Interop 與其他非同步模式和類型
.NET 中非同步模式的簡短歷程記錄：
- .NET Framework 1.0 導入了 IAsyncResult 模式，亦稱為 Asynchronous Programming Model (APM)或
Begin/End模式。 - .NET Framework 2.0 加入了 Event-based Asynchronous Pattern (EAP)。
- .NET Framework 4 引進了工作型非同步模式 (TAP) (取代 APM 和 EAP)，並可讓您從較早的模式輕鬆建置移轉常式。
工作與非同步程式設計模型 (APM)
從 APM 到 TAP
由於非同步程式設計模型 (APM) 模式結構嚴謹，因此很容易就能建置包裝函式將 APM 實作公開為 TAP 實作。 .NET Framework 4 和更新版本包含 FromAsync 方法多載形式的協助程式常式，可提供此轉譯。
請考慮 Stream 類別以及其 BeginRead 與 EndRead 方法，此類方法代表同步 Read 方法的 APM 對應項目：
public int Read(byte[] buffer, int offset, int count)
Public Function Read(buffer As Byte(), offset As Integer,
count As Integer) As Integer
public IAsyncResult BeginRead(byte[] buffer, int offset,
int count, AsyncCallback callback,
object state)
Public Function BeginRead(buffer As Byte, offset As Integer,
count As Integer, callback As AsyncCallback,
state As Object) As IAsyncResult
public int EndRead(IAsyncResult asyncResult)
Public Function EndRead(asyncResult As IAsyncResult) As Integer
您可以使用 TaskFactory<TResult>.FromAsync 方法來實作這項作業的 TAP 包裝函式，如下所示：
public static Task<int> ReadAsync(this Stream stream,
byte[] buffer, int offset,
int count)
{
if (stream == null)
throw new ArgumentNullException("stream");
return Task<int>.Factory.FromAsync(stream.BeginRead,
stream.EndRead, buffer,
offset, count, null);
}
<Extension()>
Public Function ReadAsync(strm As Stream,
buffer As Byte(), offset As Integer,
count As Integer) As Task(Of Integer)
If strm Is Nothing Then
Throw New ArgumentNullException("stream")
End If
Return Task(Of Integer).Factory.FromAsync(AddressOf strm.BeginRead,
AddressOf strm.EndRead, buffer,
offset, count, Nothing)
End Function
這個實作相似下面這個：
public static Task<int> ReadAsync(this Stream stream,
byte [] buffer, int offset,
int count)
{
if (stream == null)
throw new ArgumentNullException("stream");
var tcs = new TaskCompletionSource<int>();
stream.BeginRead(buffer, offset, count, iar =>
{
try {
tcs.TrySetResult(stream.EndRead(iar));
}
catch(OperationCanceledException) {
tcs.TrySetCanceled();
}
catch(Exception exc) {
tcs.TrySetException(exc);
}
}, null);
return tcs.Task;
}
<Extension()>
Public Function ReadAsync(stream As Stream, buffer As Byte(), _
offset As Integer, count As Integer) _
As Task(Of Integer)
If stream Is Nothing Then
Throw New ArgumentNullException("stream")
End If
Dim tcs As New TaskCompletionSource(Of Integer)()
stream.BeginRead(buffer, offset, count,
Sub(iar)
Try
tcs.TrySetResult(stream.EndRead(iar))
Catch e As OperationCanceledException
tcs.TrySetCanceled()
Catch e As Exception
tcs.TrySetException(e)
End Try
End Sub, Nothing)
Return tcs.Task
End Function
從 TAP 到 APM
如果您現有的基礎結構預期 APM 模式，則您也需進行 TAP 實作，並在預期 APM 實作的位置加以使用。 由於可以組成工作，並且 Task 類別會實作 IAsyncResult，因此您可以使用簡單的 Helper 函式來完成這項作業。 下列程式碼會使用 Task<TResult> 類別的擴充功能，但是您可以針對非泛型工作使用幾乎完全相同的函式。
public static IAsyncResult AsApm<T>(this Task<T> task,
AsyncCallback callback,
object state)
{
if (task == null)
throw new ArgumentNullException("task");
var tcs = new TaskCompletionSource<T>(state);
task.ContinueWith(t =>
{
if (t.IsFaulted)
tcs.TrySetException(t.Exception.InnerExceptions);
else if (t.IsCanceled)
tcs.TrySetCanceled();
else
tcs.TrySetResult(t.Result);
if (callback != null)
callback(tcs.Task);
}, TaskScheduler.Default);
return tcs.Task;
}
<Extension()>
Public Function AsApm(Of T)(task As Task(Of T),
callback As AsyncCallback,
state As Object) As IAsyncResult
If task Is Nothing Then
Throw New ArgumentNullException("task")
End If
Dim tcs As New TaskCompletionSource(Of T)(state)
task.ContinueWith(Sub(antecedent)
If antecedent.IsFaulted Then
tcs.TrySetException(antecedent.Exception.InnerExceptions)
ElseIf antecedent.IsCanceled Then
tcs.TrySetCanceled()
Else
tcs.TrySetResult(antecedent.Result)
End If
If callback IsNot Nothing Then
callback(tcs.Task)
End If
End Sub, TaskScheduler.Default)
Return tcs.Task
End Function
現在，考量一下有下列 TAP 實作的案例：
public static Task<String> DownloadStringAsync(Uri url)
Public Shared Function DownloadStringAsync(url As Uri) As Task(Of String)
