Multithreading in WPF

If you’re unfamiliar with multithreading, be sure to check out my previous entries on the topic.

In WPF, creating a thread is as easy as it is with C#. You can find an example on that here.  Alternatively, you could use the BackgroundWorker, which basically will create a thread and will give you a generalized, simplified interface in which to interact with it for a common threading task: doing an extra task in the background (such as downloading or progress bar updating).

In an earlier post, I used a mysterious method to enable responsiveness in the UI while loading a bunch of content (in that case, images).

This mystical object is called The Dispatcher.


No, this isn’t an edge-of-your-seat thrill ride movie that smacks explosions, swords, and alien guts into your M&M-filled mouth. It is an object used to manage the work for threads within WPF.  It maintains a queue of work items that are requested of any given thread, based on their order and priority.  This is the object you want to get to know if you’re going to be playing with your UI on a separate thread.

As mentioned previously, UI objects can’t be accessed outside of the threads that created them.  You can, however, use a separate thread to determine what changes you’ll be making and to what objects you will make them, then use their thread to actually apply that change.  In order to do this, use the object’s dispatcher to schedule the work on their queue.

For example, take a look at the code for loading images mentioned above:

private void LoadImage(string fname)
	// instantiate and initialize the image source
	BitmapImage bmi = new BitmapImage();
	bmi.UriSource = new Uri(fname, UriKind.Relative);
	bmi.Freeze();		// freeze the image source, used to move it across the thread
	// this method tells the separate thread to run the following method to run on the UI thread
	// the (ThreadStart)delegate(){ } notation is a shorthand for creating a method and a delegate for that method
	TheImage.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, (ThreadStart)delegate ()
		TheImage.Source = bmi;

This method creates the BitmampImage object on a separate thread, leaving the main thread free for user input, and freezes it so that it can be used on another thread.  It then uses TheImage’s Dispatcher to modify TheImage on its own thread by calling its dispatcher’s BeginInvoke method.

There are two ways to invoke using the dispatcher: BeginInvoke() and Invoke().  BeginInvoke() will queue the work for the dispatcher and continue the separate thread’s execution.  It puts in the request for the UI thread to execute the delegate, then continues on it merry way for its own execution.  This is useful when your separate thread does not rely on what it is requesting the UI thread to do.

The Invoke() method will wait until the delegate is executed and returns.  If you are modifying anything that you need or if the modification must be done before continuing in the separate thread, you should go with this one.

The Dispatcher is something you’ll get pretty cozy with if you plan on changing your UI elements from a separate thread.  If you’re just doing a progress bar or something else that is rather predictable, you can skip it by using the BackgroundWorker’s ReportProgress method and ProgressChanged event.  Just be sure to give it some time if you are calling the dispatcher often.

In case you didn’t notice all of my linking to previous posts, you may want to check out the rest of my posts on multithreading.

Introduction to Threads

Threading is a pretty core concept, but I felt it might be necessary to give it a brief explanation before moving on to some WPF-specific concerns for threads.  If you are already familiar with threads, these aren’t the droids you’re looking for.

If you think about the flow of a simple application that does not use multithreading, you’ll find that you can basically draw a line on a sheet of paper and label the events of the process as a straight line of consecutive actions:

simple_console_eventsThis method is often very stifling to the developer, as it limits what you can do.  At times, you want another action to be able to perform simultaneously.  A classic case is to allow a user to cancel an action.  In this case, the user is allowed to interact with the process while it is busy.  That is, the line of execution cannot be drawn so straight:

Threaded ExecutionYou can perform this (seemingly) concurrent operation by using threads.  Threads are similar to processes in that they are a set of instructions for the computer to perform; however, processes and threads differ in that separate threads share resources under the same process, while processes are independent of each other.  You can think of separate threads as workers who share materials and tools, but have separate instructions to perform at the same time.

Every application has at least one thread: the main thread of execution.  This is represented by the green line above; it is the thread that starts the application and is the top of the family tree of the cute little baby threads that are spawned during your application’s execution.  When you have an action that you wish to perform concurrently with the main thread, you create a new thread, give it a set of tasks, and start its execution.  In C#, simply instantiate a Thread object using a delegate and start it.

Thread worker = new Thread(delegate()
      // actions to perform on separate thread

A delegate is a reference type to a function that serves to give the thread an action to perform. So, if you think of a thread as a worker separate from the main thread, you can think of a delegate as that worker’s to-do list.

In C#, there is also a Delegate class. This class is used as a base for derived delegates – you can use this if you want to have a custom delegate, which is useful for defining to-do lists that have specific requirements. For example, events use custom delegates in order to ensure that the event handler has the proper information when it reacts to the event. So, if you want to create a MouseDown event handler, you must define a handler that accepts the sender of the event, as well as information regarding the mouse’s state.

Seems simple enough, right?  Well, things can actually get quite complicated.  I’ll address some complexities of multithreading in another post.