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LockRecursionPolicy Enumeration

Specifies whether a lock can be entered multiple times by the same thread.

Namespace:  System.Threading
Assembly:  System.Core (in System.Core.dll)

'Declaration
Public Enumeration LockRecursionPolicy

Member nameDescription
Supported by Portable Class LibrarySupported in .NET for Windows Store appsNoRecursionIf a thread tries to enter a lock recursively, an exception is thrown. Some classes may allow certain recursions when this setting is in effect.
Supported by Portable Class LibrarySupported in .NET for Windows Store appsSupportsRecursionA thread can enter a lock recursively. Some classes may restrict this capability.

The default recursion policy depends on the type of lock. For the default policy and the precise behavior of lock recursion for any given lock type, see the documentation for the type. For example, the ReaderWriterLockSlim class does not allow a thread to enter the lock in write mode if it already entered the lock in read mode, regardless of the lock policy setting, in order to reduce the chance of deadlocks.

Currently only one lock uses this enumeration:

The following example shows two exception scenarios, one that depends on the LockRecursionPolicy setting and one that does not.

In the first scenario, the thread enters the lock in read mode and then tries to enter read mode recursively. If the ReaderWriterLockSlim is created by using the default constructor, which sets recursion policy to NoRecursion, an exception is thrown. If SupportsRecursion is used to create the ReaderWriterLockSlim, no exception is thrown.

In the second scenario, the thread enters the lock in read mode and then tries to enter the lock in write mode. LockRecursionException is thrown regardless of the lock recursion policy.

Imports System
Imports System.Threading

Class Example
    ' By default, the lock recursion policy for a new  
    ' ReaderWriterLockSlim does not allow recursion. 
    Private Shared rwls As New ReaderWriterLockSlim()
    Private Shared rwlsWithRecursion _
        As New ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion)

    Shared Sub ThreadProc() 

        Console.WriteLine("1. Enter the read lock recursively.")
        ReadRecursive(rwls)
        ReadRecursive(rwlsWithRecursion)

        Console.WriteLine(vbCrLf & _
            "2. Enter the write lock recursively from the read lock.")
        ReadWriteRecursive(rwls)
        ReadWriteRecursive(rwlsWithRecursion)

    End Sub  

    Shared Sub ReadRecursive(ByVal rwls As ReaderWriterLockSlim)

        Console.WriteLine("LockRecursionPolicy.{0}:", rwls.RecursionPolicy)
        rwls.EnterReadLock()

        Try
            rwls.EnterReadLock()
            Console.WriteLine(vbTab & _
                "The read lock was entered recursively.")
            rwls.ExitReadLock()
        Catch lre As LockRecursionException
            Console.WriteLine(vbTab & "{0}: {1}", _
                lre.GetType().Name, lre.Message)
        End Try

        rwls.ExitReadLock()
    End Sub 

    Shared Sub ReadWriteRecursive(ByVal rwls As ReaderWriterLockSlim)

        Console.WriteLine("LockRecursionPolicy.{0}:", rwls.RecursionPolicy)
        rwls.EnterReadLock()

        Try
            rwls.EnterWriteLock()
            Console.WriteLine(vbTab & _
                "The write lock was entered recursively.")
        Catch lre As LockRecursionException
            Console.WriteLine(vbTab & "{0}: {1}", _
                lre.GetType().Name, lre.Message)
        End Try

        rwls.ExitReadLock()
    End Sub 

    Shared Sub Main() 

        Dim t As New Thread(AddressOf ThreadProc)
        t.Start()
        t.Join()

        ' Dispose of ReaderWriterLockSlim objects' unmanaged resources. 
        If rwls IsNot Nothing Then rwls.Dispose()
        If rwlsWithRecursion IsNot Nothing Then _
           rwlsWithRecursion.Dispose()
    End Sub  
End Class  

' This code example produces output similar to the following: 

'1. Enter the read lock recursively. 
'LockRecursionPolicy.NoRecursion: 
'        LockRecursionException: Recursive read lock acquisitions not allowed in this mode. 
'LockRecursionPolicy.SupportsRecursion: 
'        The read lock was entered recursively. 

'2. Enter the write lock recursively from the read lock. 
'LockRecursionPolicy.NoRecursion: 
'        LockRecursionException: Write lock may not be acquired with read lock held. This pattern is prone to deadlocks. Consider using the upgrade lock. 
'LockRecursionPolicy.SupportsRecursion: 
'        LockRecursionException: Write lock may not be acquired with read lock held. This pattern is prone to deadlocks. Consider using the upgrade lock. 
' 

.NET Framework

Supported in: 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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