Develop Reference

Why is "InvokeOnAppThread" used?

(The question is following but please read the info below as well)
According to the documentation InvokeOnAppThread executes a callback item on application thread.
Browsing more into the documentation , it seems there is the "UI Thread" and the "application thread".
However the link they offered for more information on this, is not there.
So, is there by default two threads on any Unity program? Can someone explain to me about InvokeOnAppThread? Why and when it should be used?
(I have already searched and looked on pages dealing with threading in Unity. It is clear there is a UI Thread by default and it seems multithreading is not recommended since other threads are not allowed access to the UI Thread. As I understand this can be solved by using callbacks)- so I have already done research on this
The "Application thread" mentioned in the docs is not mentioned anywhere else
Just to avoid duplicates I have already searched in SO if there is already an answer to this. The search output 0 results, so it is clear to the best of my efforts this is not a duplicate question

It looks like this is the page that should be reached from the broken link.
From that page:
Now, let’s take a closer look at AppCallbacks class. When you create
it, you specify that your game will run on different thread, for
backward compatibility reasons you can also specify that your
application can run on UI thread, but that’s not recommended, because
there’s a restriction from Microsoft - if your application won’t
become responsive after 5 seconds you’ll fail to pass WACK (Windows
Application Certification), read more here -
http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh184840(v=vs.105).aspx,
imagine if your first level is pretty big, it might take significant
amount of time to load it, because your application is running on UI
thread, UI will be unresponsive until your level is fully loaded.
That’s why it’s recommend to always run your game on different thread.
Essentially the 'UI thread' is the main thread from the point of view of the WinRT/UWP runtime. In order not to block that thread, all Unity-specific code - MonoBehaviour scripts, coroutines, etc - runs on a separate thread, the 'application thread,' which is the main thread from the point of view of the Unity engine.
You would use InvokeOnUIThread if you were on Unity's main thread and you wanted to do something on the Windows UI thread (eg, create a native pop-up). You'd call InvokeOnAppThread if you were on the UI thread and wanted to marshal back to the Unity main thread (eg, start a coroutine, instantiate a GameObject).

I'm not familiar with Unity, but this is a typical requirement with WinForms development in whatever framework you are using.
You can find detailed description of the issue in the Control.InvokeRequired property documentation and also in this MSDN page.
What the explanations say is that events for a control should only be handled by the thread that created them because otherwise they are not thread safe. This is a little different than the guidance I have understood in the past - your main UI thread is just listening for events and handing them off to a ThreadPool for processing so that user responsiveness (ie. responding to mouse/keyboard events) never halts because the app is doing something.
The Control.InvokeRequired check is supposed to tell you if you are on the UI thread so that you can queue another event that should execute on a different thread so the main UI handling thread does not block. The implementation details may have changed somewhat since I last worked with this, though.

Processing Windows Messages in blocked UI Thread?

This is possibly related to ProgressBar updates in blocked UI thread but is a little different.
While troubleshooting a crashing WinForms control (a DevExpress treelist) a member of our team encountered an unusual situation, and I'm wondering if anyone can help us understand what's going on. English is not his first language, so I'm posting on his behalf.
Please see this screenshot from Visual Studio 2005.
Note the following points:
The main UI thread is stopped and is currently in a DevExpress control draw method.
The code shown on screen is from a point earlier in the same call-stack. This code is in the data layer and was called in response to the control's request for an image to display for the tree node. (perhaps also originating from a Paint handler)
The displayed code is from earlier in the callstack, on the main UI thread, and is currently waiting on a lock! Since remote systems can send events which are processed on background threads in the data model (i.e., data models are sync'd between client and servers), we lock to keep the data collections thread safe.
As the callstack shows, we continued to process paint messages on the UI thread, while we would expect the thread to be blocked.
This is very difficult to replicate, and I have not been able to do so using a simpler test project on my own box. When this situation arises, however, the result is that the DevExpress control's internal state can be messed up, causing the control to crash. This doesn't really seem like a bug in the control, since it was no doubt written with the assumption that these paint methods are running only on the UI thread. What we see here makes it look like the UI thread is acting like two threads.
It would seem possible that this is merely a Visual Studio bug in the presentation of the callstack, except that this whole endeavor is resulting from an effort to troubleshoot the occasional crash of the control in the released app (in which case it shows as a big red X in the UI), so it seems the problem is not isolated to the debug environment.
Alright, that was complicated, but hopefully made sense. Any ideas?

