Monitor.Enter Method
Definition
Important
Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
Acquires an exclusive lock on a specified object.
Overloads
| Enter(Object) |
Acquires an exclusive lock on the specified object. |
| Enter(Object, Boolean) |
Acquires an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken. |
Enter(Object)
- Source:
- Monitor.cs
- Source:
- Monitor.cs
- Source:
- Monitor.cs
Acquires an exclusive lock on the specified object.
public:
static void Enter(System::Object ^ obj);public static void Enter (object obj);static member Enter : obj -> unitPublic Shared Sub Enter (obj As Object)Parameters
- obj
- Object
The object on which to acquire the monitor lock.
Exceptions
The obj parameter is null.
Examples
The following example demonstrates how to use the Enter method.
#using <System.dll>
using namespace System;
using namespace System::Threading;
using namespace System::Collections::Generic;
using namespace System::Text;
generic <typename T> public ref class SafeQueue
{
private:
// A queue that is protected by Monitor.
Queue<T>^ m_inputQueue;
public:
SafeQueue()
{
m_inputQueue = gcnew Queue<T>();
};
// Lock the queue and add an element.
void Enqueue(T qValue)
{
// Request the lock, and block until it is obtained.
Monitor::Enter(m_inputQueue);
try
{
// When the lock is obtained, add an element.
m_inputQueue->Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
};
// Try to add an element to the queue: Add the element to the queue
// only if the lock is immediately available.
bool TryEnqueue(T qValue)
{
// Request the lock.
if (Monitor::TryEnter(m_inputQueue))
{
try
{
m_inputQueue->Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
return true;
}
else
{
return false;
}
};
// Try to add an element to the queue: Add the element to the queue
// only if the lock becomes available during the specified time
// interval.
bool TryEnqueue(T qValue, int waitTime)
{
// Request the lock.
if (Monitor::TryEnter(m_inputQueue, waitTime))
{
try
{
m_inputQueue->Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
return true;
}
else
{
return false;
}
};
// Lock the queue and dequeue an element.
T Dequeue()
{
T retval;
// Request the lock, and block until it is obtained.
Monitor::Enter(m_inputQueue);
try
{
// When the lock is obtained, dequeue an element.
retval = m_inputQueue->Dequeue();
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
return retval;
};
// Delete all elements that equal the given object.
int Remove(T qValue)
{
int removedCt = 0;
// Wait until the lock is available and lock the queue.
Monitor::Enter(m_inputQueue);
try
{
int counter = m_inputQueue->Count;
while (counter > 0)
// Check each element.
{
T elem = m_inputQueue->Dequeue();
if (!elem->Equals(qValue))
{
m_inputQueue->Enqueue(elem);
}
else
{
// Keep a count of items removed.
removedCt += 1;
}
counter = counter - 1;
}
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
return removedCt;
};
// Print all queue elements.
String^ PrintAllElements()
{
StringBuilder^ output = gcnew StringBuilder();
// Lock the queue.
Monitor::Enter(m_inputQueue);
try
{
for each ( T elem in m_inputQueue )
{
// Print the next element.
output->AppendLine(elem->ToString());
}
}
finally
{
// Ensure that the lock is released.
Monitor::Exit(m_inputQueue);
}
return output->ToString();
};
};
public ref class Example
{
private:
static SafeQueue<int>^ q = gcnew SafeQueue<int>();
static int threadsRunning = 0;
static array<array<int>^>^ results = gcnew array<array<int>^>(3);
static void ThreadProc(Object^ state)
{
DateTime finish = DateTime::Now.AddSeconds(10);
Random^ rand = gcnew Random();
array<int>^ result = { 0, 0, 0, 0, 0, 0, 0, 0, 0 };
int threadNum = (int) state;
while (DateTime::Now < finish)
{
int what = rand->Next(250);
int how = rand->Next(100);
if (how < 16)
{
q->Enqueue(what);
result[(int)ThreadResultIndex::EnqueueCt] += 1;
}
else if (how < 32)
{
if (q->TryEnqueue(what))
{
result[(int)ThreadResultIndex::TryEnqueueSucceedCt] += 1;
}
else
{
result[(int)ThreadResultIndex::TryEnqueueFailCt] += 1;
}
}
else if (how < 48)
{
// Even a very small wait significantly increases the success
// rate of the conditional enqueue operation.
