TPL 和传统 .NET 异步编程
.NET Framework 提供以下两种执行 I/O 绑定和计算绑定异步操作的标准模式：
异步编程模型 (APM)，在该模型中异步操作由一对 Begin/End 方法（如 FileStream.BeginRead 和 Stream.EndRead）表示。
基于事件的异步模式 (EAP)，在该模式中异步操作由名为“操作名称Async”和“操作名称Completed”的方法/事件对（例如 WebClient.DownloadStringAsync 和 WebClient.DownloadStringCompleted）表示。 （EAP 是在 .NET Framework 2.0 版中引入的）。
任务并行库 (TPL) 可通过各种方式与任一异步模式结合使用。 可以将 APM 和 EAP 操作以 Task 的形式向库使用者公开，或者可以公开 APM 模式，但使用 Task 对象在内部实现它们。 在这两种方案中，通过使用 Task 对象，可以简化代码并利用以下有用的功能：
在任务启动后，可以随时以任务延续的形式注册回调。
通过使用 ContinueWhenAll 和 ContinueWhenAny 方法或者 WaitAll 方法或 WaitAny 方法，协调多个为了响应 Begin_ 方法而执行的操作。
在同一 Task 对象中封装异步 I/O 绑定和计算绑定操作。
监视 Task 对象的状态。
使用 TaskCompletionSource<TResult> 将操作的状态封送到 Task 对象。
在任务中包装 APM 操作
System.Threading.Tasks.TaskFactory 和 System.Threading.Tasks.TaskFactory<TResult> 类都提供了 FromAsync 和 FromAsync 方法的多种重载，这些重载使您能够在一个 Task 实例或 Task<TResult> 实例中封装一个 APM Begin/End 方法对。 各种重载可容纳任何具有零到三个输入参数的 Begin/End 方法对。
对于具有返回值的 End 方法（在 Visual Basic 中为 Function）的对，在创建 Task<TResult> 的 TaskFactory<TResult> 中使用这些方法。 对于返回 void 的 End 方法（在 Visual Basic 中为 Sub），在创建 Task 的 TaskFactory 中使用这些方法。
对于 Begin 方法有三个以上的参数或包含 ref 或 out 参数的少数情况，提供了仅封装 End 方法的附加 FromAsync 重载。
下面的代码示例演示与 FileStream.BeginRead 和 FileStream.EndRead 方法匹配的 FromAsync 重载的签名。 此重载采用三个输入参数，如下所示。
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)
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
)
第一个参数是与 FileStream.BeginRead 方法的签名匹配的 Func<T1, T2, T3, T4, T5, TResult> 委托。 第二个参数是采用 IAsyncResult 并返回 TResult 的 Func<T, TResult> 委托。 由于 EndRead 返回一个整数，因此编译器将 TResult 的类型推断为 Int32，将任务的类型推断为 Task<Int32>。 最后四个参数等同于 FileStream.BeginRead 方法中的那些参数：
在其中存储文件数据的缓冲区。
缓冲区中开始写入数据的偏移量。
要从文件中读取的最大数据量。
用于存储要传递给回调的用户定义状态数据的可选对象。
将 ContinueWith 用于回调功能
如果需要访问文件中的数据，而不只是访问字节数，则使用 FromAsync 方法是不够的。 而应当使用 Task<String>，其 Result 属性包含文件数据。 可以通过向原始任务添加延续任务来执行此操作。 延续任务执行 AsyncCallback 委托通常执行的工作。 当前面的任务完成并且数据缓冲区已填充时，会调用它。 （FileStream 对象应在返回之前关闭）。
下面的示例演示如何返回 Task<String>，它封装了 FileStream 类的 BeginRead/EndRead 对。
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) 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
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);
}
});
}
然后可以按如下所示调用该方法。
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
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);
}
提供自定义状态数据
在典型的 IAsyncResult 操作中，如果 AsyncCallback 委托需要一些自定义状态数据，您必须通过 Begin 方法中的最后一个参数传递它，以使这些数据可以打包到 IAsyncResult 对象中，该对象最终传递给回调方法。 如果使用 FromAsync 方法，则通常不需要这些操作。 如果自定义数据对于延续任务是已知的，则可以直接在延续委托中捕获它。 下面的示例类似于前面的示例，但延续任务不检查前面的任务的 Result 属性，而是检查延续任务的用户委托可直接访问的自定义状态数据。
Public Function GetFileStringAsync2(ByVal path As String) As Task(Of String)
Dim fi = New FileInfo(path)
Dim data(fi.Length) 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
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);
}
});
}
同步多个 FromAsync 任务
当与 FromAsync 方法结合使用时，静态 ContinueWhenAll 和 ContinueWhenAny 方法可提供更高的灵活性。 下面的示例演示如何启动多个异步 I/O 操作，然后在您执行延续任务之前等待所有这些操作完成。
Public Function GetMultiFileData(ByVal filesToRead As String()) As Task(Of String)
Dim fs As FileStream
Dim tasks(filesToRead.Length) 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