而且您想要提供此 APM 實作：
public IAsyncResult BeginDownloadString(Uri url,
AsyncCallback callback,
object state)
Public Function BeginDownloadString(url As Uri,
callback As AsyncCallback,
state As Object) As IAsyncResult
public string EndDownloadString(IAsyncResult asyncResult)
Public Function EndDownloadString(asyncResult As IAsyncResult) As String
下列範例將示範如何移轉至 APM：
public IAsyncResult BeginDownloadString(Uri url,
AsyncCallback callback,
object state)
{
return DownloadStringAsync(url).AsApm(callback, state);
}
public string EndDownloadString(IAsyncResult asyncResult)
{
return ((Task<string>)asyncResult).Result;
}
Public Function BeginDownloadString(url As Uri,
callback As AsyncCallback,
state As Object) As IAsyncResult
Return DownloadStringAsync(url).AsApm(callback, state)
End Function
Public Function EndDownloadString(asyncResult As IAsyncResult) As String
Return CType(asyncResult, Task(Of String)).Result
End Function
工作與事件架構非同步模式 (EAP)
包裝 Event-based Asynchronous Pattern (EAP) 實作會比包裝 APM 模式更為複雜，原因是 EAP 模式擁有較多的變化，結構也不如 APM 模式嚴謹。 供示範所用，下列程式碼包裝了 DownloadStringAsync 方法。 DownloadStringAsync 會接受 URI、在下載時引發 DownloadProgressChanged 事件，以報告多個進行中的統計資料，然後在完成時引發 DownloadStringCompleted 。 最終結果是字串，其中包含指定之 URI 頁面的內容。
public static Task<string> DownloadStringAsync(Uri url)
{
var tcs = new TaskCompletionSource<string>();
var wc = new WebClient();
wc.DownloadStringCompleted += (s,e) =>
{
if (e.Error != null)
tcs.TrySetException(e.Error);
else if (e.Cancelled)
tcs.TrySetCanceled();
else
tcs.TrySetResult(e.Result);
};
wc.DownloadStringAsync(url);
return tcs.Task;
}
Public Shared Function DownloadStringAsync(url As Uri) As Task(Of String)
Dim tcs As New TaskCompletionSource(Of String)()
Dim wc As New WebClient()
AddHandler wc.DownloadStringCompleted, Sub(s, e)
If e.Error IsNot Nothing Then
tcs.TrySetException(e.Error)
ElseIf e.Cancelled Then
tcs.TrySetCanceled()
Else
tcs.TrySetResult(e.Result)
End If
End Sub
wc.DownloadStringAsync(url)
Return tcs.Task
End Function
工作與等候控制代碼
從等候控制代碼到 TAP
雖然等候控制代碼不會實作非同步模式，但進階開發人員可能會在等候控制代碼設定完成後，使用 WaitHandle 類別與 ThreadPool.RegisterWaitForSingleObject 方法，用於非同步通知。 您可以包裝 RegisterWaitForSingleObject 方法，以啟用等候控制代碼上任何同步等候的工作式替代方案：
public static Task WaitOneAsync(this WaitHandle waitHandle)
{
if (waitHandle == null)
throw new ArgumentNullException("waitHandle");
var tcs = new TaskCompletionSource<bool>();
var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle,
delegate { tcs.TrySetResult(true); }, null, -1, true);
var t = tcs.Task;
t.ContinueWith( (antecedent) => rwh.Unregister(null));
return t;
}
<Extension()>
Public Function WaitOneAsync(waitHandle As WaitHandle) As Task
If waitHandle Is Nothing Then
Throw New ArgumentNullException("waitHandle")
End If
Dim tcs As New TaskCompletionSource(Of Boolean)()
Dim rwh As RegisteredWaitHandle = ThreadPool.RegisterWaitForSingleObject(waitHandle,
Sub(state, timedOut)
tcs.TrySetResult(True)
End Sub, Nothing, -1, True)
Dim t = tcs.Task
t.ContinueWith(Sub(antecedent)
rwh.Unregister(Nothing)
End Sub)
Return t
End Function
有了此方法，您可以使用非同步方法中現有的 WaitHandle 實作。 例如，如果您想要節流處理任何特定時間執行的非同步作業數目，可以利用旗號 (System.Threading.SemaphoreSlim 物件)。 您可以節流處理至 N，即同時執行的作業數，方法是透過將旗號的計數初始化至 N，在任何想執行作業時等待旗號，並在完成作業時釋放旗號：
static int N = 3;
static SemaphoreSlim m_throttle = new SemaphoreSlim(N, N);
static async Task DoOperation()
{
await m_throttle.WaitAsync();
// do work
m_throttle.Release();
}
Shared N As Integer = 3
Shared m_throttle As New SemaphoreSlim(N, N)
Shared Async Function DoOperation() As Task
Await m_throttle.WaitAsync()
' Do work.
m_throttle.Release()
End Function
您也可以建置非同步的旗號，而該旗號不會依賴等候控制代碼，並會完全與工作一同合作。 若要這樣做，您可以使用像是 Consuming the Task-based Asynchronous Pattern 中所討論的技術，用以在 Task。
從 TAP 到等候控制代碼
如先前所述， Task 類別會實作 IAsyncResult，且該實作會公開 IAsyncResult.AsyncWaitHandle 屬性，而該屬性會傳回 Task 完成時將進行設定的等候控制代碼。 您可以取得 WaitHandle 的 Task ，如下所示：
WaitHandle wh = ((IAsyncResult)task).AsyncWaitHandle;
Dim wh As WaitHandle = CType(task, IAsyncResult).AsyncWaitHandle
