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Monitor.Enter Method (Object)

Acquires an exclusive lock on the specified object.

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

static member Enter : 
        obj:Object -> unit 

Parameters

obj
Type: System.Object
The object on which to acquire the monitor lock.

ExceptionCondition
ArgumentNullException

The obj parameter is a null reference (Nothing in Visual Basic).

Use Enter to acquire the Monitor on the object passed as the parameter. If another thread has executed an Enter on the object but has not yet executed the corresponding Exit, the current thread will block until the other thread releases the object. It is legal for the same thread to invoke Enter more than once without it blocking; however, an equal number of Exit calls must be invoked before other threads waiting on the object will unblock.

Use Monitor to lock objects (that is, reference types), not value types. When you pass a value type variable to Enter, it is boxed as an object. If you pass the same variable to Enter again, it is boxed as a separate object, and the thread does not block. In this case, the code that Monitor is supposedly protecting is not protected. Furthermore, when you pass the variable to Exit, still another separate object is created. Because the object passed to Exit is different from the object passed to Enter, Monitor throws SynchronizationLockException. For more information, see the conceptual topic Monitors.

Interrupt can interrupt threads that are waiting to enter a Monitor on an object. A ThreadInterruptedException will be thrown.

Use a C# tryfinally block (TryFinally in Visual Basic) to ensure that you release the monitor, or use the C# lock statement (SyncLock statement in Visual Basic), which wraps the Enter and Exit methods in a tryfinally block.

The following example demonstrates how to use the Enter method.

No code example is currently available or this language may not be supported.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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