Could you explain STA and MTA

Question

Can you explain STA and MTA in your own words   Also  what are apartment threads and do they pertain only to COM  If so  why

User · Accepted Answer

The COM threading model is called an  apartment  model  where the execution context of initialized COM objects is associated with either a single thread  Single Thread Apartment  or many threads  Multi Thread Apartment   In this model  a COM object  once initialized in an apartment  is part of that apartment for the duration of its runtime   The STA model is used for COM objects that are not thread safe  That means they do not handle their own synchronization  A common use of this is a UI component  So if another thread needs to interact with the object  such as pushing a button in a form  then the message is marshalled onto the STA thread  The windows forms message pumping system is an example of this   If the COM object can handle its own synchronization then the MTA model can be used where multiple threads are allowed to interact with the object without marshalled calls

User · Answer

I find the existing explanations too gobbledygook  Here s my explanation in plain English   STA  If a thread creates a COM object that s set to STA  when calling CoCreateXXX you can pass a flag that sets the COM object to STA mode   then only this thread can access this COM object  that s what STA means - Single Threaded Apartment   other thread trying to call methods on this COM object is under the hood silently turned into delivering messages to the thread that creates owns  the COM object  This is very much like the fact that only the thread that created a UI control can access it directly  And this mechanism is meant to prevent complicated lock unlock operations   MTA  If a thread creates a COM object that s set to MTA  then pretty much every thread can directly call methods on it   That s pretty much the gist of it  Although technically there re some details I didn t mention  such as in the  STA  paragraph  the creator thread must itself be STA  But this is pretty much all you have to know to understand STA MTA NA

User · Answer

Each EXE which hosts COM or OLE controls defines it s apartment state   The apartment state is by default STA  and for most programs should be STA    STA - All OLE controls by necessity must live in a STA   STA means that your COM-object must be always manipulated on the UI thread and cannot be passed to other threads  much like any UI element in MFC    However  your program can still have many threads   MTA - You can manipulate the COM object on any thread in your program

User · Answer

As my understanding  the  Apartment  is used to protect the COM objects from multi-threading issues   If a COM object is not thread-safe  it should declare it as a STA object  Then only the thread who creates it can access it  The creation thread should declare itself as a STA thread  Under the hood  the thread stores the STA information in its TLS Thread Local Storage   We call this behavior as that the thread enters a STA apartment  When other threads want to access this COM object  it should marshal the access to the creation thread  Basically  the creation thread uses messages mechanism to process the in-bound calls   If a COM object is thread-safe  it should declare it as a MTA object  The MTA object can be accessed by multi-threads

User · Answer

Code that calls COM object dlls  for example  to read proprietary data files   may work fine in a user interface but hang mysteriously from a service   The reason is that as of  Net 2 0 user interfaces assume STA  thread-safe  while services assume MTA   before that  services assumed STA    Having to create an STA thread for every COM call in a service can add significant overhead

User · Answer

STA  Single Threaded Apartment  is basically the concept that only one thread will interact with your code at a time   Calls into your apartment are marshaled via windows messages  using a non-visible  window   This allows calls to be queued and wait for operations to complete   MTA  Multi Threaded Apartment  is where many threads can all operate at the same time and the onus is on you as the developer to handle the thread security   There is a lot more to learn about threading models in COM  but if you are having trouble understanding what they are then I would say that understanding what the STA is and how it works would be the best starting place because most COM objects are STA   s   Apartment Threads  if a thread lives in the same apartment as the object it is using then it is an apartment thread   I think this is only a COM concept because it is only a way of talking about the objects and threads they interact with

User · Answer

It s all down to how calls to objects are handled  and how much protection they need  COM objects can ask the runtime to protect them against being called by multiple threads at the same time  those that don t can potentially be called concurrently from different threads  so they have to protect their own data   In addition  it s also necessary for the runtime to prevent a COM object call from blocking the user interface  if a call is made from a user interface thread   An apartment is a place for objects to live  and they contain one or more threads  The apartment defines what happens when calls are made  Calls to objects in an apartment will be received and processed on any thread in that apartment  with the exception that a call by a thread already in the right apartment is processed by itself  i e  a direct call to the object    Threads can be either in a Single-Threaded Apartment  in which case they are the only thread in that apartment  or in a Multi-Threaded Apartment  They specify which when the thread initializes COM for that thread   The STA is primarily for compatibility with the user interface  which is tied to a specific thread  An STA receives notifications of calls to process by receiving a window message to a hidden window  when it makes an outbound call  it starts a modal message loop to prevent other window messages being processed  You can specify a message filter to be called  so that your application can respond to other messages   By contrast all MTA threads share a single MTA for the process  COM may start a new worker thread to handle an incoming call if no threads are available  up to a pool limit  Threads making outbound calls simply block   For simplicity we ll consider only objects implemented in DLLs  which advertise in the registry what they support  by setting the ThreadingModel value for their class s key  There are four options    Main thread  ThreadingModel value not present   The object is created on the host s main UI thread  and all calls are marshalled to that thread  The class factory will only be called on that thread  Apartment  This indicates that the class can run on any single-threaded-mode thread  If the thread that creates it is an STA thread  the object will run on that thread  otherwise it will be created in the main STA - if no main STA exists  an STA thread will be created for it   This means MTA threads that create Apartment objects will be marshalling all calls to a different thread   The class factory can be called concurrently by multiple STA threads so it must protect its internal data against this  Free  This indicates a class designed to run in the MTA  It will always load in the MTA  even if created by an STA thread  which again means the STA thread s calls will be marshalled  This is because a Free object is generally written with the expectation that it can block  Both  These classes are flexible and load in whichever apartment they re created from  They must be written to fit both sets of requirements  however  they must protect their internal state against concurrent calls  in case they re loaded in the MTA  but must not block  in case they re loaded in an STA    From the  NET Framework  basically just use  STAThread  on any thread that creates UI  Worker threads should use the MTA  unless they re going to use Apartment-marked COM components  in which case use the STA to avoid marshalling overhead and scalability problems if the same component is called from multiple threads  as each thread will have to wait for the component in turn   It s much easier all around if you use a separate COM object per thread  whether the component is in the STA or MTA

[.net] Could you explain STA and MTA?

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