if (q->TryEnqueue(what, 10))
{
result[(int)ThreadResultIndex::TryEnqueueWaitSucceedCt] += 1;
}
else
{
result[(int)ThreadResultIndex::TryEnqueueWaitFailCt] += 1;
}
}
else if (how < 96)
{
result[(int)ThreadResultIndex::DequeueCt] += 1;
try
{
q->Dequeue();
}
catch (Exception^ ex)
{
result[(int)ThreadResultIndex::DequeueExCt] += 1;
}
}
else
{
result[(int)ThreadResultIndex::RemoveCt] += 1;
result[(int)ThreadResultIndex::RemovedCt] += q->Remove(what);
}
}
results[threadNum] = result;
if (0 == Interlocked::Decrement(threadsRunning))
{
StringBuilder^ sb = gcnew StringBuilder(
" Thread 1 Thread 2 Thread 3 Total\n");
for (int row = 0; row < 9; row++)
{
int total = 0;
sb->Append(titles[row]);
for(int col = 0; col < 3; col++)
{
sb->Append(String::Format("{0,9}", results[col][row]));
total += results[col][row];
}
sb->AppendLine(String::Format("{0,9}", total));
}
Console::WriteLine(sb->ToString());
}
};
static array<String^>^ titles = {
"Enqueue ",
"TryEnqueue succeeded ",
"TryEnqueue failed ",
"TryEnqueue(T, wait) succeeded ",
"TryEnqueue(T, wait) failed ",
"Dequeue attempts ",
"Dequeue exceptions ",
"Remove operations ",
"Queue elements removed "};
enum class ThreadResultIndex
{
EnqueueCt,
TryEnqueueSucceedCt,
TryEnqueueFailCt,
TryEnqueueWaitSucceedCt,
TryEnqueueWaitFailCt,
DequeueCt,
DequeueExCt,
RemoveCt,
RemovedCt
};
public:
static void Demo()
{
Console::WriteLine("Working...");
for(int i = 0; i < 3; i++)
{
Thread^ t = gcnew Thread(gcnew ParameterizedThreadStart(Example::ThreadProc));
t->Start(i);
Interlocked::Increment(threadsRunning);
}
};
};
void main()
{
Example::Demo();
}
/* This example produces output similar to the following:
Working...
Thread 1 Thread 2 Thread 3 Total
Enqueue 274718 513514 337895 1126127
TryEnqueue succeeded 274502 513516 337480 1125498
TryEnqueue failed 119 235 141 495
TryEnqueue(T, wait) succeeded 274552 513116 338532 1126200
TryEnqueue(T, wait) failed 0 1 0 1
Dequeue attempts 824038 1541866 1015006 3380910
Dequeue exceptions 12828 23416 14799 51043
Remove operations 68746 128218 84306 281270
Queue elements removed 11464 22024 14470 47958
Queue elements removed 2921 4690 2982 10593
*/
using System;
using System.Threading;
using System.Collections.Generic;
using System.Text;
class SafeQueue<T>
{
// A queue that is protected by Monitor.
private Queue<T> m_inputQueue = new Queue<T>();
// Lock the queue and add an element.
public void Enqueue(T qValue)
{
// Request the lock, and block until it is obtained.
Monitor.Enter(m_inputQueue);
try
{
// When the lock is obtained, add an element.
m_inputQueue.Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
}
// Try to add an element to the queue: Add the element to the queue
// only if the lock is immediately available.
public bool TryEnqueue(T qValue)
{
// Request the lock.
if (Monitor.TryEnter(m_inputQueue))
{
try
{
m_inputQueue.Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
return true;
}
else
{
return false;
}
}
// Try to add an element to the queue: Add the element to the queue
// only if the lock becomes available during the specified time
// interval.
public bool TryEnqueue(T qValue, int waitTime)
{
// Request the lock.
if (Monitor.TryEnter(m_inputQueue, waitTime))
{
try
{
m_inputQueue.Enqueue(qValue);
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
return true;
}
else
{
return false;
}
}
// Lock the queue and dequeue an element.
public T Dequeue()
{
T retval;
// Request the lock, and block until it is obtained.
