如何：在数据流块收到数据时执行操作

项目
05/09/2023

在接收数据时，执行数据流块类型会调用用户提供的委托。 System.Threading.Tasks.Dataflow.ActionBlock<TInput>、System.Threading.Tasks.Dataflow.TransformBlock<TInput,TOutput> 和 System.Threading.Tasks.Dataflow.TransformManyBlock<TInput,TOutput> 类是执行数据流块类型。当为执行数据流块提供工作函数时，可以使用 delegate 关键字（Visual Basic 中为 Sub）、Action<T>、Func<T,TResult> 或 lambda 表达式。本文档描述如何使用 Func<T,TResult> 和 lambda 表达式在执行块中执行操作。

注意

TPL 数据流库（System.Threading.Tasks.Dataflow 命名空间）不随 .NET 一起分发。若要在 Visual Studio 中安装 System.Threading.Tasks.Dataflow 命名空间，请打开项目，选择“项目”菜单中的“管理 NuGet 包”，再在线搜索 System.Threading.Tasks.Dataflow 包。或者，若要使用 .NET Core CLI 进行安装，请运行 dotnet add package System.Threading.Tasks.Dataflow。

示例

下面的示例使用数据流从磁盘读取一个文件并计算该文件中等于零的字节的数量。它使用 TransformBlock<TInput,TOutput> 读取文件并计算零字节的数量，并使用 ActionBlock<TInput> 将零字节的数量输出到控制台。当块收到数据时，TransformBlock<TInput,TOutput> 对象会指定一个 Func<T,TResult> 对象来执行工作。 ActionBlock<TInput> 对象使用 lambda 表达式将读取到的零字节的数量输出到控制台。

using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using System.Threading.Tasks.Dataflow;

// Demonstrates how to provide delegates to exectution dataflow blocks.
class DataflowExecutionBlocks
{
   // Computes the number of zero bytes that the provided file
   // contains.
   static int CountBytes(string path)
   {
      byte[] buffer = new byte[1024];
      int totalZeroBytesRead = 0;
      using (var fileStream = File.OpenRead(path))
      {
         int bytesRead = 0;
         do
         {
            bytesRead = fileStream.Read(buffer, 0, buffer.Length);
            totalZeroBytesRead += buffer.Count(b => b == 0);
         } while (bytesRead > 0);
      }

      return totalZeroBytesRead;
   }

   static void Main(string[] args)
   {
      // Create a temporary file on disk.
      string tempFile = Path.GetTempFileName();

      // Write random data to the temporary file.
      using (var fileStream = File.OpenWrite(tempFile))
      {
         Random rand = new Random();
         byte[] buffer = new byte[1024];
         for (int i = 0; i < 512; i++)
         {
            rand.NextBytes(buffer);
            fileStream.Write(buffer, 0, buffer.Length);
         }
      }

      // Create an ActionBlock<int> object that prints to the console
      // the number of bytes read.
      var printResult = new ActionBlock<int>(zeroBytesRead =>
      {
         Console.WriteLine("{0} contains {1} zero bytes.",
            Path.GetFileName(tempFile), zeroBytesRead);
      });

      // Create a TransformBlock<string, int> object that calls the
      // CountBytes function and returns its result.
      var countBytes = new TransformBlock<string, int>(
         new Func<string, int>(CountBytes));

      // Link the TransformBlock<string, int> object to the
      // ActionBlock<int> object.
      countBytes.LinkTo(printResult);

      // Create a continuation task that completes the ActionBlock<int>
      // object when the TransformBlock<string, int> finishes.
      countBytes.Completion.ContinueWith(delegate { printResult.Complete(); });

      // Post the path to the temporary file to the
      // TransformBlock<string, int> object.
      countBytes.Post(tempFile);

      // Requests completion of the TransformBlock<string, int> object.
      countBytes.Complete();

      // Wait for the ActionBlock<int> object to print the message.
      printResult.Completion.Wait();

      // Delete the temporary file.
      File.Delete(tempFile);
   }
}

/* Sample output:
tmp4FBE.tmp contains 2081 zero bytes.
*/

Imports System.IO
Imports System.Linq
Imports System.Threading.Tasks
Imports System.Threading.Tasks.Dataflow

' Demonstrates how to provide delegates to exectution dataflow blocks.
Friend Class DataflowExecutionBlocks
    ' Computes the number of zero bytes that the provided file
    ' contains.
    Private Shared Function CountBytes(ByVal path As String) As Integer
        Dim buffer(1023) As Byte
        Dim totalZeroBytesRead As Integer = 0
        Using fileStream = File.OpenRead(path)
            Dim bytesRead As Integer = 0
            Do
                bytesRead = fileStream.Read(buffer, 0, buffer.Length)
                totalZeroBytesRead += buffer.Count(Function(b) b = 0)
            Loop While bytesRead > 0
        End Using

        Return totalZeroBytesRead
    End Function

    Shared Sub Main(ByVal args() As String)
        ' Create a temporary file on disk.
        Dim tempFile As String = Path.GetTempFileName()

        ' Write random data to the temporary file.
        Using fileStream = File.OpenWrite(tempFile)
            Dim rand As New Random()
            Dim buffer(1023) As Byte
            For i As Integer = 0 To 511
                rand.NextBytes(buffer)
                fileStream.Write(buffer, 0, buffer.Length)
            Next i
        End Using

