LockRecursionPolicy Enumerazione

Definizione

public enum class LockRecursionPolicy

public enum LockRecursionPolicy

type LockRecursionPolicy = 

Public Enum LockRecursionPolicy

Ereditarietà

Object

ValueType

Enum

LockRecursionPolicy

Campi

Esempio

L'esempio seguente illustra due scenari di eccezione, uno che dipende dall'impostazione LockRecursionPolicy e da quello che non lo fa.

Nel primo scenario, il thread entra nel blocco in modalità di lettura e quindi tenta di accedere in modalità di lettura in modo ricorsivo. Se viene ReaderWriterLockSlim creato utilizzando il costruttore senza parametri, che imposta i criteri di ricorsione su NoRecursion, viene generata un'eccezione. Se supportsRecursion viene usato per creare , ReaderWriterLockSlimnon viene generata alcuna eccezione.

Nel secondo scenario, il thread entra nel blocco in modalità di lettura e quindi tenta di immettere il blocco in modalità di scrittura. LockRecursionException viene generata indipendentemente dal criterio di ricorsione del blocco.

using System;
using System.Threading;

class Example
{
    // By default, the lock recursion policy for a new 
    // ReaderWriterLockSlim does not allow recursion.
    private static ReaderWriterLockSlim rwls = new ReaderWriterLockSlim();
    private static ReaderWriterLockSlim rwlsWithRecursion =
        new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

    static void ThreadProc()
    { 
        Console.WriteLine("1. Enter the read lock recursively.");
        ReadRecursive(rwls);
        ReadRecursive(rwlsWithRecursion);

        Console.WriteLine("\n2. Enter the write lock recursively from the read lock.");
        ReadWriteRecursive(rwls);
        ReadWriteRecursive(rwlsWithRecursion);
    } 

    static void ReadRecursive(ReaderWriterLockSlim rwls)
    {
        Console.WriteLine("LockRecursionPolicy.{0}:", rwls.RecursionPolicy);
        rwls.EnterReadLock();

        try {
            rwls.EnterReadLock();
            Console.WriteLine("\nThe read lock was entered recursively.");
            rwls.ExitReadLock();
        }    
        catch (LockRecursionException lre) {
            Console.WriteLine("\n{0}: {1}",
                lre.GetType().Name, lre.Message);
        }

        rwls.ExitReadLock();
    }

    static void ReadWriteRecursive(ReaderWriterLockSlim rwls)
    {
        Console.WriteLine("LockRecursionPolicy.{0}:", rwls.RecursionPolicy);
        rwls.EnterReadLock();

        try {
            rwls.EnterWriteLock();
            Console.WriteLine("\nThe write lock was entered recursively.");
        }
        catch (LockRecursionException lre) {
            Console.WriteLine("\n{0}: {1}", 
                lre.GetType().Name, lre.Message);
        }

        rwls.ExitReadLock();
    }

    static void Main() 
    {
        Thread t = new Thread(ThreadProc);
        t.Start();
        t.Join();
        
        // Dispose of ReaderWriterLockSlim objects' unmanaged resources.
        if (rwls != null) rwls.Dispose();
        if (rwlsWithRecursion != null) rwlsWithRecursion.Dispose();
    } 
} 
// This example displays 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 i
//    s prone to deadlocks. Please ensure that read locks are released before taking a write loc
//    k. If an upgrade is necessary, use an upgrade lock in place of the read lock.
//    LockRecursionPolicy.SupportsRecursion:
//    
//    LockRecursionException: Write lock may not be acquired with read lock held. This pattern i
//    s prone to deadlocks. Please ensure that read locks are released before taking a write loc
//    k. If an upgrade is necessary, use an upgrade lock in place of the read lock.

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.
'

Commenti

Il criterio di ricorsione predefinito dipende dal tipo di blocco. Per i criteri predefiniti e il comportamento preciso della ricorsione di blocco per qualsiasi tipo di blocco specifico, vedere la documentazione relativa al tipo. Ad esempio, la ReaderWriterLockSlim classe non consente a un thread di accedere al blocco in modalità di scrittura se è già entrato nel blocco in modalità di lettura, indipendentemente dall'impostazione dei criteri di blocco, per ridurre la probabilità di deadlock.

Attualmente solo un blocco usa questa enumerazione:

• ReaderWriterLockSlim. Per altre informazioni, vedere la proprietà RecursionPolicy.

Si applica a