Artikel
11/18/2011

Monitor.Enter Method (Object, Boolean%)

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Acquires an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.

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

Syntax

'Declaration
<SecuritySafeCriticalAttribute> _
Public Shared Sub Enter ( _
    obj As Object, _
    ByRef lockTaken As Boolean _
)

[SecuritySafeCriticalAttribute]
public static void Enter(
    Object obj,
    ref bool lockTaken
)

Parameters

obj
Type: System.Object
The object on which to wait.

lockTaken
Type: System.Boolean%
The result of the attempt to acquire the lock, passed by reference. The input must be false. The output is true if the lock is acquired; otherwise, the output is false. The output is set even if an exception occurs during the attempt to acquire the lock.

Exceptions

Exception	Condition
ArgumentException	The input to lockTaken is true.
ArgumentNullException	The obj parameter is nulla null reference (Nothing in Visual Basic).

Remarks

Use Enter to acquire the Monitor on the object passed as the obj 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.

If the lock was not taken because an exception was thrown, the variable specified for the lockTaken parameter is false after this method ends. This allows the program to determine, in all cases, whether it is necessary to release the lock. If this method returns without throwing an exception, the variable specified for the lockTaken parameter is always true, and there is no need to test it.

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, 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.

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

Examples

The following code shows the basic pattern for using the Enter(Object, Boolean%) method overload. This overload always sets the value of the variable that is passed to the ref parameter (ByRef in Visual Basic) lockTaken, even if the method throws an exception, so the value of the variable is a reliable way to test whether the lock has to be released.

Dim acquiredLock As Boolean = False

Try
    Monitor.Enter(lockObject, acquiredLock)

    ' Code that accesses resources that are protected by the lock.

Finally
    If acquiredLock Then
        Monitor.Exit(lockObject)
    End If
End Try

bool acquiredLock = false;

try
{
    Monitor.Enter(lockObject, ref acquiredLock);

    // Code that accesses resources that are protected by the lock.

}
finally
{
    if (acquiredLock)
    {
        Monitor.Exit(lockObject);
    }
}

Version Information

Silverlight

Supported in: 5, 4

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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