How does lock work exactly

Question

I see that for using objects which are not thread safe we wrap the code with a lock like this   private static readonly Object obj   new Object     lock  obj           thread unsafe code     So what happens when multiple threads access the same code  let s assume that it is running in a ASP NET web application   Are they queued  If so how long will they wait    What is the performance impact because of using locks

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lock is actually hidden Monitor class

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The lock statement is translated to calls to the Enter and Exit methods of Monitor   The lock statement will wait indefinitely for the locking object to be released

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The lock statement is translated by C  3 0 to the following   var temp   obj   Monitor Enter temp    try          body   finally       Monitor Exit temp       In C  4 0 this has changed and it is now generated as follows   bool lockWasTaken   false  var temp   obj  try       Monitor Enter temp  ref lockWasTaken          body   finally       if  lockWasTaken                Monitor Exit temp              You can find more info about what Monitor Enter does here  To quote MSDN      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    The Monitor Enter method will wait infinitely  it will not time out

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Locks will block other threads from executing the code contained in the lock block   The threads will have to wait until the thread inside the lock block has completed and the lock is released   This does have a negative impact on performance in a multithreaded environment   If you do need to do this you should make sure the code within the lock block can process very quickly   You should try to avoid expensive activities like accessing a database etc

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Its simpler than you think   According to Microsoft  The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section  If another thread tries to enter a locked code  it will wait  block  until the object is released   The lock keyword calls Enter at the start of the block and Exit at the end of the block  lock keyword actually handles Monitor class at back end   For example   private static readonly Object obj   new Object     lock  obj           critical section     In the above code  first the thread enters a critical section  and then it will lock obj  When another thread tries to enter  it will also try to lock obj  which is already locked by the first thread  Second thread will have to wait for the first thread to release obj  When the first thread leaves  then another thread will lock obj and will enter the critical section

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According to Microsoft s MSDN  the lock is equivalent to  object   lockObj   x  bool   lockWasTaken   false  try       System Threading Monitor Enter   lockObj  ref   lockWasTaken          Your code      finally       if    lockWasTaken  System Threading Monitor Exit   lockObj      If you need to create locks in runtime  you can use open source DynaLock  You can create new locks in run-time and specify boundaries to the locks with context concept  DynaLock is open-source and source code is available at GitHub

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The performance impact depends on the way you lock  You can find a good list of optimizations here  http   www thinkingparallel com 2007 07 31 10-ways-to-reduce-lock-contention-in-threaded-programs   Basically you should try to lock as little as possible  since it puts your waiting code to sleep  If you have some heavy calculations or long lasting code  e g  file upload  in a lock it results in a huge performance loss

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No  they are not queued  they are sleeping  A lock statement of the form  lock  x         where x is an expression of a reference-type  is precisely equivalent to  var temp   x  System Threading Monitor Enter temp    try          finally   System Threading Monitor Exit temp       You just need to know that they are waiting to each other  and only one thread will enter to lock block  the others will wait     Monitor is written fully in  net so it is enough fast  also look at  class Monitor with reflector for more details

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The part within the lock statement can only be executed by one thread  so all other threads will wait indefinitely for it the thread holding the lock to finish  This can result in a so-called deadlock

[c#] How does lock work exactly?

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