 例如：FileStream.BeginRead 與 Stream.EndRead。
事件架構異步模式 （EAP），其中異步作是由名為
<OperationName>Async和<OperationName>Completed的方法/事件組表示。 例如：WebClient.DownloadStringAsync 與 WebClient.DownloadStringCompleted。
工作平行程式庫（TPL）可以以多種方式與任何一種異步模式搭配使用。 您可以將 APM 和 EAP 作業公開為連結庫取用者 Task 物件，也可以公開 APM 模式，但使用 Task 對象在內部實作它們。 在這兩種情況下，使用 Task 物件，您可以簡化程序代碼，並利用下列實用的功能：
在工作啟動後的任何時間，您都可以以工作延續的形式註冊回呼函式。
使用 ContinueWhenAll 和 ContinueWhenAny 方法，或 WaitAll 和 WaitAny 方法來協調多個作業，以回應
Begin_方法。在相同的
Task物件中封裝異步 I/O 系結和計算系結作業。監視
Task物件的狀態。使用 TaskCompletionSource<TResult>將作業的狀態封送至
Task物件。
將 APM 作業包裝在任務中
System.Threading.Tasks.TaskFactory 和 System.Threading.Tasks.TaskFactory<TResult> 類別都提供數個多載的 TaskFactory.FromAsync 和 TaskFactory<TResult>.FromAsync 方法，可讓您在一個 Task 或 Task<TResult> 實例中封裝 APM 開始/結束方法組。 各種多載可容納任何從零到三個輸入參數的開始/結束方法組。
對於具有傳回值的 End 方法（在 Visual Basic 中為 Function）之配對，請使用 TaskFactory<TResult> 中可建立 Task<TResult>的方法。 針對傳回 void 的 End 方法（在 Visual Basic 中的 Sub），請使用 TaskFactory 中那些能建立 Task的方法。
對於 Begin 方法有三個以上的參數或包含 ref 或 out 參數的少數案例，會提供只封裝 End 方法的其他 FromAsync 多載。
下列範例顯示符合 FileStream.BeginRead 和 FileStream.EndRead 方法之 FromAsync 多載的簽章。
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)
此多載採用三個輸入參數，如下所示。 第一個參數是 Func<T1,T2,T3,T4,T5,TResult> 委託，其簽章符合 FileStream.BeginRead 方法。 第二個參數是接受 IAsyncResult 並傳回 TResult的 Func<T,TResult> 委派。 因為 EndRead 傳回整數，編譯程式會將 TResult 的類型推斷為 Int32，並將工作的類型推斷為 Task。 最後四個參數與 FileStream.BeginRead 方法中的參數相同：
要在其中儲存檔案數據的緩衝區。
緩衝區中開始寫入數據的位移。
要從檔案讀取的數據量上限。
可選物件，儲存要傳遞至回呼函式的使用者定義狀態資料。
請使用 ContinueWith 來實現回呼功能
如果您需要存取檔案中的數據，而不只是位元組數目，則 FromAsync 方法是不夠的。 請改用 Task，其 Result 屬性包含檔案數據。 您可以藉由在原始工作中新增接續來完成此項工作。 接續會執行通常由 AsyncCallback 代理人執行的工作。 當前項完成且數據緩衝區已填滿時，就會叫用它。 （傳回之前，應該關閉 FileStream 對象。）
下列範例顯示如何傳回一個 Task，以封裝 FileStream 類別中的 BeginRead/EndRead 組合。
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 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
提供自定義狀態數據
在一般 IAsyncResult 作業中，如果您的 AsyncCallback 委派需要一些自定義狀態數據，您必須透過 Begin 方法中的最後一個參數傳入，以便將數據封裝到最終傳遞至回呼方法的 IAsyncResult 物件。 使用 FromAsync 方法時，通常不需要這一項。 如果接續已知自定義數據，則可以直接在接續委派中擷取。 下列範例與上一個範例類似，但接續會檢查直接可供接續之使用者委派存取的自定義狀態數據，而不是檢查前項的 Result 屬性。
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FromAsync工作
靜態 ContinueWhenAll 和 ContinueWhenAny 方法可在搭配 FromAsync 方法使用時，提供額外的彈性。 下列範例示範如何起始多個異步 I/O 作業，然後等候所有作業完成，再執行接續。
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
FromAsync 任務僅適用於 End 方法
在少數情況下，Begin 方法需要三個以上的輸入參數，或具有 ref 或 out 參數，您可以使用 FromAsync 多載，例如，TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult,TResult>)，只代表 End 方法。 這些方法也可以用在您收到 IAsyncResult 的任何情境中，並希望將其封裝在任務中。
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FromAsync工作
由 FromAsync 方法傳回的工作的狀態為 WaitingForActivation，且會在工作建立後於某個時間點由系統啟動。 如果您嘗試在這類工作上呼叫 Start，將會引發例外狀況。
