C# 5.0 Programmer's Reference

Asynchronous Methods ❘ 145
callback that it automatically invokes when the task has finished. One really nice feature of this “virtual
callback” is that it executes on the UI thread so it can manipulate controls directly without using
the form’s Invoke method.
The AsyncAwait example program, which is available for download on this book’s website, is
similar to the HandleCallback example program, but it uses the async and await keywords
instead of a callback.
The following code shows the event handler that executes when you click the program’s Count
button. The async and await keywords are highlighted in bold.
// Count on a separate thread.
private async void countButton_Click(object sender, EventArgs e)
{
// Disable the button.
countButton.Enabled = false;
countLabel.Text = "";
countButton.Refresh();
countLabel.Refresh();
// Start the Count method on a new thread.
Func countDelegate = Count;
Task task = new Task(countDelegate, 5);
task.Start();
// Count to 3.
for (int i = 1; i