Partitioner(Of TSource) Class

.NET Framework (current version)

Represents a particular manner of splitting a data source into multiple partitions.

Namespace:   System.Collections.Concurrent
Assembly:  mscorlib (in mscorlib.dll)

  System.Collections.Concurrent.Partitioner(Of TSource)
    System.Collections.Concurrent.OrderablePartitioner(Of TSource)

<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,
	ExternalThreading := True)>
Public MustInherit Class Partitioner(Of TSource)

Type Parameters


Type of the elements in the collection.

System_CAPS_protmethodPartitioner(Of TSource)()

Creates a new partitioner instance.


Gets whether additional partitions can be created dynamically.


Determines whether the specified object is equal to the current object.(Inherited from Object.)


Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)


Creates an object that can partition the underlying collection into a variable number of partitions.


Serves as the default hash function. (Inherited from Object.)


Partitions the underlying collection into the given number of partitions.


Gets the Type of the current instance.(Inherited from Object.)


Creates a shallow copy of the current Object.(Inherited from Object.)


Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubmethodAsParallel(Of TSource)()

Overloaded. Enables parallelization of a query, as sourced by a custom partitioner that is responsible for splitting the input sequence into partitions.(Defined by ParallelEnumerable.)

The following example shows how to implement a partitioner that returns a single element at a time:

Imports System.Collections.Concurrent
Imports System.Threading
Imports System.Threading.Tasks

Module PartitionerDemo
    ' Simple partitioner that will extract one item at a time, in a thread-safe fashion,
    ' from the underlying collection.
    Class SingleElementPartitioner(Of T)
        Inherits Partitioner(Of T)
        ' The collection being wrapped by this Partitioner
        Private m_referenceEnumerable As IEnumerable(Of T)

        ' Internal class that serves as a shared enumerable for the
        ' underlying collection.
        Private Class InternalEnumerable
            Implements IEnumerable(Of T)
            Implements IDisposable

            Private m_reader As IEnumerator(Of T)
            Private m_disposed As Boolean = False

            ' These two are used to implement Dispose() when static partitioning is being performed
            Private m_activeEnumerators As Integer
            Private m_downcountEnumerators As Boolean

            ' "downcountEnumerators" will be true for static partitioning, false for
            ' dynamic partitioning. 
            Public Sub New(ByVal reader As IEnumerator(Of T), ByVal downcountEnumerators As Boolean)
                m_reader = reader
                m_activeEnumerators = 0
                m_downcountEnumerators = downcountEnumerators
            End Sub

            Public Function GetEnumerator() As IEnumerator(Of T) Implements IEnumerable(Of T).GetEnumerator
                If m_disposed Then
                    Throw New ObjectDisposedException("InternalEnumerable: Can't call GetEnumerator() after disposing")
                End If

                ' For static partitioning, keep track of the number of active enumerators.
                If m_downcountEnumerators Then
                End If

                Return New InternalEnumerator(m_reader, Me)
            End Function

            Private Function GetEnumerator2() As IEnumerator Implements IEnumerable.GetEnumerator
                Return DirectCast(Me, IEnumerable(Of T)).GetEnumerator()
            End Function

            Public Sub Dispose() Implements IDisposable.Dispose
                If Not m_disposed Then
                    ' Only dispose the source enumerator if you are doing dynamic partitioning
                    If Not m_downcountEnumerators Then
                    End If
                    m_disposed = True
                End If
            End Sub

            ' Called from Dispose() method of spawned InternalEnumerator. During
            ' static partitioning, the source enumerator will be automatically
            ' disposed once all requested InternalEnumerators have been disposed.
            Public Sub DisposeEnumerator()
                If m_downcountEnumerators Then
                    If Interlocked.Decrement(m_activeEnumerators) = 0 Then
                    End If
                End If
            End Sub
        End Class

        ' Internal class that serves as a shared enumerator for 
        ' the underlying collection.
        Private Class InternalEnumerator
            Implements IEnumerator(Of T)

            Private m_current As T
            Private m_source As IEnumerator(Of T)
            Private m_controllingEnumerable As InternalEnumerable
            Private m_disposed As Boolean = False

            Public Sub New(ByVal source As IEnumerator(Of T), ByVal controllingEnumerable As InternalEnumerable)
                m_source = source
                m_current = Nothing
                m_controllingEnumerable = controllingEnumerable
            End Sub

            Private ReadOnly Property Current2() As Object Implements IEnumerator.Current
                    Return m_current
                End Get
            End Property

            Private ReadOnly Property Current() As T Implements IEnumerator(Of T).Current
                    Return m_current
                End Get
            End Property

            Private Sub Reset() Implements IEnumerator.Reset
                Throw New NotSupportedException("Reset() not supported")
            End Sub

            ' This method is the crux of this class. Under lock, it calls
            ' MoveNext() on the underlying enumerator and grabs Current.
            Private Function MoveNext() As Boolean Implements IEnumerator.MoveNext
                Dim rval As Boolean = False
                SyncLock m_source
                    rval = m_source.MoveNext()
                    m_current = If(rval, m_source.Current, Nothing)
                End SyncLock
                Return rval
            End Function

            Private Sub Dispose() Implements IDisposable.Dispose
                If Not m_disposed Then
                    ' Delegate to parent enumerable's DisposeEnumerator() method
                    m_disposed = True
                End If
            End Sub