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();
});
}
仅用于 End 方法的 FromAsync 任务
对于 Begin 方法需要三个以上的输入参数或者具有 ref 或 out 参数这些很少出现的情况，您可以使用 FromAsync 重载，例如仅代表 End 方法的 TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult, TResult>)。 这些方法还可以在向您传递了 IAsyncResult，且您想要在某个任务中封装它的任何情况下使用。
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
static Task<String> ReturnTaskFromAsyncResult()
{
IAsyncResult ar = DoSomethingAsynchronously();
Task<String> t = Task<string>.Factory.FromAsync(ar, _ =>
{
return (string)ar.AsyncState;
});
return t;
}
启动和取消 FromAsync 任务
FromAsync 方法返回的任务具有 WaitingForActivation 状态，并将在创建该任务后的某一时间由系统启动。 如果尝试在这样的任务上调用 Start，将引发异常。
不能取消 FromAsync 任务，这是因为基础 .NET Framework API 当前不支持文件或网络 I/O 的进程内取消。 您可以向封装 FromAsync 调用的方法添加取消功能，但您只能在调用 FromAsync 之前或它完成之后（例如在延续任务中）响应取消。
某些支持 EAP 的类（例如 WebClient）支持取消，您可以通过使用取消标记集成原生取消功能。
将复杂的 EAP 操作公开为任务
TPL 不提供任何与 FromAsync 系列方法包装 IAsyncResult 模式的方式相同的方法，来专门用于封装基于事件的异步操作。 但是，TPL 提供 System.Threading.Tasks.TaskCompletionSource<TResult> 类，该类可用于以 Task<TResult> 的形式表示任意一组操作。 这些操作可以是同步或异步操作，且可以 I/O 绑定或计算绑定，或者两者同时进行。
下面的示例演示如何使用 TaskCompletionSource<TResult> 以基本 Task 的形式将一组异步 WebClient 操作公开到客户端代码。 该方法使您可以输入 Web URL 数组、要搜索的术语或名称，然后返回搜索术语在每个站点出现的次数。
Class SimpleWebExample
Dim tcs As New TaskCompletionSource(Of String())
Dim nameToSearch As String
Dim token As CancellationToken
Dim results As New List(Of String)
Dim m_lock As Object
Dim count As Integer
Dim addresses() As String
Public Function GetWordCountsSimplified(ByVal urls() As String, ByVal str As String, ByVal token As CancellationToken) As Task(Of String())
Dim webClients() As WebClient
ReDim webClients(urls.Length)
' 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 Not wc Is Nothing Then
wc.CancelAsync()
End If
Next
End Sub)
For i As Integer = 0 To urls.Length
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 Not args.Error Is 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.
results.Add(String.Format("{0} has {1} instances of {2}", args.UserState, nameCount, NAME))
End If
SyncLock (m_lock)
count = count + 1
If (count = addresses.Length) Then
tcs.TrySetResult(results.ToArray())
End If
End SyncLock
End Sub
End Class
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.
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.
lock (m_lock)
{
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;
}
有关更完整的示例（该示例包括附加的异常处理和演示如何从客户端代码中调用该方法），请参阅如何：在任务中包装 EAP 模式。
请记住，TaskCompletionSource<TResult> 创建的任何任务将由 TaskCompletionSource 启动，因此，用户代码不应在该任务上调用 Start 方法。
使用任务实现 APM 模式
在某些情况下，可能需要通过在 API 中使用 Begin/End 方法对来直接公开 IAsyncResult 模式。 例如，您可能希望维护与现有 API 的一致性，或者您可能有需要此模式的自动工具。 在这种情况下，可以使用 Task 来简化在内部实现 APM 模式的方式。
下面的示例演示如何使用任务来为长时间运行的计算绑定方法实现一个 APM begin/end 方法对。
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
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);
}
}
使用 StreamExtensions 代码示例
MSDN 网站上的 Samples for Parallel Programming with the .NET Framework 4（使用 .NET Framework 4 并行编程的示例）中的 Streamextensions.cs 文件包含一些引用实现，这些实现将 Task 对象用于异步文件和网络 I/O。