Monitor.Enter(m_inputQueue);
try
{
// When the lock is obtained, dequeue an element.
retval = m_inputQueue.Dequeue();
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
return retval;
}
// Delete all elements that equal the given object.
public int Remove(T qValue)
{
int removedCt = 0;
// Wait until the lock is available and lock the queue.
Monitor.Enter(m_inputQueue);
try
{
int counter = m_inputQueue.Count;
while (counter > 0)
// Check each element.
{
T elem = m_inputQueue.Dequeue();
if (!elem.Equals(qValue))
{
m_inputQueue.Enqueue(elem);
}
else
{
// Keep a count of items removed.
removedCt += 1;
}
counter = counter - 1;
}
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
return removedCt;
}
// Print all queue elements.
public string PrintAllElements()
{
StringBuilder output = new StringBuilder();
// Lock the queue.
Monitor.Enter(m_inputQueue);
try
{
foreach( T elem in m_inputQueue )
{
// Print the next element.
output.AppendLine(elem.ToString());
}
}
finally
{
// Ensure that the lock is released.
Monitor.Exit(m_inputQueue);
}
return output.ToString();
}
}
public class Example
{
private static SafeQueue<int> q = new SafeQueue<int>();
private static int threadsRunning = 0;
private static int[][] results = new int[3][];
static void Main()
{
Console.WriteLine("Working...");
for(int i = 0; i < 3; i++)
{
Thread t = new Thread(ThreadProc);
t.Start(i);
Interlocked.Increment(ref threadsRunning);
}
}
private static void ThreadProc(object state)
{
DateTime finish = DateTime.Now.AddSeconds(10);
Random rand = new Random();
int[] result = { 0, 0, 0, 0, 0, 0, 0, 0, 0 };
int threadNum = (int) state;
while (DateTime.Now < finish)
{
int what = rand.Next(250);
int how = rand.Next(100);
if (how < 16)
{
q.Enqueue(what);
result[(int)ThreadResultIndex.EnqueueCt] += 1;
}
else if (how < 32)
{
if (q.TryEnqueue(what))
{
result[(int)ThreadResultIndex.TryEnqueueSucceedCt] += 1;
}
else
{
result[(int)ThreadResultIndex.TryEnqueueFailCt] += 1;
}
}
else if (how < 48)
{
// Even a very small wait significantly increases the success
// rate of the conditional enqueue operation.
if (q.TryEnqueue(what, 10))
{
result[(int)ThreadResultIndex.TryEnqueueWaitSucceedCt] += 1;
}
else
{
result[(int)ThreadResultIndex.TryEnqueueWaitFailCt] += 1;
}
}
else if (how < 96)
{
result[(int)ThreadResultIndex.DequeueCt] += 1;
try
{
q.Dequeue();
}
catch
{
result[(int)ThreadResultIndex.DequeueExCt] += 1;
}
}
else
{
result[(int)ThreadResultIndex.RemoveCt] += 1;
result[(int)ThreadResultIndex.RemovedCt] += q.Remove(what);
}
}
results[threadNum] = result;
if (0 == Interlocked.Decrement(ref threadsRunning))
{
StringBuilder sb = new StringBuilder(
" Thread 1 Thread 2 Thread 3 Total\n");
for(int row = 0; row < 9; row++)
{
int total = 0;
sb.Append(titles[row]);
for(int col = 0; col < 3; col++)
{
sb.Append(String.Format("{0,9}", results[col][row]));
total += results[col][row];
}
sb.AppendLine(String.Format("{0,9}", total));
}
Console.WriteLine(sb.ToString());
}
}
private static string[] titles = {
"Enqueue ",
"TryEnqueue succeeded ",
"TryEnqueue failed ",
"TryEnqueue(T, wait) succeeded ",
"TryEnqueue(T, wait) failed ",
"Dequeue attempts ",
"Dequeue exceptions ",
"Remove operations ",
"Queue elements removed "};
private enum ThreadResultIndex
{
EnqueueCt,
TryEnqueueSucceedCt,
TryEnqueueFailCt,
TryEnqueueWaitSucceedCt,
TryEnqueueWaitFailCt,
DequeueCt,
DequeueExCt,
RemoveCt,
RemovedCt
};
}
/* This example produces output similar to the following:
Working...
