How to get rid of Dispatcher.Invoke

• Download ContextSwitching.zip - 8.7 KB

Introduction

After struggling with switching context between threads using Dispatcher I'm happy to be able to convey the following solution using a Task instead.  

Using the code

I wrap the task in an execute method that takes an Action.  

//
// private method to switch context.
//
private void Execute(Action action)
{
  try
  {
    // Invoke action on UI thread
    var task = Task.Factory.StartNew(() =>
               {
                 if (action != null)
                    action();
               }, CancellationToken.None, TaskCreationOptions.None, taskScheduler);
    if (task.IsFaulted)
      throw new AggregateException(task.Exception);
  }
  catch (Exception e)
  {
    if (Debugger.IsAttached) Debugger.Break();
    // Handle exception here.
  }
} 

To use the method you need to set the task scheduler within the UI thread and the SynchronizationContext must be set. 

// Must be set on the UI thread
taskScheduler = TaskScheduler.FromCurrentSynchronizationContext();  

// Example 
// Invoke event LogonStatusChanged on the UI thread.
private void InvokeLoginStatusChanged(Object sender, EventArgs e)
{
  if (LogonStatusChanged == null) return;
  Execute(() => LogonStatusChanged(this, e));
}  

Points of Interest

Using a Task  to switch context between threads is tricky when it comes to Unit tests. In those cases you can create you own Synchronization context using the following code.

// Creating you own Synchronization context
[TestInitialize]
private void TestSetup()
{
  SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
}  

Note that the Synchronization Context is not created right away when you start your application.
If you reference TaskScheduler.FromCurrentSynchronizationContext() in the App.cs constructor it will always be null. 

In stead move any initialization to OnStartup. 

protected override void OnStartup(StartupEventArgs e)
{
  // Initialize here
  base.OnStartup(e);
}  

History 

2013-12-6: The initial version.  
2013-12-8: Added an example.