Updating wpf ui from another thread. Threading Model.



Updating wpf ui from another thread

Updating wpf ui from another thread

Exit without first calling Monitor. Enter on the same object throws an exception. The lockTaken overloads The code that we just demonstrated is exactly what the C 1. Consider the unlikely event of an exception being thrown within the implementation of Monitor.

Enter, or between the call to Monitor. Enter and the try block due, perhaps, to Abort being called on that thread — or an OutOfMemoryException being thrown. In such a scenario, the lock may or may not be taken.

This will result in a leaked lock. To avoid this danger, CLR 4. The method then returns true if a lock was obtained, or false if no lock was obtained because the method timed out.

Choosing the Synchronization Object Any object visible to each of the partaking threads can be used as a synchronizing object, subject to one hard rule: The synchronizing object is typically private because this helps to encapsulate the locking logic and is typically an instance or static field.

Add "Item 1" ; The containing object this — or even its type — can also be used as a synchronization object: A lock on a type may also seep through application domain boundaries within the same process.

You can also lock on local variables captured by lambda expressions or anonymous methods. In other words, x. ToString will not block because another thread has called lock x ; both threads must call lock x in order for blocking to occur.

When to Lock As a basic rule, you need to lock around accessing any writable shared field. Even in the simplest case — an assignment operation on a single field — you must consider synchronization. In the following class, neither the Increment nor the Assign method is thread-safe: The atomicity provided by a lock is violated if an exception is thrown within a lock block. For example, consider the following: In this case, we could avoid the problem by calling GetBankFee earlier.

Instruction atomicity is a different, although analogous concept: A thread can repeatedly lock the same object in a nested reentrant fashion: Enter locker ; Monitor. Exit locker ; In these scenarios, the object is unlocked only when the outermost lock statement has exited — or a matching number of Monitor.

Exit statements have executed. Nested locking is useful when one method calls another within a lock: Deadlocks A deadlock happens when two threads each wait for a resource held by the other, so neither can proceed. The easiest way to illustrate this is with two locks: The CLR, in a standard hosting environment, is not like SQL Server and does not automatically detect and resolve deadlocks by terminating one of the offenders.

Under the SQL CLR integration host, however, deadlocks are automatically detected and a [catchable] exception is thrown on one of the threads. Deadlocking is one of the hardest problems in multithreading — especially when there are many interrelated objects. Fundamentally, the hard problem is that you can't be sure what locks your caller has taken out.

So, you might innocently lock private field a within your class x, unaware that your caller or caller's caller has already locked field b within class y. Meanwhile, another thread is doing the reverse — creating a deadlock.

Ironically, the problem is exacerbated by good object-oriented design patterns, because such patterns create call chains that are not determined until runtime. A better strategy is to be wary of locking around calling methods in objects that may have references back to your own object. Relying more on declarative and data parallelism , immutable types , and nonblocking synchronization constructs , can lessen the need for locking.

Here is an alternative way to perceive the problem: This is not a fault in the CLR or. NET Framework, but a fundamental limitation of locking in general. The problems of locking are being addressed in various research projects, including Software Transactional Memory.

Another deadlocking scenario arises when calling Dispatcher. Invoke in a WPF application or Control. Invoke in a Windows Forms application while in possession of a lock.

This can often be fixed simply by calling BeginInvoke instead of Invoke. Alternatively, you can release your lock before calling Invoke, although this won't work if your caller took out the lock. Performance Locking is fast: If it is contended, the consequential context switch moves the overhead closer to the microsecond region, although it may be longer before the thread is actually rescheduled.

Locking can degrade concurrency if locks are held for too long. This can also increase the chance of deadlock. A Mutex is like a C lock, but it can work across multiple processes. In other words, Mutex can be computer-wide as well as application-wide. Acquiring and releasing an uncontended Mutex takes a few microseconds — about 50 times slower than a lock. Closing or disposing a Mutex automatically releases it. Just as with the lock statement, a Mutex can be released only from the same thread that obtained it.

A common use for a cross-process Mutex is to ensure that only one instance of a program can run at a time. WriteLine "Another app instance is running.

Semaphore A semaphore is like a nightclub: Then, for each person that leaves, one person enters from the head of the queue. The constructor requires a minimum of two arguments: Any thread can call Release on a Semaphore, whereas with Mutex and lock, only the thread that obtained the lock can release it. There are two functionally similar versions of this class: The latter was introduced in Framework 4.

It cannot, however, be used for interprocess signaling. Semaphore incurs about 1 microsecond in calling WaitOne or Release; SemaphoreSlim incurs about a quarter of that. Semaphores can be useful in limiting concurrency — preventing too many threads from executing a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once: A Semaphore, if named, can span processes in the same way as a Mutex.