I would strongly recommend against locking the UI to wait for background processing. Consider something like multiple buffering. You can probably get this behavior fairly easily by utilizing thread-safe collections in .NET 4, but if that's not an option there are versions of those in the Parallel Extensions released prior to v4.

What about altering the synchronization scheme so that you don't need to acquire exclusive locks to read data?
In situations where you can be sure that a read will always produce consistent data even when it happens while the data is also being written, you might be able to get away with having no lock statements for getters. Otherwise there's ReaderWriterLockSlim, which permits multiple concurrent readers but still allows you to stop the presses for write operations.
It doesn't fix everything, but it does at least reduce the number of opportunities for deadlocks.

We see something similar in our project. The stack trace looks like the pump's event loop is called while the UI thread is waiting on a lock. This could happen if Monitor.enter has some special behavior when called on the UI thread. I believe this is what's happening, but I haven't found any documentation to back it up yet.
Probably has something to do with synchronization contexts :)

Find which module is blocking the UI thread

This is a two part question:
I am working on a big project where multiple plugins developed by different teams are loaded inside one common container shell. At times I can see that my UI updates are blocked as there are multiple parallel UI updates, i want to know if there is a way to find which component is blocking the ui thread
In .net how can i create a separate UI thread which requires dedicated UI intensive work?
Much Appreciate your help. Thanks.

Use the debugger. Debug + Break All when you notice it blocking. Then Debug + Windows + Threads and select the main thread. The call stack window shows you what it is doing.
A corner case is where these plugins are using a lot of calls to Control.Begin/Invoke or Dispatcher.Begin/Invoke. Your UI thread is not blocked in this case, it is just being overwhelmed by requests to dispatch the delegate targets. And doesn't get around to doing its normal duties anymore, like repainting the windows and responding mouse and keyboard events. There's little you can do about this beyond working with the plugin authors to get them to mend their ways.
You've already got an UI thread, the thread that created the first window. Creating additional threads that have their own windows is possible but causes unsolvable problems with window Z-order (a window will disappear underneath the window of another app) and generous helpings of window interop threading misery.

Visual Studio 2010's (in the higher SKUs) include features to check for this. If you run your program under the Concurrency Profiler, you can see exactly which threads are waiting on which locks when the deadlock occurs. In addition, it will highlight the deadlock (I believe in bright red) to make it easy to track down.

One approach you can take (though it may require a bit of redesign) is to disallow all plugin logic from running in the UI thread. All operations that require updates to the UI must be routed through well-defined service interfaces that can interpret, dispatch and perhaps even throttle the UI updates. This is only practical if your plugins are not deeply UI-centric and you have a service model that allows you to isolate the data being manipulated by the plugins from the visualization of that data. Without knowing more about your application, I can't give more concrete recommendations.

Here are two possible solutions to the problem that I came up with quickly. I am sure there are other equally valid solutions though.
Option 1: Instead of using the push model (via the ISynchronizeInvoke methods) switch to a pull (or poll) model in which the UI queries the plugin for updates. This has the following advantages.
It breaks the tight coupling between the UI and worker/plugin threads that Control.Invoke imposes.
It puts the responsibility of updating the UI thread on the UI thread where it should belong anyway.
The UI thread gets to dictate when and how often the update should take place.
There is no risk of the UI message pump being overrun as would be the case with the marshaling techniques initiated by the worker/plugin thread.
The worker/plugin thread does not have to wait for an acknowledgement that the update was performed before proceeding with its next steps (ie. you get more throughput on both the UI and worker/plugin threads).
Option 2: Have the plugin accept an ISynchronizeInvoke instance instead of an actual Form or Control. This special synchronizing object will be implemented using a dedicated thread and a queue that acts as buffer between the plugin and the UI. It will accept update messages via the normal Invoke or BeginInvoke methods, which means you can keep the plugin architecture and interfaces mostly intact, and then forward those messages on to the UI after some type of filtering, merging, and throttling operations have occurred. The number of update messages existing in the synchronizing object will ebb and flow as the UI and plugin threads work load changes. It could be smart enough to change its forwarding strategy as the rate of messages increase.

Error handling patterns for multithreaded apps using WF?

