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ReaderWriterLockSlim.EnterReadLock Method

Tries to enter the lock in read mode.

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

public void EnterReadLock()


The RecursionPolicy property is LockRecursionPolicy.NoRecursion and the current thread has already entered read mode.


The recursion number would exceed the capacity of the counter. This limit is so large that applications should never encounter it.

This method blocks until the calling thread enters the lock, and therefore might never return. Use the TryEnterReadLock method to block for a specified interval, and then return if the calling thread has not entered read mode during that interval.

Multiple threads can enter read mode at the same time.

If one or more threads are waiting to enter write mode, a thread that calls the EnterReadLock method blocks until those threads have either timed out or entered write mode and then exited from it.


If a lock allows recursion, a thread that has entered the lock in read mode can enter read mode recursively, even if other threads are waiting to enter write mode.

At most one thread can be in upgradeable mode while other threads are in read mode. If additional threads are waiting to enter upgradeable mode, and there are no threads waiting to enter write mode, threads that call the EnterReadLock method enter read mode immediately and do not block.

The following example shows how to use the EnterReadLock method to enter the lock in read mode. The method shown in the example retrieves the value associated with a key. If the key is not found, the exception thrown by the inner Dictionary<TKey, TValue> is allowed to terminate the method. A finally block is used to execute the ExitReadLock method, ensuring that the caller exits read mode.

This code is part of a larger example provided for the ReaderWriterLockSlim class.

private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
private Dictionary<int, string> innerCache = new Dictionary<int, string>();


public string Read(int key)
        return innerCache[key];

.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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