您無法取消 FromAsync 工作，因為基礎 .NET API 目前不支援檔案或網路 I/O 的進行中取消。 您可以將取消功能新增至封裝 FromAsync 呼叫的方法，但是您只能在呼叫 FromAsync 之前或完成之後回應取消作業（例如，在接續工作中）。
某些支援 EAP 的類別，例如，WebClient支援取消，而且您可以使用取消令牌來整合該原生取消功能。
將複雜的 EAP 作業呈現作為任務
TPL 不提供任何特別設計來封裝事件型異步作的方法，其方式與 FromAsync 系列方法包裝 IAsyncResult 模式相同。 不過，TPL 提供 System.Threading.Tasks.TaskCompletionSource<TResult> 類別，可用來將任何任意的作業集表示為 Task<TResult>。 作業可能是同步或異步的，而且可能是 I/O 系結或計算系結，或是兩者。
下列範例示範如何使用 TaskCompletionSource<TResult>，將一組異步 WebClient 作業公開給客戶端程式碼，作為一個基本的 Task<TResult>。 方法可讓您輸入 Web URL 的數位，以及要搜尋的字詞或名稱，然後傳回每個網站上搜尋字詞的次數。
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代碼呼叫 方法，請參閱 如何：在工作中包裝 EAP 模式。
請記住，TaskCompletionSource<TResult> 所建立的任何工作都會由該 TaskCompletionSource 啟動，因此，用戶程式代碼不應該在該工作上呼叫 Start 方法。
使用任務實作 APM 模式
在某些情況下，建議您在 API 中使用開始/結束方法組，直接公開 IAsyncResult 模式。 例如，您可能想要維持與現有 API 的一致性，或者您可能有需要此模式的自動化工具。 在這種情況下，您可以使用 Task 對象來簡化內部實作 APM 模式的方式。
下列範例示範如何使用任務來實作長時間執行的計算密集型方法的 APM Begin/End 方法組。
class Calculator
{
public IAsyncResult BeginCalculate(int decimalPlaces, AsyncCallback ac, object state)
{
Console.WriteLine($"Calling BeginCalculate on thread {Thread.CurrentThread.ManagedThreadId}");
Task<string> f = Task<string>.Factory.StartNew(_ => Compute(decimalPlaces), state);
if (ac != null) f.ContinueWith((res) => ac(f));
return f;
}
public string Compute(int numPlaces)
{
Console.WriteLine($"Calling compute on thread {Thread.CurrentThread.ManagedThreadId}");
// Simulating some heavy work.
Thread.SpinWait(500000000);
// Actual implementation left as exercise for the reader.
// Several examples are available on the Web.
return "3.14159265358979323846264338327950288";
}
public string EndCalculate(IAsyncResult ar)
{
Console.WriteLine($"Calling EndCalculate on thread {Thread.CurrentThread.ManagedThreadId}");
return ((Task<string>)ar).Result;
}
}
public class CalculatorClient
{
static int decimalPlaces = 12;
public static void Main()
{
Calculator calc = new Calculator();
int places = 35;
AsyncCallback callBack = new AsyncCallback(PrintResult);
IAsyncResult ar = calc.BeginCalculate(places, callBack, calc);
// Do some work on this thread while the calculator is busy.
Console.WriteLine("Working...");
Thread.SpinWait(500000);
Console.ReadLine();
}
public static void PrintResult(IAsyncResult result)
{
Calculator c = (Calculator)result.AsyncState;
string piString = c.EndCalculate(result);
Console.WriteLine($"Calling PrintResult on thread {Thread.CurrentThread.ManagedThreadId}; result = {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(antecedent) 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 implementation 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 calculator 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 範例程序代碼
StreamExtensions.cs 檔案，在 .NET Standard 平行延伸模組額外 存放庫中，包含數個參考實作，這些參考實作會針對異步檔案和網路 I/O 使用 Task 物件。
