What is the difference between ManualResetEvent and AutoResetEvent in NET

Question

I have read the documentation on this and I think I understand  An AutoResetEvent resets when the code passes through event WaitOne    but a ManualResetEvent does not   Is this correct

User · Answer

Taken from C  3 0 Nutshell book  by   Joseph Albahari   Threading in C  - Free E-Book  A ManualResetEvent is a variation on AutoResetEvent  It differs in that it doesn t automatically reset after a thread is let through on a WaitOne call  and so functions like a gate  calling Set opens the gate  allowing any number of threads that WaitOne at the gate through  calling Reset closes the gate  causing  potentially  a queue of waiters to accumulate until its next opened   One could simulate this functionality with a boolean  gateOpen  field  declared with the volatile keyword  in combination with  spin-sleeping      repeatedly checking the flag  and then sleeping for a short period of time   ManualResetEvents are sometimes used to signal that a particular operation is complete  or that a thread s completed initialization and is ready to perform work

User · Answer

autoResetEvent WaitOne     is similar to  try      manualResetEvent WaitOne      finally      manualResetEvent Reset        as an atomic operation

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AutoResetEvent maintains a boolean variable in memory  If the boolean variable is false then it blocks the thread and if the boolean variable is true it unblocks the thread   When we instantiate an AutoResetEvent object  we pass the default value of boolean value in the constructor  Below is the syntax of instantiate an AutoResetEvent object   AutoResetEvent autoResetEvent   new AutoResetEvent false     WaitOne method  This method blocks the current thread and wait for the signal by other thread  WaitOne method puts the current thread into a Sleep thread state  WaitOne method returns true if it receives the signal else returns false   autoResetEvent WaitOne      Second overload of WaitOne method wait for the specified number of seconds  If it does not get any signal thread continues its work   static void ThreadMethod         while  autoResetEvent WaitOne TimeSpan FromSeconds 2                  Console WriteLine  Continue            Thread Sleep TimeSpan FromSeconds 1               Console WriteLine  Thread got signal        We called WaitOne method by passing the 2 seconds as arguments  In the while loop  it wait for the signal for 2 seconds then it continues its work  When the thread got the signal WaitOne returns true and exits the loop and print the  Thread got signal    Set method  AutoResetEvent Set method sent the signal to the waiting thread to proceed its work  Below is the syntax of calling Set method   autoResetEvent Set        ManualResetEvent maintains a boolean variable in memory  When the boolean variable is false then it blocks all threads and when the boolean variable is true it unblocks all threads   When we instantiate a ManualResetEvent  we initialize it with default boolean value   ManualResetEvent manualResetEvent   new ManualResetEvent false     In the above code  we initialize the ManualResetEvent with false value  that means all the threads which calls the WaitOne method will block until some thread calls the Set   method   If we initialize ManualResetEvent with true value  all the threads which calls the WaitOne method will not block and free to proceed further   WaitOne Method  This method blocks the current thread and wait for the signal by other thread  It returns true if its receives a signal else returns false   Below is the syntax of calling WaitOne method   manualResetEvent WaitOne      In the second overload of WaitOne method  we can specify the time interval till the current thread wait for the signal  If within time internal  it does not receives a signal it returns false and goes into the next line of method   Below is the syntax of calling WaitOne method with time interval   bool isSignalled   manualResetEvent WaitOne TimeSpan FromSeconds 5      We have specify 5 seconds into the WaitOne method  If the manualResetEvent object does not receives a signal between 5 seconds  it set the isSignalled variable to false   Set Method  This method is used for sending the signal to all waiting threads  Set   Method set the ManualResetEvent object boolean variable to true  All the waiting threads are unblocked and proceed further   Below is the syntax of calling Set   method   manualResetEvent Set      Reset Method  Once we call the Set   method on the ManualResetEvent object  its boolean remains true  To reset the value we can use Reset   method  Reset method change the boolean value to false   Below is the syntax of calling Reset method   manualResetEvent Reset      We must immediately call Reset method after calling Set method if we want to send signal to threads multiple times