I was writing up a long, detailed question, but just scrapped it in favor of a simpler question that I didn't find an answer to here.
Brief app description:
I have a WPF app that spawns several threads, and each thread executes its own WF. What are some of the best ways to handle errors in the threads and WF that will allow user interaction from the GUI side? I definitely plan to handle any low level exceptions in the thread, because I don't want the thread to exit.
Summary of questions:
How have you implemented communication between WF and the thread that starts it? There is WorkflowTerminated, but I don't want the workflow to exit -- I need to fix the problem and let it continue. I assume the only option is using a FaultHandler, but was wondering if there's another way to do it without using an activity block. I am hoping there's a framework out there that I just haven't found yet.
The error from WF needs to get caught by the thread, which then needs to display the error in the GUI. The user will then make a logical choice for recovery, which should then be sent back to the thread, and then to WF. Again, is there something existing out there that I should take a look at?
Even buzzwords / keywords that accomplish what I am describing would be really helpful, and I can do the legwork on researching each of them. However, any additional insight is always welcome. :)

What's worked for me in multi-threaded WPF apps is to have the errant thread invoke a callback method that passes the exception and other info back to the UI thread. Callbacks can have return values, so if your thread can block while waiting for the user to respond, then that can work for you. Remember that the callback will run on the thread that calls it, so any UI updates have to be done via the control's dispatcher. You will have to decide whether all of the threads use the same callback and what kind of synchronization you'll need if there's a chance that multiple threads can throw exceptions simultaneously.

Here's how I ended up solving this problem. But first a little background info:
User clicks a button in the GUI that causes the candy packager to start running. This is done via a command binding in the ViewModel, which then calls a low-level function in the Model. The function in the model launches a thread and executes a state machine.
At some point, the machine will fail. When it does, I compile information about the error and possible (known) recovery methods. I put this into an object and then pass it to the GUI via a callback interface. In the meantime, the worker thread is stuck waiting for an Event to get set.
Eventually, the candy worker will notice the error and will click a button telling the system what to do. This results in two things: 1) it flags one of the recovery methods as the preferred one, and 2) sets the event. Now the worker thread continues on, checks for the preferred error recovery method and transitions into the respective state in the state machine.
This works very well (so far). The part I know is totally lame is the manner in which it checks for the preferred error recovery method. I am essentially setting a string variable, and then comparing this string to a list of known strings. Ultra lame, but I'm not sure of a better way to do this, other than using an enum. Does anyone have recommendations for me?

Multithreading: how to update UI to indicate progress

I've been working on the same project now since Christmas 2008. I've been asked to take it from a Console Application (which just prints out trace statements), to a full Windows App. Sure, that's fine. The only thing is there are parts of the App that can take several minutes to almost an hour to run. I need to multithread it to show the user status, or errors. But I have no idea where to begin.
I've aready built a little UI in WPF. It's very basic, but I'd like to expand it as I need to. The app works by selecting a source, choosing a destination, and clicking start. I would like a listbox to update as the process goes along. Much in the same way SQL Server Installs, each step has a green check mark by its name as it completes.
How does a newbie start multithreading? What libraries should I check out? Any advice would be greatly appreciated.
p.s. I'm currently reading about this library, http://www.codeplex.com/smartthreadpool
#Martin: Here is how my app is constructed:
Engine: Runs all major components in pre-defined order
Excel: Library I wrote to wrap COM to open/read/close/save Workbooks
Library: Library which understands different types of workbook formats (5 total)
Business Classes: Classes I've written to translate Excel data and prep it for Access
Db Library: A Library I've written which uses ADO.NET to read in Access data
AppSettings: you get the idea
Serialier: Save data in-case of app crash
I use everything from LINQ to ADO.NET to get data, transform it, and then output it.
My main requirement is that I want to update my UI to indicate progress
#Frank: What happens if something in the Background Worker throws an Exception (handled or otherwise)? How does my application recieve notice?
#Eric Lippert: Yes, I'm investigating that right now. Before I complicate things.
Let me know if you need more info. Currently I've running this application from a Unit Test, so I guess callig it a Console Application isn't true. I use Resharper to do this. I'm the only person right now who uses the app, but I'd like a more attractive interface

