.NET 中异步模式的简要历史记录：
- .NET Framework 1.0 引入了 IAsyncResult 模式，否则称为 异步编程模型（APM）或
Begin/End模式。 - .NET Framework 2.0 添加了基于事件的异步模式（EAP）。
- .NET Framework 4 引入了 基于任务的异步模式 （TAP），该模式取代了 APM 和 EAP，并提供了从早期模式轻松生成迁移例程的功能。
任务和异步编程模型 （APM）
从 APM 到 TAP
由于 异步编程模型（APM）模式 是结构化的，因此构建包装器很容易将 APM 实现公开为 TAP 实现。 .NET Framework 4 及更高版本包含采用 FromAsync 方法重载形式的帮助器例程来实现这种转换。
请考虑 Stream 类及其 BeginRead 和 EndRead 方法，它们代表与同步 Read 方法对应的 APM：
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
可以使用TaskFactory<TResult>.FromAsync方法为此操作实现一个TAP包装器，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
此实现类似于以下内容：
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
从 TAP 到 APM
如果现有的基础结构需要 APM 模式，则还需要采用 TAP 实现并在需要 APM 实现的地方使用它。 由于任务可以组合， Task 类实现 IAsyncResult，因此可以使用简单的帮助程序函数来执行此作。 以下代码使用类的 Task<TResult> 扩展，但可以对非泛型任务使用几乎相同的函数。
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
现在，请考虑具有以下 TAP 实现的用例：
public static Task<String> DownloadStringAsync(Uri url)
Public Shared Function DownloadStringAsync(url As Uri) As Task(Of String)
并且想要提供此 APM 实现：
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
以下示例演示了如何迁移到 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
任务和基于事件的异步模式（EAP）
包装 基于事件的异步模式（EAP） 实现比包装 APM 模式更为复杂，因为 EAP 模式的变体和结构比 APM 模式少。 为了演示，以下代码包装了 DownloadStringAsync 该方法。 DownloadStringAsync 接受 URI，在下载时引发 DownloadProgressChanged 事件，以便报告有关进度的多个统计信息，并在事件完成后引发 DownloadStringCompleted 事件。 最终结果是一个字符串，其中包含指定 URI 处的页面内容。
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
任务和等待句柄
从等待句柄到 TAP
尽管等待句柄未实现异步模式，但高级开发人员可以在设置等待句柄时使用 WaitHandle 类和 ThreadPool.RegisterWaitForSingleObject 异步通知的方法。 可以包装 RegisterWaitForSingleObject 方法以在等待句柄中启用针对任何同步等待的基于任务的替代方法：
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
使用此方法，可以在异步方法中使用现有 WaitHandle 实现。 例如，如果要限制在任何特定时间执行的异步操作数量，可以利用信号量（对象System.Threading.SemaphoreSlim）。 可以将并发运行的操作数目限制到 N，方法为：初始化到 N 的信号量的数目、在想要执行操作时等待信号量，并在完成操作时释放信号量：
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
还可以构建不依赖等待句柄就完全可以处理任务的异步信号量。 为此，您可以使用在基于任务的异步模式中讨论的技术来在Task上构建数据结构。
从 TAP 到等待句柄
如前所述，Task 类实现了 IAsyncResult，该实现包含一个公开的 IAsyncResult.AsyncWaitHandle 属性，该属性返回一个等待句柄，当 Task 完成时此句柄将被设置。 可以获得 WaitHandle 的 Task ，如下所示：
WaitHandle wh = ((IAsyncResult)task).AsyncWaitHandle;
Dim wh As WaitHandle = CType(task, IAsyncResult).AsyncWaitHandle