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OK  normally it does not a good practice to add 2 answers in same thread  but I did not want to edit delete my previous answer  since it can help on another manner   Now  I created  much more comprehensive  and easy to understand  run-to-learn console app snippet below   Just run the examples on two different consoles  and observe behaviour  You will get much more clear idea there what is happening behind the scenes   Manual Reset Event  using System  using System Threading   namespace ConsoleApplicationDotNetBasics ThreadingExamples       public class ManualResetEventSample               private readonly ManualResetEvent  manualReset   new ManualResetEvent false            public void RunAll                         new Thread Worker1  Start                new Thread Worker2  Start                new Thread Worker3  Start                Console WriteLine  All Threads Scheduled to RUN   ThreadId   0    Thread CurrentThread ManagedThreadId               Console WriteLine  Main Thread is waiting for 15 seconds  observe 3 thread behaviour  All threads run once and stopped  Why  Because they call WaitOne   internally  They will wait until signals arrive  down below                 Thread Sleep 15000               Console WriteLine  1- Main will call ManualResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                manualReset Set                Thread Sleep 2000               Console WriteLine  2- Main will call ManualResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                manualReset Set                Thread Sleep 2000               Console WriteLine  3- Main will call ManualResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                manualReset Set                Thread Sleep 2000               Console WriteLine  4- Main will call ManualResetEvent Reset   in 5 seconds  watch out                 Thread Sleep 5000                manualReset Reset                Thread Sleep 2000               Console WriteLine  It ran one more time  Why  Even Reset Sets the state of the event to nonsignaled  false   causing threads to block  this will initial the state  and threads will run again until they WaitOne                   Thread Sleep 10000               Console WriteLine                Console WriteLine  This will go so on  Everytime you call Set    ManualResetEvent will let ALL threads to run  So if you want synchronization between them  consider using AutoReset event  or simply user TPL  Task Parallel Library                  Thread Sleep 5000               Console WriteLine  Main thread reached to end  ThreadId   0    Thread CurrentThread ManagedThreadId                       public void Worker1                         for  int i   1  i  lt   10  i                                  Console WriteLine  Worker1 is running  0  10  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 5000                      this gets blocked until  autoReset gets signal                  manualReset WaitOne                              Console WriteLine  Worker1 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId                     public void Worker2                         for  int i   1  i  lt   10  i                                  Console WriteLine  Worker2 is running  0  10  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 5000                      this gets blocked until  autoReset gets signal                  manualReset WaitOne                              Console WriteLine  Worker2 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId                     public void Worker3                         for  int i   1  i  lt   10  i                                  Console WriteLine  Worker3 is running  0  10  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 5000                      this gets blocked until  autoReset gets signal                  manualReset WaitOne                              Console WriteLine  Worker3 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId                          Auto Reset Event  using System  using System Threading   namespace ConsoleApplicationDotNetBasics ThreadingExamples       public class AutoResetEventSample               private readonly AutoResetEvent  autoReset   new AutoResetEvent false            public void RunAll                         new Thread Worker1  Start                new Thread Worker2  Start                new Thread Worker3  Start                Console WriteLine  All Threads Scheduled to RUN   ThreadId   0    Thread CurrentThread ManagedThreadId               Console WriteLine  Main Thread is waiting for 15 seconds  observe 3 thread behaviour  All threads run once and stopped  Why  Because they call WaitOne   internally  They will wait until signals arrive  down below                 Thread Sleep 15000               Console WriteLine  1- Main will call AutoResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                autoReset Set                Thread Sleep 2000               Console WriteLine  2- Main will call AutoResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                autoReset Set                Thread Sleep 2000               Console WriteLine  3- Main will call AutoResetEvent Set   in 5 seconds  watch out                 Thread Sleep 5000                autoReset Set                Thread Sleep 2000               Console WriteLine  4- Main will call AutoResetEvent Reset   in 5 seconds  watch out                 Thread Sleep 5000                autoReset Reset                Thread Sleep 2000               Console WriteLine  Nothing happened  Why  Becasuse Reset Sets the state of the event to nonsignaled  causing threads to block  Since they are already blocked  it will not affect anything                 Thread Sleep 10000               Console WriteLine  This will go so on  Everytime you call Set    AutoResetEvent will let another thread to run  It will make it automatically  so you do not need to worry about thread running order  unless you want it manually                 Thread Sleep 5000               Console WriteLine  Main thread reached to end  ThreadId   0    Thread CurrentThread ManagedThreadId                       public void Worker1                         for  int i   1  i  lt   5  i                                  Console WriteLine  Worker1 is running  0  5  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 500                      this gets blocked until  autoReset gets signal                  autoReset WaitOne                              Console WriteLine  Worker1 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId                     public void Worker2                         for  int i   1  i  lt   5  i                                  Console WriteLine  Worker2 is running  0  5  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 500                      this gets blocked until  autoReset gets signal                  autoReset WaitOne                              Console WriteLine  Worker2 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId                     public void Worker3                         for  int i   1  i  lt   5  i                                  Console WriteLine  Worker3 is running  0  5  ThreadId   1     i  Thread CurrentThread ManagedThreadId                   Thread Sleep 500                      this gets blocked until  autoReset gets signal                  autoReset WaitOne                              Console WriteLine  Worker3 is DONE  ThreadId   0    Thread CurrentThread ManagedThreadId

User · Answer

Yes  This is absolutely correct   You could see ManualResetEvent as a way to indicate state  Something is on  Set  or off  Reset   An occurrence with some duration   Any thread waiting for that state to happen can proceed   An AutoResetEvent is more comparable to a signal  A one shot indication that something has happened  An occurrence without any duration  Typically but not necessarily the  something  that has happened is small and needs to be handled by a single thread - hence the automatic reset after a single thread have consumed the event

User · Answer

Taken from C  3 0 Nutshell book  by   Joseph Albahari   Threading in C  - Free E-Book  A ManualResetEvent is a variation on AutoResetEvent  It differs in that it doesn t automatically reset after a thread is let through on a WaitOne call  and so functions like a gate  calling Set opens the gate  allowing any number of threads that WaitOne at the gate through  calling Reset closes the gate  causing  potentially  a queue of waiters to accumulate until its next opened   One could simulate this functionality with a boolean  gateOpen  field  declared with the volatile keyword  in combination with  spin-sleeping      repeatedly checking the flag  and then sleeping for a short period of time   ManualResetEvents are sometimes used to signal that a particular operation is complete  or that a thread s completed initialization and is ready to perform work

