LockRecursionPolicy Enumeration

Definition

public enum class LockRecursionPolicy

public enum LockRecursionPolicy

type LockRecursionPolicy = 

Public Enum LockRecursionPolicy

Vererbung

Object

ValueType

Enum

LockRecursionPolicy

Felder

Beispiele

Das folgende Beispiel zeigt zwei Ausnahmeszenarien, eine, die von der LockRecursionPolicy Einstellung abhängig ist und die nicht.

Im ersten Szenario gibt der Thread die Sperre im Lesemodus ein und versucht dann, den Lesemodus rekursiv einzugeben. Wenn dies ReaderWriterLockSlim mithilfe des parameterlosen Konstruktors erstellt wird, das Rekursionsrichtlinie auf NoRecursion festlegt, wird eine Ausnahme ausgelöst. Wenn SupportsRecursion verwendet wird, um die Erstellung zu ReaderWriterLockSlimerstellen, wird keine Ausnahme ausgelöst.

Im zweiten Szenario gibt der Thread die Sperre im Lesemodus ein und versucht dann, die Sperre im Schreibmodus einzugeben. LockRecursionException wird unabhängig von der Sperres-Rekursionsrichtlinie ausgelöst.

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.
'

Hinweise

Die Standard-Rekursionsrichtlinie hängt vom Sperrtyp ab. Für die Standardrichtlinie und das genaue Verhalten der Sperr rekursion für jeden bestimmten Sperrtyp finden Sie in der Dokumentation für den Typ. Die Klasse erlaubt beispielsweise ReaderWriterLockSlim keinen Thread, den Sperrmodus in den Schreibmodus einzugeben, wenn er bereits den Sperrmodus eingegeben hat, unabhängig von der Sperrrichtlinieneinstellung, um die Chance von Deadlocks zu verringern.

Derzeit verwendet nur eine Sperre diese Enumeration:

• ReaderWriterLockSlim. Weitere Informationen finden Sie in den Ausführungen zur RecursionPolicy-Eigenschaft.

Gilt für