Threading in C Part 2. Threading in CJoseph Albahari. W5hXB.png' alt='Wpf Updating Ui From Background Thread' title='Wpf Updating Ui From Background Thread' />Last updated 2. Translations. Japanese. Download PDFPart 2 Basic Synchronization. So far, weve described how to start a task on a thread. Weve also described how. The next step is synchronization. Synchronization. is particularly important when threads access the same data its surprisingly. Synchronization constructs can be divided into four. Simple blocking methods. These wait for another thread to finish or for a period. Sleep, Join. and Task. Wait are simple blocking methods. Locking constructs. These limit the number of threads that can perform some. The architecture of WPF spans across both managed code and native code components. However, the public API exposed is only available via managed code. Exclusive locking. The standard exclusive locking constructs are lock Monitor. EnterMonitor. Exit, Mutex. Spin. Lock. The nonexclusive locking constructs. Hi, I have an ObservableCollection in my ViewModel that Im binding to the ItemsSource of a ComboBox. The problem Im having is in the GetDatabases method. Semaphore, Semaphore. Slim, and the readerwriter locks. Signaling constructs. These allow a thread to pause until receiving a. There are. two commonly used signaling devices event. Monitors WaitPulse. Framework 4. 0 introduces the Countdown. Event and Barrier classes. Nonblocking synchronization constructs. These protect access to a common field by calling upon. The CLR and C provide the following nonblocking. Thread. Memory. Barrier, Thread. Volatile. Read, Thread. Volatile. Write, the. Interlocked. class. Blocking is essential to all but the last category. Lets. briefly examine this concept. A thread is deemed blocked when its execution is. Sleeping or. waiting for another to end via Join or End. Invoke. A blocked thread immediately yields its processor time slice, and from then on. You can. test for a thread being blocked via its Thread. State property. bool blocked some. Thread. Thread. State Thread. State. Wait. Sleep. Join 0. More than the coolest LINQ tool. LINQPad is now the ultimate. C scratchpad Interactive development in a standalone executable Written by the author of this article. FREEGiven that a threads state may change in between testing. When a thread blocks or unblocks, the operating system. This incurs an. overhead of a few microseconds. Unblocking happens in one of four ways the computers. Thread. Interruptby being aborted via Thread. Abort. A thread is not deemed blocked if its execution is paused. Suspend method. Sometimes a thread must pause until a certain condition is. Signaling and locking. However, there is a simpler alternative a thread can await a. For. while proceed. Date. Time. Now lt next. Start. Time. In general, this is very wasteful on processor time as. CLR and operating system are concerned, the thread is performing an. Sometimes a hybrid between blocking and spinning is used. Thread. Sleep 1. Although inelegant, this is in general far more. Problems can arise, though, due to. Proper use. of locking and signaling. Spinning very briefly can be effective when you. The. NET. Framework provides special methods and classes to assist these are covered. You can query a threads execution status via its Thread. State property. This returns a flags enum of type Thread. State, which combines three layers of data in a. Most values, however, are redundant, unused, or deprecated. The following diagram shows one layer The following code strips a Thread. State. to one of the four most useful values Unstarted, Running, Wait. Sleep. Join, and Stopped. Thread. State Simple. Thread. State Thread. State ts. return ts Thread. State. Unstarted. Thread. State. Wait. Sleep. Join. Thread. State. Stopped. The Thread. State property is. Thread. State. and acting on that information. Exclusive locking is used to ensure that only one thread. The two main exclusive locking. Mutex. Of the two, the lock construct is faster and more. Mutex, though, has a niche in that its. In this section, well start with the lock. Mutex. and semaphores for nonexclusive locking. Later. well cover readerwriter locks. From Framework 4. Spin. Lock. struct for high concurrency scenarios. Lets start with the following class. Thread. Unsafe. static int val. Go. if val. 2 0 Console. Write. Line val. This class is not thread safe if Go. Console. Write. Line. Heres how lock can fix the. Thread. Safe. static readonly object locker new object. Go. lock locker. Console. Write. Line val. Only one thread can lock the synchronizing object in this. If more than. one thread contends the lock, they are queued on a ready queue and granted. Windows and the CLR mean that the fairness of the queue can. Exclusive locks are sometimes said to enforce serialized access to whatevers protected by the. In this. case, were protecting the logic inside the Go. A thread blocked while awaiting a contended lock has a Thread. Maschine The Hip Hop Beat Makers Missing Manual. State of Wait. Sleep. Join. In Interrupt. Abort, we describe how a blocked thread can be forcibly released via. This is a fairly heavy duty technique that might be used in. Cs lock statement is in fact. Monitor. Enter. and Monitor. Exit, with a tryfinally block. Heres a simplified version of whats. Go method of the. Monitor. Enter locker. Console. Write. Line