        End Class

        ' Constructor just grabs the collection to wrap
        Public Sub New(ByVal enumerable As IEnumerable(Of T))

            ' Verify that the source IEnumerable is not null
            If enumerable Is Nothing Then
                Throw New ArgumentNullException("enumerable")
            End If

            m_referenceEnumerable = enumerable
        End Sub

        ' Produces a list of "numPartitions" IEnumerators that can each be
        ' used to traverse the underlying collection in a thread-safe manner.
        ' This will return a static number of enumerators, as opposed to
        ' GetDynamicPartitions(), the result of which can be used to produce
        ' any number of enumerators.
        Public Overloads Overrides Function GetPartitions(ByVal numPartitions As Integer) As IList(Of IEnumerator(Of T))
            If numPartitions < 1 Then
                Throw New ArgumentOutOfRangeException("NumPartitions")
            End If

            Dim list As New List(Of IEnumerator(Of T))(numPartitions)

            ' Since we are doing static partitioning, create an InternalEnumerable with reference
            ' counting of spawned InternalEnumerators turned on. Once all of the spawned enumerators
            ' are disposed, dynamicPartitions will be disposed.
            Dim dynamicPartitions = New InternalEnumerable(m_referenceEnumerable.GetEnumerator(), True)
            For i As Integer = 0 To numPartitions - 1

            Return list
        End Function

        ' Returns an instance of our internal Enumerable class. GetEnumerator()
        ' can then be called on that (multiple times) to produce shared enumerators.
        Public Overloads Overrides Function GetDynamicPartitions() As IEnumerable(Of T)
            ' Since we are doing dynamic partitioning, create an InternalEnumerable with reference
            ' counting of spawned InternalEnumerators turned off. This returned InternalEnumerable
            ' will need to be explicitly disposed.
            Return New InternalEnumerable(m_referenceEnumerable.GetEnumerator(), False)
        End Function

        ' Must be set to true if GetDynamicPartitions() is supported.
        Public Overloads Overrides ReadOnly Property SupportsDynamicPartitions() As Boolean
                Return True
            End Get
        End Property
    End Class

    Class Program
        ' Test our SingleElementPartitioner(T) class
        Shared Sub Main()
            ' Our sample collection
            Dim collection As String() = New String() {"red", "orange", "yellow", "green", "blue", "indigo", _
            "violet", "black", "white", "grey"}

            ' Instantiate a partitioner for our collection
            Dim myPart As New SingleElementPartitioner(Of String)(Collection)

            ' Simple test with ForEach
            Console.WriteLine("Testing with Parallel.ForEach")
                                 Console.WriteLine(" item = {0}, thread id = {1}", item, Thread.CurrentThread.ManagedThreadId)
                             End Sub)

            ' Demonstrate the use of static partitioning, which really means
            ' "using a static number of partitioners". The partitioners themselves
            ' may still be "dynamic" in the sense that their outputs may not be
            ' deterministic.

            ' Perform static partitioning of collection
            Dim staticPartitions = myPart.GetPartitions(2)
            Dim index As Integer = 0

            Console.WriteLine("Static Partitioning, 2 partitions, 2 tasks:")

            ' Action will consume from static partitions
            Dim staticAction As Action =
                    Dim myIndex As Integer = Interlocked.Increment(index) - 1
                    ' compute your index
                    Dim myItems = staticPartitions(myIndex)
                    ' grab your static partition
                    Dim id As Integer = Thread.CurrentThread.ManagedThreadId
                    ' cache your thread id
                    ' Enumerate through your static partition
                    While myItems.MoveNext()
                        ' guarantees that multiple threads have a chance to run
                        Console.WriteLine(" item = {0}, thread id = {1}", myItems.Current, Thread.CurrentThread.ManagedThreadId)
                    End While

                End Sub

            ' Spawn off 2 actions to consume 2 static partitions
            Parallel.Invoke(staticAction, staticAction)

            ' Demonstrate the use of dynamic partitioning

            ' Grab an IEnumerable which can then be used to generate multiple
            ' shared IEnumerables.
            Dim dynamicPartitions = myPart.GetDynamicPartitions()

            Console.WriteLine("Dynamic Partitioning, 3 tasks:")

            ' Action will consume from dynamic partitions
            Dim dynamicAction As Action =
                    ' Grab an enumerator from the dynamic partitioner
                    Dim enumerator = dynamicPartitions.GetEnumerator()
                    Dim id As Integer = Thread.CurrentThread.ManagedThreadId
                    ' cache our thread id
                    ' Enumerate through your dynamic enumerator
                    While enumerator.MoveNext()
                        ' guarantees that multiple threads will have a chance to run
                        Console.WriteLine(" item = {0}, thread id = {1}", enumerator.Current, id)
                    End While

                End Sub

            ' Spawn 3 concurrent actions to consume the dynamic partitions
            Parallel.Invoke(dynamicAction, dynamicAction, dynamicAction)

            ' Clean up
            If TypeOf dynamicPartitions Is IDisposable Then
                DirectCast(dynamicPartitions, IDisposable).Dispose()
            End If
        End Sub
    End Class

End Module

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1

The static methods on Partitioner(Of TSource) are all thread-safe and may be used concurrently from multiple threads. However, while a created partitioner is in use, the underlying data source should not be modified, whether from the same thread that is using a partitioner or from a separate thread.

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