Synchronization Contexts

Execute code on the UI thread after performing background work

This example shows how to update a UI component from a background thread by using a SynchronizationContext

void Button_Click(object sender, EventArgs args)
{
    SynchronizationContext context = SynchronizationContext.Current;
    Task.Run(() =>
    {
        for(int i = 0; i < 10; i++) 
        {
            Thread.Sleep(500); //simulate work being done
            context.Post(ShowProgress, "Work complete on item " + i);
        }
    }
}

void UpdateCallback(object state)
{
    // UI can be safely updated as this method is only called from the UI thread
    this.MyTextBox.Text = state as string;
}

In this example, if you tried to directly update MyTextBox.Text inside the for loop, you would get a threading error. By posting the UpdateCallback action to the SynchronizationContext, the text box is updated on the same thread as the rest of the UI.

In practice, progress updates should be performed using an instance of System.IProgress<T>. The default implementation System.Progress<T> automatically captures the synchronisation context it is created on.

Remarks

A Synchronization Context is an abstraction that allows consuming to code to pass units of work to a scheduler, without requiring awareness of how the work will be scheduled.

Synchronization contexts are traditionally used to ensure that code is run on a specific thread. In WPF and Winforms applications, a SynchronizationContext representing the UI thread is provided by the presentation framework. In this way SynchronizationContext can be thought of as a producer-consumer pattern for delegates. A worker thread will produce executable code (the delegate) and queue it or consumption by the UI message loop.

The Task Parallel Library provides features for automatically capturing and using synchronization contexts.