val. Monitor. Exit locker. Calling Monitor. Exit without. Monitor. Enter on the same object. The code that we just demonstrated is exactly what the C. Theres a subtle vulnerability in this code, however. Consider the unlikely event of an exception being thrown within the implementation. Monitor. Enter, or between the call to Monitor. Enter and the try block. Abort being. called on that thread or an Out. Of. Memory. Exception. In such a scenario, the lock may or may not be taken. If the. lock is taken, it wont be released because well never enter the tryfinally block. This will. result in a leaked lock. To avoid this danger, CLR 4. Monitor. Enter. public static void Enter object obj, ref bool lock. Taken. lock. Taken is false after this. Enter method throws an. Heres the correct pattern of use which is exactly how C. Taken false. Monitor. Enter locker, ref lock. Taken. Do your stuff. Taken Monitor. Exit locker. Try. Enter. Monitor also provides a Try. Enter method that allows a timeout to be specified. Time. Span. The method. Try. Enter can also be called with no argument. As with the Enter method, its. CLR 4. 0 to accept a lock. Taken. argument. Any object visible to each of the partaking threads can be. The synchronizing object is typically private because this. The synchronizing object can double as the object its protecting, as. Thread. Safe. List lt string list new List lt string. Add Item 1. A field dedicated for the purpose of locking such as locker, in the example prior allows precise control over. The containing object this or even its type can also be used as a. Widget.     For protecting access to statics. The disadvantage of locking in this way is that youre not. A lock on a type may also seep through application domain boundaries within. You can also lock on local variables captured by lambda. Locking doesnt restrict access to the synchronizing object. In other words, x. To. String will. As a basic rule, you need to lock around accessing any. Even in the simplest case an assignment operation. In the following class. Increment nor the Assign. Thread. Unsafe. static int x. Increment x. Assign    x 1. Here are thread safe versions of Increment. Thread. Safe. static readonly object locker new object. Increment lock locker x. Assign    lock locker x 1. Synchronization, we explain how this need arises, and how the memory. Interlocked. class can provide alternatives to locking in these situations. If a group of variables are always read and written within. Lets suppose fields x and y are always read and. One can say x and y are accessed atomically, because the code block cannot. Youll never get a division by zero error, providing x and y are always accessed. The atomicity provided by a lock is violated if an. For. example, consider the following. Balance, check. Balance. Transfer decimal amount. Balance amount. Balance amount Get. Whats New in WPF Version 4. Thistopic contains information about new and enhanced features in Windows Presentation Foundation WPF version 4. This topic contains the following sections Ribbon control WPF 4. Ribbon control that hosts a Quick Access Toolbar, Application Menu, and tabs. For more information, see the Ribbon Overview. Improved performance when displaying large sets of grouped data UI virtualization occurs when a subset of user interface UI elements are generated from a larger number of data items based on which items are visible on the screen. The Virtualizing. Panel defines the Is. Virtualizing. When. Grouping attached property that enables UI Virtualization for grouped data. For more information about grouping data, see How to Sort and Group Data Using a View in XAML. For more information about virtualizing grouped data, see the Is. Virtualizing. When. Grouping attached property. New features for the Virtualizing. Panel. You can specify whether a Virtualizing. Panel, such as the Virtualizing. Stack. Panel, displays partial items by using the Scroll. Unit attached property. If Scroll. Unit is set to Item, the Virtualizing. Panel will only display items that are completely visible. If Scroll. Unit is set to Pixel, the Virtualizing. Panel can display partially visible items. You can specify the size of the cache before and after the viewport when the Virtualizing. Panel is virtualizing by using the Cache. Length attached property. The cache is the amount of space above or below the viewport in which items are not virtualized. Using a cache to avoid generating UI elements as theyre scrolled into view can improve performance. The cache is populated at a lower priority so that the application does not become unresponsive during the operation. The Cache. Length. Unit property determines the unit of measurement that is used by Cache. Length. Binding to static properties You can use static properties as the source of a data binding. The data binding engine recognizes when the propertys value changes if a static event is raised. For example, if the class Some. Class defines a static property called My. Property, Some. Class can define a static event that is raised when the value of My. Property changes. The static event can use either of the following signatures. Event. Handler My. Property. Changed public static event Event. Handlerlt Property. Changed. Event. Args Static. Property. Changed Note that in the first case, the class exposes a static event named Property. Name. Changed that passes Event. Args to