Monitor.TryEnter Method (Object, Int32, Boolean)

 
System_CAPS_noteNote

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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)

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.

Exception Condition
ArgumentException

The input to lockTaken is true.

ArgumentNullException

The obj parameter is null.

ArgumentOutOfRangeException

millisecondsTimeout is negative, and not equal to Infinite.

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.

System_CAPS_noteNote

Use Monitor to lock objects (that is, reference types), not value types. For more information, see the Monitor class topic.

To ensure that the thread does not enter the critical section, you should examine the value of lockTaken and execute code in the critical section only if its value is true. The following code fragment shows the pattern used to call this method. Note that you should call Exit in a finally block to ensure that the calling thread releases its lock on the critical section if an exception occurs.

var lockObj = new Object();
int timeout = 500;
bool lockTaken = false;

try {
   Monitor.TryEnter(lockObj, timeout, ref lockTaken);
   if (lockTaken) {
      // The critical section.
   }
   else {
      // The lock was not acquired.
   }
}
finally {
   // Ensure that the lock is released.
   if (lockTaken) {
      Monitor.Exit(lockObj);
   }   
}

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.

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);
    }
}

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 4.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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