Articolo
11/18/2011

Monitor.TryEnter Method (Object, Int32, Boolean%)

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Attempts, for the specified number of milliseconds, 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, _
    millisecondsTimeout As Integer, _
    ByRef lockTaken As Boolean _
)

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

Parameters

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

millisecondsTimeout
Type: System.Int32
The number of milliseconds to wait for 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).
ArgumentOutOfRangeException	millisecondsTimeout is negative, and not equal to Infinite.

Remarks

If the millisecondsTimeout parameter equals Infinite, this method is equivalent to Enter(Object). If millisecondsTimeout equals 0, this method is equivalent to TryEnter(Object).

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.

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, 500, 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, 500, 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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