Articolo
11/18/2011

Monitor.TryEnter Method (Object, Boolean%)

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Attempts to acquire 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 TryEnter ( _
    obj As Object, _
    ByRef lockTaken As Boolean _
)

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

Parameters

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

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

If successful, this method acquires an exclusive lock on the obj parameter. This method returns immediately, whether or not the lock is available.

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.

This method is similar to Enter(Object, Boolean%), but it will never block. If the thread cannot enter without blocking, the method returns false, and the thread does not enter the critical section.

Note:
Use Monitor to lock objects (that is, reference types), not value types. For more information, see Enter and the conceptual topic Monitors.

Examples

The following code shows the basic pattern for using the TryEnter(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.TryEnter(lockObject, acquiredLock)
    If acquiredLock Then

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

    Else

        ' Code to deal with the fact that the lock was not acquired.

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

bool acquiredLock = false;

try
{
    Monitor.TryEnter(lockObject, ref acquiredLock);
    if (acquiredLock)
    {

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

    }
    else
    {

        // Code to deal with the fact that the lock was not acquired.

    }
}
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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