Thread 1 Thread 2 Thread 3 Total
Enqueue 277382 515209 308464 1101055
TryEnqueue succeeded 276873 514621 308099 1099593
TryEnqueue failed 109 181 134 424
TryEnqueue(T, wait) succeeded 276913 514434 307607 1098954
TryEnqueue(T, wait) failed 2 0 0 2
Dequeue attempts 830980 1544081 924164 3299225
Dequeue exceptions 12102 21589 13539 47230
Remove operations 69550 129479 77351 276380
Queue elements removed 11957 22572 13043 47572
*/
Imports System.Threading
Imports System.Collections.Generic
Imports System.Text
Class SafeQueue(Of T)
' A queue that is protected by Monitor.
Private m_inputQueue As New Queue(Of T)
' Lock the queue and add an element.
Public Sub Enqueue(ByVal qValue As T)
' Request the lock, and block until it is obtained.
Monitor.Enter(m_inputQueue)
Try
' When the lock is obtained, add an element.
m_inputQueue.Enqueue(qValue)
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
End Sub
' Try to add an element to the queue: Add the element to the queue
' only if the lock is immediately available.
Public Function TryEnqueue(ByVal qValue As T) As Boolean
' Request the lock.
If Monitor.TryEnter(m_inputQueue) Then
Try
m_inputQueue.Enqueue(qValue)
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
Return True
Else
Return False
End If
End Function
' Try to add an element to the queue: Add the element to the queue
' only if the lock becomes available during the specified time
' interval.
Public Function TryEnqueue(ByVal qValue As T, ByVal waitTime As Integer) As Boolean
' Request the lock.
If Monitor.TryEnter(m_inputQueue, waitTime) Then
Try
m_inputQueue.Enqueue(qValue)
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
Return True
Else
Return False
End If
End Function
' Lock the queue and dequeue an element.
Public Function Dequeue() As T
Dim retval As T
' Request the lock, and block until it is obtained.
Monitor.Enter(m_inputQueue)
Try
' When the lock is obtained, dequeue an element.
retval = m_inputQueue.Dequeue()
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
Return retval
End Function
' Delete all elements that equal the given object.
Public Function Remove(ByVal qValue As T) As Integer
Dim removedCt As Integer = 0
' Wait until the lock is available and lock the queue.
Monitor.Enter(m_inputQueue)
Try
Dim counter As Integer = m_inputQueue.Count
While (counter > 0)
'Check each element.
Dim elem As T = m_inputQueue.Dequeue()
If Not elem.Equals(qValue) Then
m_inputQueue.Enqueue(elem)
Else
' Keep a count of items removed.
removedCt += 1
End If
counter = counter - 1
End While
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
Return removedCt
End Function
' Print all queue elements.
Public Function PrintAllElements() As String
Dim output As New StringBuilder()
'Lock the queue.
Monitor.Enter(m_inputQueue)
Try
For Each elem As T In m_inputQueue
' Print the next element.
output.AppendLine(elem.ToString())
Next
Finally
' Ensure that the lock is released.
Monitor.Exit(m_inputQueue)
End Try
Return output.ToString()
End Function
End Class
Public Class Example
Private Shared q As New SafeQueue(Of Integer)
Private Shared threadsRunning As Integer = 0
Private Shared results(2)() As Integer
Friend Shared Sub Main()
Console.WriteLine("Working...")
