Synchronization Context in Async-Await

Pseudocode for async/await keywords

Consider a simple asynchronous method:

async Task Foo()
{
    Bar();
    await Baz();
    Qux();
}

Simplifying, we can say that this code actually means the following:

Task Foo()
{
    Bar();
    Task t = Baz();
    var context = SynchronizationContext.Current;
    t.ContinueWith(task) =>
    {
        if (context == null)
            Qux();
        else
            context.Post((obj) => Qux(), null);
    }, TaskScheduler.Current);

    return t;
}

It means that async/await keywords use current synchronization context if it exists. I.e. you can write library code that would work correctly in UI, Web, and Console applications.

Source article.

Disabling synchronization context

To disable synchronization context you should call the ConfigureAwait method:

async Task() Foo()
{
    await Task.Run(() => Console.WriteLine("Test"));
}

. . .

Foo().ConfigureAwait(false);

ConfigureAwait provides a means to avoid the default SynchronizationContext capturing behavior; passing false for the flowContext parameter prevents the SynchronizationContext from being used to resume execution after the await.

Quote from It’s All About the SynchronizationContext.

Why SynchronizationContext is so important?

Consider this example:

private void button1_Click(object sender, EventArgs e)
{
    label1.Text = RunTooLong();
}

This method will freeze UI application until the RunTooLong will be completed. The application will be unresponsive.

You can try run inner code asynchronously:

private void button1_Click(object sender, EventArgs e)
{
    Task.Run(() => label1.Text = RunTooLong());
}

But this code won’t execute because inner body may be run on non-UI thread and it shouldn’t change UI properties directly:

private void button1_Click(object sender, EventArgs e)
{
    Task.Run(() =>
    {
        var label1Text = RunTooLong();

        if (label1.InvokeRequired)
            lable1.BeginInvoke((Action) delegate() { label1.Text = label1Text; });
        else
            label1.Text = label1Text;
    });
}

Now don’t forget always to use this pattern. Or, try SynchronizationContext.Post that will make it for you:

private void button1_Click(object sender, EventArgs e)
{
    Task.Run(() =>
    {
        var label1Text = RunTooLong();
        SynchronizationContext.Current.Post((obj) =>
        {
            label1.Text = label1    Text);
        }, null);
    });
}