the event handler. In the second case, the class exposes a static event named Static. Property. Changed that passes Property. Changed. Event. Args to the event handler. A class that implements the static property can choose to raise property change notifications using either method. Accessing collections on non UI Threads WPF enables you to access and modify data collections on threads other than the one that created the collection. This enables you to use a background thread to receive data from an external source, such as a database, and display the data on the UI thread. By using another thread to modify the collection, your user interface remains responsive to user interaction. Synchronously and Asynchronously validating data The INotify. Data. Error. Info interface enables data entity classes to implement custom validation rules and expose validation results asynchronously. This interface also supports custom error objects, multiple errors per property, cross property errors, and entity level errors. For more information, see INotify. Data. Error. Info. Automatically updating the source of a data binding If you use a data binding to update a data source, you can use the Delay property to specify an amount of time to pass after the property changes on the target before the source updates. For example, suppose that you have a Slider that has its Value property data two way bound to a property of a data object and the Update. Source. Trigger property is set to Property. Changed. In this example, when the user moves the Slider, the source updates for each pixel that the Slider moves. The source object typically needs the value of the slider only when the sliders Value stops changing. To prevent updating the source too often, use Delay to specify that the source should not be updated until a certain amount of time elapses after the thumb stops moving. Binding to types that Implement ICustom. Type. Provider WPF supports data binding to objects that implement ICustom. Type. Provider, also known as custom types. You can use custom types in the following cases. As a Property. Path in a data binding. For example, the Path property of a Binding can reference a property of a custom type. As the value of the Data. Type property. As a type that determines the automatically generated columns in a Data. Grid. Retrieving data binding information from a binding expression In certain cases, you might get the Binding. Expression of a Binding and need information about the source and target objects of the binding. New APIs have been added to enable you to get the source or target object or the associated property. When you have a Binding. Expression, use the following APIs to get information about the target and source. Checking for a valid Data. Context object There are cases where the Data. Context of an item container in an Items. Control becomes disconnected. An item container is the UI element that displays an item in an Items. Control. When an Items. Control is data bound to a collection, an item container is generated for each item. In some cases, item containers are removed from the visual tree. Two typical cases where an item container is removed are when an item is removed from the underlying collection and when virtualization is enabled on the Items. Control. In these cases, the Data. Context property of the item container will be set to the sentinel object that is returned by the Binding. Operations. Disconnected. Source static property. You should check whether the Data. Context is equal to the Disconnected. Source object before accessing the Data. Context of an item container. Repositioning data as the datas values change Live shaping A collection of data can be grouped, sorted, or filtered. WPF 4. 5 enables the data to be rearranged when the data is modified. For example, suppose that an application uses a Data. Grid to list stocks in a stock market and the stocks are sorted by stock value. If live sorting is enabled on the stocks Collection. View, a stocks position in the Data. Grid moves when the value of the stock becomes greater or less than another stocks value. For more information, see the ICollection. View. Live. Shaping interface. Improved Support for Establishing a Weak Reference to an Event Implementing the weak event pattern is now easier because subscribers to events can participate in it without implementing an extra interface. The generic Weak. Event. Manager class also enables subscribers to participate in the weak event pattern if a dedicated Weak. Event. Manager does not exist for a certain event. For more information, see Weak Event Patterns. New methods for the Dispatcher class The Dispatcher class defines new methods for synchronous and asynchronous operations. The synchronous Invoke method defines overloads that take an Action or Funclt TResult parameter. The new asynchronous method, Invoke. Async, also takes an Action or Funclt TResult as the callback parameter and returns a Dispatcher. Operation or Dispatcher. Operationlt TResult. The Dispatcher. Operation and Dispatcher. Operationlt TResult classes define a Task property. When you call Invoke. Async, you can use the await keyword with either the Dispatcher. Operation or the associated Task. If you need to wait synchronously for the Task that is returned by a Dispatcher. Operation or Dispatcher. Operationlt TResult, call the Dispatcher. Operation. Wait extension method.