        ' Create an ActionBlock<int> object that prints to the console 
        ' the number of bytes read.
        Dim printResult = New ActionBlock(Of Integer)(Sub(zeroBytesRead) Console.WriteLine("{0} contains {1} zero bytes.", Path.GetFileName(tempFile), zeroBytesRead))

        ' Create a TransformBlock<string, int> object that calls the 
        ' CountBytes function and returns its result.
        Dim countBytes = New TransformBlock(Of String, Integer)(New Func(Of String, Integer)(AddressOf DataflowExecutionBlocks.CountBytes))

        ' Link the TransformBlock<string, int> object to the 
        ' ActionBlock<int> object.
        countBytes.LinkTo(printResult)

        ' Create a continuation task that completes the ActionBlock<int>
        ' object when the TransformBlock<string, int> finishes.
        countBytes.Completion.ContinueWith(Sub() printResult.Complete())

        ' Post the path to the temporary file to the 
        ' TransformBlock<string, int> object.
        countBytes.Post(tempFile)

        ' Requests completion of the TransformBlock<string, int> object.
        countBytes.Complete()

        ' Wait for the ActionBlock<int> object to print the message.
        printResult.Completion.Wait()

        ' Delete the temporary file.
        File.Delete(tempFile)
    End Sub
End Class

' Sample output:
'tmp4FBE.tmp contains 2081 zero bytes.
'

虽然可以为 TransformBlock<TInput,TOutput> 对象提供 Lambda 表达式，但是本示例使用 Func<T,TResult> 来允许其他代码使用 CountBytes 方法。因为要执行的工作是此任务特有的，使用其他代码不可能有用，所以 ActionBlock<TInput> 对象使用 lambda 表达式。有关 lambda 表达式如何在任务并行库中工作的详细信息，请参阅 PLINQ 和 TPL 中的 Lambda 表达式。

Dataflow 文档中的“委托类型摘要”汇总了可以提供给 ActionBlock<TInput>、TransformBlock<TInput,TOutput> 和 TransformManyBlock<TInput,TOutput> 对象的委托类型。该表还指出委托类型是同步执行还是异步执行。

可靠编程

此示例为 Func<T,TResult> 对象提供类型 TransformBlock<TInput,TOutput> 的委托，以同步执行数据流块的任务。为了使数据流块异步执行操作，请为数据流块提供类型 Func<T, Task<TResult>> 的委托。当数据流块异步执行操作时，数据流块的任务只有在返回的 Task<TResult> 对象完成时才会完成。下面的示例修改了 CountBytes 方法，并使用 CountBytes 和 await 运算符（Visual Basic 中为 Async 和 Await）异步计算所提供文件中为零的字节的总数。 ReadAsync 方法异步执行文件读取操作。

// Asynchronously computes the number of zero bytes that the provided file
// contains.
static async Task<int> CountBytesAsync(string path)
{
   byte[] buffer = new byte[1024];
   int totalZeroBytesRead = 0;
   using (var fileStream = new FileStream(
      path, FileMode.Open, FileAccess.Read, FileShare.Read, 0x1000, true))
   {
      int bytesRead = 0;
      do
      {
         // Asynchronously read from the file stream.
         bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length);
         totalZeroBytesRead += buffer.Count(b => b == 0);
      } while (bytesRead > 0);
   }

   return totalZeroBytesRead;
}

' Asynchronously computes the number of zero bytes that the provided file 
' contains.
Private Shared async Function CountBytesAsync(ByVal path As String) As Task(Of Integer)
    Dim buffer(1023) As Byte
    Dim totalZeroBytesRead As Integer = 0
    Using fileStream = New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, &H1000, True)
        Dim bytesRead As Integer = 0
        Do
            ' Asynchronously read from the file stream.
            bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length)
            totalZeroBytesRead += buffer.Count(Function(b) b = 0)
        Loop While bytesRead > 0
    End Using

    Return totalZeroBytesRead
End Function

还可以使用异步的 lambda 表达式在执行数据流块中执行操作。下面的示例修改了上一示例中使用的 TransformBlock<TInput,TOutput> 对象，以便使用 lambda 表达式异步执行工作。

// Create a TransformBlock<string, int> object that calls the
// CountBytes function and returns its result.
var countBytesAsync = new TransformBlock<string, int>(async path =>
{
   byte[] buffer = new byte[1024];
   int totalZeroBytesRead = 0;
   using (var fileStream = new FileStream(
      path, FileMode.Open, FileAccess.Read, FileShare.Read, 0x1000, true))
   {
      int bytesRead = 0;
      do
      {
         // Asynchronously read from the file stream.
         bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length);
         totalZeroBytesRead += buffer.Count(b => b == 0);
      } while (bytesRead > 0);
   }

   return totalZeroBytesRead;
});

' Create a TransformBlock<string, int> object that calls the 
' CountBytes function and returns its result.
Dim countBytesAsync = New TransformBlock(Of String, Integer)(async Function(path)
                                                                 ' Asynchronously read from the file stream.
                                                                 Dim buffer(1023) As Byte
                                                                 Dim totalZeroBytesRead As Integer = 0
                                                                 Using fileStream = New FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, &H1000, True)
                                                                     Dim bytesRead As Integer = 0
                                                                     Do
                                                                         bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length)
                                                                         totalZeroBytesRead += buffer.Count(Function(b) b = 0)
                                                                     Loop While bytesRead > 0
                                                                 End Using
                                                                 Return totalZeroBytesRead
                                                             End Function)

请参阅

数据流

通过

如何：在数据流块收到数据时执行操作

示例

可靠编程

请参阅

其他资源