.NET 提供以下两种标准模式，用于执行 I/O 绑定和计算绑定异步作：
异步编程模型 （APM），其中异步作由一对开始/结束方法表示。 例如：FileStream.BeginRead 和 Stream.EndRead。
基于事件的异步模式（EAP），在这种模式中，异步操作由方法/事件对表示，这些方法/事件对命名为
<OperationName>Async和<OperationName>Completed。 例如：WebClient.DownloadStringAsync 和 WebClient.DownloadStringCompleted。
任务并行库（TPL）可以与任一异步模式结合使用，以各种方式使用。 可以将 APM 和 EAP 作为对象公开给 Task 库消费者，也可以公开 APM 模式，但使用 Task 对象在内部实现它们。 在这两种情况下，通过使用 Task 对象，可以简化代码并利用以下有用的功能：
在任务开始后随时以任务延续形式注册回调。
使用
Begin_和ContinueWhenAll方法，或者ContinueWhenAny和WaitAll方法，协调多个操作，以响应WaitAny方法。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
Task将操作状态封送处理至 TaskCompletionSource<TResult> 对象。
在 Task 中包装 APM 操作
System.Threading.Tasks.TaskFactory类和System.Threading.Tasks.TaskFactory<TResult>类都提供了多个TaskFactory.FromAsync和TaskFactory<TResult>.FromAsync方法的重载，这些重载允许您在一个Task或Task<TResult>实例中封装一个 APM 开始/结束方法对。 各种重载能够处理从零到三个输入参数的任意开始和结束方法组合。
对于具有返回值（在 Visual Basic 中为 End）的 Function 方法的对，使用 TaskFactory<TResult> 中创建 Task<TResult> 的方法。 对于具有返回 void（在 Visual Basic 中为 End）的 Sub 方法，使用 TaskFactory 中创建 Task 的方法。
在极少情况下，如果 Begin 方法具有三个以上参数或包含 ref 或 out 参数，则提供仅封装 FromAsync 方法的其他 End 重载。
下面的示例显示了匹配 FromAsync 和 FileStream.BeginRead 方法的 FileStream.EndRead 重载的签名。
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)
此重载采用三个输入参数，如下所示。 第一个参数是匹配 Func<T1,T2,T3,T4,T5,TResult> 方法签名的 FileStream.BeginRead 委托。 第二个参数使用 Func<T,TResult> 并返回 IAsyncResult 的 TResult 委托。 由于EndRead返回一个整数，因此编译器将 as TResult 的类型Int32和任务的类型推断为 Task。 最后四个参数与方法中的 FileStream.BeginRead 参数相同：
要在其中存储文件数据的缓冲区。
开始写入数据的缓冲区的偏移量。
要从文件读取的最大数据量。
一个可选对象，用于存储要传递给回调的用户定义状态数据。
使用 ContinueWith 实现回调功能
如果需要访问文件中的数据，而不是仅访问字节数，则 FromAsync 该方法是不够的。 请改用 Task，其 Result 属性包含文件数据。 可以通过向原始任务添加延续来实现这种操作。 延续执行通常由 AsyncCallback 委托执行的任务。 先前任务完成且填充了数据缓冲区后调用此操作。 在返回之前应关闭 FileStream 对象。
下面的示例演示如何返回封装 Task 类的 BeginRead/EndRead 对的 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
然后，可以调用该方法，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
提供自定义状态数据
在典型IAsyncResult操作中，如果AsyncCallback委托需要一些自定义状态数据，你必须通过Begin方法的最后一个参数传入数据，这样数据才能被打包到最终传递给回调方法的IAsyncResult对象中。 使用FromAsync方法时，通常不需要这样做。 如果延续知道自定义数据，可直接在延续委托中捕获它。 下面的示例与以前的示例类似，但延续检查此延续的用户委托可直接访问的自定义状态数据，而不是检查历史任务的 Result 属性。
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
同步多个 FromAsync 任务
静态 ContinueWhenAll 和 ContinueWhenAny 方法在与 FromAsync 方法结合使用时提供了额外的灵活性。 以下示例演示如何启动多个异步 I/O操作，并等待所有操作完成后再执行后续步骤。
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
仅用于 End 方法的 FromAsync 任务
在极少情况下，如果 Begin 方法需要三个以上的输入参数，或具有 ref 或 out 参数，可以使用仅表示 FromAsync 方法的 TaskFactory<TResult>.FromAsync(IAsyncResult, Func<IAsyncResult,TResult>) 重载，例如，End。 这些方法还可用于传递 IAsyncResult 并将其封装到 Task 的任何方案中。
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
启动和取消 FromAsync 任务
方法返回 FromAsync 的任务的状态为 WaitingForActivation 系统将在创建任务后的某些时间点启动。 如果尝试调用此类任务上的“启动”，将引发异常。
无法取消 FromAsync 任务，因为基础 .NET API 目前不支持取消正在进行中的文件或网络 I/O。 可以将取消功能添加到封装 FromAsync 调用的方法中，但只能在调用 FromAsync 之前或在调用完成之后响应取消（例如，在延续任务中）。
一些支持 EAP 的类（如 WebClient）不支持取消，但可以通过使用取消标记集成该本机取消功能。
将复杂的 EAP 操作公开为任务
TPL 不提供任何专门用于封装基于事件的异步操作的方法，其方式与FromAsync方法系列封装IAsyncResult模式的方式相同。 但是，TPL 确实提供了 System.Threading.Tasks.TaskCompletionSource<TResult> 类，可以用来将任意一组操作表示为 Task<TResult>。 操作可以是同步的或异步的，可以是 I/O 绑定或计算绑定，或两者兼具。
下面的示例显示如何使用 TaskCompletionSource<TResult> 将一组异步 WebClient 操作作为基础 Task<TResult> 向客户端代码公开。 该方法允许你输入 Web URL 数组，以及要搜索的术语或名称，然后返回每个网站上搜索词的次数。
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
有关包含额外异常处理的更完整示例，并演示如何从客户端代码调用该方法，请参阅如何：在任务中包装EAP模式。
请记住，由某个 TaskCompletionSource<TResult> 任务创建的任何任务都将由该 TaskCompletionSource 任务启动，因此，用户代码不应对该任务调用 Start 该方法。
使用任务实现 APM 模式
在某些场景中，可能希望在 API 中使用起始/结束方法对来直接呈现 IAsyncResult 模式。 例如，你可能想要保持与现有 API 的一致性，或者你可能具有需要此模式的自动化工具。 在这种情况下，可以使用 Task 对象来简化 APM 模式在内部实现的方式。
下面的示例显示如何使用任务实现长时间运行计算密集型方法的 APM begin/end 方法对。
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 示例代码
.NET Standard parallel extensions extras 存储库中的 StreamExtensions.cs 文件包含将 Task 对象用于异步文件和网络 I/O 的若干参考实现。