Thread Safety By the same author: A program or method is thread-safe if it has no indeterminacy in the face of any multithreading scenario. Thread safety is achieved primarily with locking and by reducing the possibilities for thread interaction. General-purpose types are rarely thread-safe in their entirety, for the following reasons: The development burden in full thread safety can be significant, particularly if a type has many fields each field is a potential for interaction in an arbitrarily multithreaded context.

Thread safety can entail a performance cost payable, in part, whether or not the type is actually used by multiple threads. A thread-safe type does not necessarily make the program using it thread-safe, and often the work involved in the latter makes the former redundant.

Thread safety is hence usually implemented just where it needs to be, in order to handle a specific multithreading scenario. One is to sacrifice granularity by wrapping large sections of code — even access to an entire object — within a single exclusive lock , enforcing serialized access at a high level. This tactic is, in fact, essential if you want to use thread-unsafe third-party code or most Framework types, for that matter in a multithreaded context.

The trick is simply to use the same exclusive lock to protect access to all properties, methods, and fields on the thread-unsafe object. Primitive types aside, few. NET Framework types, when instantiated, are thread-safe for anything more than concurrent read-only access. The onus is on the developer to superimpose thread safety, typically with exclusive locks. The collections in System. Concurrent are an exception.

Another way to cheat is to minimize thread interaction by minimizing shared data. Since multiple client requests can arrive simultaneously, the server methods they call must be thread-safe.

A stateless design popular for reasons of scalability intrinsically limits the possibility of interaction, since classes do not persist data between requests. Thread interaction is then limited just to the static fields one may choose to create, for such purposes as caching commonly used data in memory and in providing infrastructure services such as authentication and auditing. The final approach in implementing thread safety is to use an automatic locking regime.

Whenever a method or property on such an object is then called, an object-wide lock is automatically taken for the whole execution of the method or property. Although this reduces the thread-safety burden, it creates problems of its own: For these reasons, manual locking is generally a better option — at least until a less simplistic automatic locking regime becomes available.

A good application of this is the. If we had two interrelated lists, we would have to choose a common object upon which to lock we could nominate one of the lists, or better: NET collections is also thread-unsafe in the sense that an exception is thrown if the list is modified during enumeration.

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Dispatcher in WPF



Updating wpf ui from another thread

Exit without first calling Monitor. Enter on the same object throws an exception. The lockTaken overloads The code that we just demonstrated is exactly what the C 1. Consider the unlikely event of an exception being thrown within the implementation of Monitor. Enter, or between the call to Monitor. Enter and the try block due, perhaps, to Abort being called on that thread — or an OutOfMemoryException being thrown.

In such a scenario, the lock may or may not be taken. This will result in a leaked lock. To avoid this danger, CLR 4. The method then returns true if a lock was obtained, or false if no lock was obtained because the method timed out. Choosing the Synchronization Object Any object visible to each of the partaking threads can be used as a synchronizing object, subject to one hard rule: The synchronizing object is typically private because this helps to encapsulate the locking logic and is typically an instance or static field.

Add "Item 1" ; The containing object this — or even its type — can also be used as a synchronization object: A lock on a type may also seep through application domain boundaries within the same process. You can also lock on local variables captured by lambda expressions or anonymous methods. In other words, x. ToString will not block because another thread has called lock x ; both threads must call lock x in order for blocking to occur.

When to Lock As a basic rule, you need to lock around accessing any writable shared field. Even in the simplest case — an assignment operation on a single field — you must consider synchronization. In the following class, neither the Increment nor the Assign method is thread-safe: The atomicity provided by a lock is violated if an exception is thrown within a lock block.

For example, consider the following: In this case, we could avoid the problem by calling GetBankFee earlier. Instruction atomicity is a different, although analogous concept: A thread can repeatedly lock the same object in a nested reentrant fashion: Enter locker ; Monitor.

Exit locker ; In these scenarios, the object is unlocked only when the outermost lock statement has exited — or a matching number of Monitor. Exit statements have executed. Nested locking is useful when one method calls another within a lock: Deadlocks A deadlock happens when two threads each wait for a resource held by the other, so neither can proceed. The easiest way to illustrate this is with two locks: The CLR, in a standard hosting environment, is not like SQL Server and does not automatically detect and resolve deadlocks by terminating one of the offenders.

Under the SQL CLR integration host, however, deadlocks are automatically detected and a [catchable] exception is thrown on one of the threads. Deadlocking is one of the hardest problems in multithreading — especially when there are many interrelated objects. Fundamentally, the hard problem is that you can't be sure what locks your caller has taken out. So, you might innocently lock private field a within your class x, unaware that your caller or caller's caller has already locked field b within class y.