For i As Integer = 0 To 2
Dim t As New Thread(AddressOf ThreadProc)
t.Start(i)
Interlocked.Increment(threadsRunning)
Next i
End Sub
Private Shared Sub ThreadProc(ByVal state As Object)
Dim finish As DateTime = DateTime.Now.AddSeconds(10)
Dim rand As New Random()
Dim result() As Integer = { 0, 0, 0, 0, 0, 0, 0, 0, 0 }
Dim threadNum As Integer = CInt(state)
While (DateTime.Now < finish)
Dim what As Integer = rand.Next(250)
Dim how As Integer = rand.Next(100)
If how < 16 Then
q.Enqueue(what)
result(ThreadResultIndex.EnqueueCt) += 1
Else If how < 32 Then
If q.TryEnqueue(what)
result(ThreadResultIndex.TryEnqueueSucceedCt) += 1
Else
result(ThreadResultIndex.TryEnqueueFailCt) += 1
End If
Else If how < 48 Then
' Even a very small wait significantly increases the success
' rate of the conditional enqueue operation.
If q.TryEnqueue(what, 10)
result(ThreadResultIndex.TryEnqueueWaitSucceedCt) += 1
Else
result(ThreadResultIndex.TryEnqueueWaitFailCt) += 1
End If
Else If how < 96 Then
result(ThreadResultIndex.DequeueCt) += 1
Try
q.Dequeue()
Catch
result(ThreadResultIndex.DequeueExCt) += 1
End Try
Else
result(ThreadResultIndex.RemoveCt) += 1
result(ThreadResultIndex.RemovedCt) += q.Remove(what)
End If
End While
results(threadNum) = result
If 0 = Interlocked.Decrement(threadsRunning) Then
Dim sb As New StringBuilder( _
" Thread 1 Thread 2 Thread 3 Total" & vbLf)
For row As Integer = 0 To 8
Dim total As Integer = 0
sb.Append(titles(row))
For col As Integer = 0 To 2
sb.Append(String.Format("{0,9}", results(col)(row)))
total += results(col)(row)
Next col
sb.AppendLine(String.Format("{0,9}", total))
Next row
Console.WriteLine(sb.ToString())
End If
End Sub
Private Shared titles() As String = { _
"Enqueue ", _
"TryEnqueue succeeded ", _
"TryEnqueue failed ", _
"TryEnqueue(T, wait) succeeded ", _
"TryEnqueue(T, wait) failed ", _
"Dequeue attempts ", _
"Dequeue exceptions ", _
"Remove operations ", _
"Queue elements removed " _
}
Private Enum ThreadResultIndex
EnqueueCt
TryEnqueueSucceedCt
TryEnqueueFailCt
TryEnqueueWaitSucceedCt
TryEnqueueWaitFailCt
DequeueCt
DequeueExCt
RemoveCt
RemovedCt
End Enum
End Class
' This example produces output similar to the following:
'
'Working...
' Thread 1 Thread 2 Thread 3 Total
'Enqueue 294357 512164 302838 1109359
'TryEnqueue succeeded 294486 512403 303117 1110006
'TryEnqueue failed 108 234 127 469
'TryEnqueue(T, wait) succeeded 294259 512796 302556 1109611
'TryEnqueue(T, wait) failed 1 1 1 3
'Dequeue attempts 882266 1537993 907795 3328054
'Dequeue exceptions 12691 21474 13480 47645
'Remove operations 74059 128715 76187 278961
'Queue elements removed 12667 22606 13219 48492
Remarks
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# try…finally block (Try…Finally 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 try…finally block.
See also
Applies to
Enter(Object, Boolean)
- Source:
- Monitor.CoreCLR.cs
- Source:
- Monitor.CoreCLR.cs
- Source:
- Monitor.CoreCLR.cs
Acquires an exclusive lock on the specified object, and atomically sets a value that indicates whether the lock was taken.
public:
static void Enter(System::Object ^ obj, bool % lockTaken);public static void Enter (object obj, ref bool lockTaken);static member Enter : obj * bool -> unitPublic Shared Sub Enter (obj As Object, ByRef lockTaken As Boolean)Parameters
- obj
- Object
The object on which to wait.
- lockTaken
- 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.
Note If no exception occurs, the output of this method is always true.
Exceptions
The input to lockTaken is true.
The obj parameter is null.
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.
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);
}
}
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
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.
Interrupt can interrupt threads that are waiting to enter a Monitor on an object. A ThreadInterruptedException will be thrown.
Applies to
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