I don't think you specify the version of the CLR you are using, but you might check out the "BackgroundWorker" control. It is a simple way to implemented multiple threads.
The best part, is that it is a part of the CLR 2.0 and up
Update in response to your update: If you want to be able to update the progress in the UI -- for example in a progress bar -- the background worker is perfect. It uses an event that I think is called: ProgressChanged to report the status. It is very elegant. Also, keep in mind that you can have as many instances that you need and can execute all the instances at the same time (if needed).
In response to your question: You could easily setup an example project and test for your question. I did find the following, here (under remarks, 2nd paragraph from the caution):
If the operation raises an exception
that your code does not handle, the
BackgroundWorker catches the exception
and passes it into the
RunWorkerCompleted event handler,
where it is exposed as the Error
property of
System.ComponentModel..::.RunWorkerCompletedEventArgs.

Threading in C# from Joseph Albahari is quite good.

This page is quite a good summary of threading.
By the sound of it you probably don't need anything very complex - if you just start the task and then want to know when it has finished, you only need a few lines of code to create a new thread and get it to run your task. Then your UI thread can bumble along and check periodically if the task has completed.

Concurrent Programming on Windows is THE best book in the existence on the subject. Written by Joe Duffy, famous Microsoft Guru of multithreading. Everything you ever need to know and more, from the way Windows thread scheduler works to .NET Parallels Extensions Library.

Remember to create your delegates to update the UI so you don't get cross-threading issues and the UI doesn't appear to freeze/lockup
Also if you need a lot of notes/power points/etc etc
Might I suggest all the lecture notes from my undergrad
http://ist.psu.edu/courses/SP04/ist411/lectures.html

The best way for a total newcomer to threading is probably the threadpool. We'll probably need to know a little more about these parts to make more in depth recommendations
EDIT::
Since we now have a little more info, I'm going to stick with my previous answer, it looks like you have a loads of tasks which need doing, the best way to do a load of tasks is to add them to the threadpool and then just keep checking if they're done, if tasks need to be done in a specific order then you can simply add the next one as the previous one finishes. The threadpool really is rather good for this kind of thing and I see no reason not to use it in this case

Jason's link is a good article. Things you need to be aware of are that the UI can only be updated by the main UI thread, you will get cross threading exceptions if you try to do it in the worker thread. The BackgroundWorker control can help you there with the events, but you should also know about Control.Invoke (or Control.Begin/EndInvoke). This can be used to execute delegates in the context of the UI thread.
Also you should read up on the gotchas of accessing the same code/variables from different threads, some of these issues can lead to bugs that are intermittent and tricky to track down.
One point to note is that the volatile keyword only guarantees 'freshness' of variable access, for example, it guarantees that each read and write of the variable will be from main memory, and not from a thread or processor cache or other 'feature' of the memory model. It doesnt stop issues like a thread being interrupted by another thread during its read-update-write process (e.g. changing the variables value). This causes errors where the 2 threads have different (or the same) values for the variable, and can lead to things like values being lost, 2 threads having the same value for the variable when they should have different values, etc. You should use a lock/monitor (or other thread sync method, wait handles, interlockedincrement/decrement etc) to prevent these types of problems, which guarantee only one thread can access the variable. (Monitor also has the advantage that it implicitly performs volatile read/write)
And as someone else has noted, you also should try to avoid blocking your UI thread whilst waiting for background threads to complete, otherwise your UI will become unresponsive. You can do this by having your worker threads raise events that your UI subscribes to that indicate progress or completion.
Matt

Typemock have a new tool called Racer for helping with Multithreading issues. It’s a bit advanced but you can get help on their forum and in other online forums (one that strangely comes to mind is stackoverflow :-) )

I'm a newbie to multithreading as well, but I agree with Frank that a background worker is probably your best options. It works through event subscriptions. Here's the basics of how you used it.
First Instantiate a new background worker
Subscribed methods in your code to the background workers major events:
DoWork: This should contain whatever code that takes a long time to process
ProgressChanged: This is envoked whenever you call ReportProgress() from inside the method subscribed to DoWork
RunWorkerCompleted: Envoked when the DoWork method has completed
When you are ready to run your time consuming process you call the RunAsync() method of the background worker. This starts DoWork method on a separate thread, which can then report it's progress back through the ProgressChanged event. Once it completed RunWorkerComplete will be evoked.
The DoWork event method can also check if the user somehow requested that the process be canceled (CanceLAsync() was called)) by checking the value of the CancelPending property.