Meanwhile, another thread is doing the reverse — creating a deadlock. Ironically, the problem is exacerbated by good object-oriented design patterns, because such patterns create call chains that are not determined until runtime.

A better strategy is to be wary of locking around calling methods in objects that may have references back to your own object.

Relying more on declarative and data parallelism , immutable types , and nonblocking synchronization constructs , can lessen the need for locking. Here is an alternative way to perceive the problem: This is not a fault in the CLR or. NET Framework, but a fundamental limitation of locking in general. The problems of locking are being addressed in various research projects, including Software Transactional Memory. Another deadlocking scenario arises when calling Dispatcher.

Invoke in a WPF application or Control. Invoke in a Windows Forms application while in possession of a lock.

This can often be fixed simply by calling BeginInvoke instead of Invoke. Alternatively, you can release your lock before calling Invoke, although this won't work if your caller took out the lock. Performance Locking is fast: If it is contended, the consequential context switch moves the overhead closer to the microsecond region, although it may be longer before the thread is actually rescheduled.

Locking can degrade concurrency if locks are held for too long. This can also increase the chance of deadlock. A Mutex is like a C lock, but it can work across multiple processes. In other words, Mutex can be computer-wide as well as application-wide. Acquiring and releasing an uncontended Mutex takes a few microseconds — about 50 times slower than a lock. Closing or disposing a Mutex automatically releases it.

Just as with the lock statement, a Mutex can be released only from the same thread that obtained it. A common use for a cross-process Mutex is to ensure that only one instance of a program can run at a time. WriteLine "Another app instance is running. Semaphore A semaphore is like a nightclub: Then, for each person that leaves, one person enters from the head of the queue. The constructor requires a minimum of two arguments: Any thread can call Release on a Semaphore, whereas with Mutex and lock, only the thread that obtained the lock can release it.

There are two functionally similar versions of this class: The latter was introduced in Framework 4. It cannot, however, be used for interprocess signaling. Semaphore incurs about 1 microsecond in calling WaitOne or Release; SemaphoreSlim incurs about a quarter of that.

Semaphores can be useful in limiting concurrency — preventing too many threads from executing a particular piece of code at once. In the following example, five threads try to enter a nightclub that allows only three threads in at once: A Semaphore, if named, can span processes in the same way as a Mutex.

Thread Safety By the same author: A program or method is thread-safe if it has no indeterminacy in the face of any multithreading scenario. Thread safety is achieved primarily with locking and by reducing the possibilities for thread interaction. General-purpose types are rarely thread-safe in their entirety, for the following reasons: The development burden in full thread safety can be significant, particularly if a type has many fields each field is a potential for interaction in an arbitrarily multithreaded context.

Thread safety can entail a performance cost payable, in part, whether or not the type is actually used by multiple threads. A thread-safe type does not necessarily make the program using it thread-safe, and often the work involved in the latter makes the former redundant. Thread safety is hence usually implemented just where it needs to be, in order to handle a specific multithreading scenario. One is to sacrifice granularity by wrapping large sections of code — even access to an entire object — within a single exclusive lock , enforcing serialized access at a high level.

This tactic is, in fact, essential if you want to use thread-unsafe third-party code or most Framework types, for that matter in a multithreaded context. The trick is simply to use the same exclusive lock to protect access to all properties, methods, and fields on the thread-unsafe object. Primitive types aside, few. NET Framework types, when instantiated, are thread-safe for anything more than concurrent read-only access. The onus is on the developer to superimpose thread safety, typically with exclusive locks.

The collections in System. Concurrent are an exception. Another way to cheat is to minimize thread interaction by minimizing shared data. Since multiple client requests can arrive simultaneously, the server methods they call must be thread-safe.

A stateless design popular for reasons of scalability intrinsically limits the possibility of interaction, since classes do not persist data between requests. Thread interaction is then limited just to the static fields one may choose to create, for such purposes as caching commonly used data in memory and in providing infrastructure services such as authentication and auditing.

The final approach in implementing thread safety is to use an automatic locking regime. Whenever a method or property on such an object is then called, an object-wide lock is automatically taken for the whole execution of the method or property. Although this reduces the thread-safety burden, it creates problems of its own: For these reasons, manual locking is generally a better option — at least until a less simplistic automatic locking regime becomes available.

A good application of this is the. If we had two interrelated lists, we would have to choose a common object upon which to lock we could nominate one of the lists, or better: NET collections is also thread-unsafe in the sense that an exception is thrown if the list is modified during enumeration.

Updating wpf ui from another thread

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1 Comments

  1. The trick is simply to use the same exclusive lock to protect access to all properties, methods, and fields on the thread-unsafe object. If only one thread can modify the UI, how do background threads interact with the user? Thread safety in static methods is something that you must explicitly code:

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