User · Answer

The short answer is yes  The most important difference is that an AutoResetEvent will only allow one single waiting thread to continue  A ManualResetEvent on the other hand will keep allowing threads  several at the same time even  to continue until you tell it to stop  Reset it

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If you want to understand AutoResetEvent and ManualResetEvent you need to understand not threading but interrupts    NET wants to conjure up low-level programming the most distant possible   An interrupts is something used in low-level programming which equals to a signal that from low became high  or viceversa   When this happens the program interrupt its normal execution and move the execution pointer to the function that handles this event   The first thing to do when an interrupt happend is to reset its state  becosa the hardware works in this way    a pin is connected to a signal and the hardware listen for it to change  the signal could have only two states   if the signal changes means that something happened and the hardware put a memory variable to the state happened  and it remain like this even if the signal change again   the program notice that variable change states and move the execution to a handling function  here the first thing to do  to be able to listen again this interrupt  is to reset this memory variable to the state not-happened    This is the difference between ManualResetEvent and AutoResetEvent  If a ManualResetEvent happen and I do not reset it  the next time it happens I will not be able to listen it

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Yes  thats right   You can get an idea by the usage of these two   If you need to tell that you are finished with some work and other  threads  waiting for this can now proceed  you should use ManualResetEvent   If you need to have mutual exclusive access to any resource  you should use AutoResetEvent

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Yes  It s like the difference between a tollbooth and a door  The ManualResetEvent is the door  which needs to be closed  reset  manually  The AutoResetEvent is a tollbooth  allowing one car to go by and automatically closing before the next one can get through

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I created simple examples to clarify understanding of ManualResetEvent vs AutoResetEvent   AutoResetEvent  lets assume you have 3 workers thread  If any of those threads will call WaitOne   all other 2 threads will stop execution and wait for signal  I am assuming they are using WaitOne    It is like  if I do not work  nobody works  In first example you can see that  autoReset Set    Thread Sleep 1000   autoReset Set      When you call Set   all threads will work and wait for signal  After 1 second I am sending second signal and they execute and wait  WaitOne     Think about these guys are soccer team players and if one player says I will wait until manager calls me  and others will wait until manager tells them to continue  Set     public class AutoResetEventSample       private AutoResetEvent autoReset   new AutoResetEvent false        public void RunAll                 new Thread Worker1  Start            new Thread Worker2  Start            new Thread Worker3  Start            autoReset Set            Thread Sleep 1000           autoReset Set            Console WriteLine  Main thread reached to end                public void Worker1                 Console WriteLine  Entered in worker 1            for  int i   0  i  lt  5  i                  Console WriteLine  Worker1 is running  0    i               Thread Sleep 2000               autoReset WaitOne                        public void Worker2                 Console WriteLine  Entered in worker 2             for  int i   0  i  lt  5  i                  Console WriteLine  Worker2 is running  0    i               Thread Sleep 2000               autoReset WaitOne                        public void Worker3                 Console WriteLine  Entered in worker 3             for  int i   0  i  lt  5  i                  Console WriteLine  Worker3 is running  0    i               Thread Sleep 2000               autoReset WaitOne                        In this example you can clearly see that when you first hit Set   it will let all threads go  then after 1 second it signals all threads to wait  As soon as you set them again regardless they are calling WaitOne   inside  they will keep running because you have to manually call Reset   to stop them all    manualReset Set    Thread Sleep 1000   manualReset Reset    Console WriteLine  Press to release all threads     Console ReadLine    manualReset Set      It is more about Referee Players relationship there regardless of any of the player is injured and wait for playing others will continue to work  If Referee says wait  Reset    then all players will wait until next signal   public class ManualResetEventSample       private ManualResetEvent manualReset   new ManualResetEvent false        public void RunAll                 new Thread Worker1  Start            new Thread Worker2  Start            new Thread Worker3  Start            manualReset Set            Thread Sleep 1000           manualReset Reset            Console WriteLine  Press to release all threads             Console ReadLine            manualReset Set            Console WriteLine  Main thread reached to end                public void Worker1                 Console WriteLine  Entered in worker 1            for  int i   0  i  lt  5  i                  Console WriteLine  Worker1 is running  0    i               Thread Sleep 2000               manualReset WaitOne                        public void Worker2                 Console WriteLine  Entered in worker 2             for  int i   0  i  lt  5  i                  Console WriteLine  Worker2 is running  0    i               Thread Sleep 2000               manualReset WaitOne                        public void Worker3                 Console WriteLine  Entered in worker 3             for  int i   0  i  lt  5  i                  Console WriteLine  Worker3 is running  0    i               Thread Sleep 2000               manualReset WaitOne

User · Answer

Just imagine that the AutoResetEvent executes WaitOne   and Reset   as a single atomic operation

[c#] What is the difference between ManualResetEvent and AutoResetEvent in .NET?

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