How does Task.CurrentId work?

Question

I am currently learning how to use Tasks, async and await in Windows Store ("Metro") apps. I stumbled upon the property Task.CurrentId and try to understand how it works.

According to MSDN it returns "An integer that was assigned by the system to the currently-executing task". So I added logging this value to my custom logger, but to my surprise none of my test apps ever logged anything but null.

Look at this example:

private async void TestButton_Click(object sender, RoutedEventArgs e)
{
    int? id1 = Task.CurrentId;

    await Task.Delay(100);
    int? id2 = Task.CurrentId;

    StorageFolder folder = ApplicationData.Current.LocalFolder;
    StorageFile file = await folder.CreateFileAsync("test.txt", 
        CreationCollisionOption.OpenIfExists);
    int? id3 = Task.CurrentId;

    await FileIO.AppendTextAsync(file, "test");
    int? id4 = Task.CurrentId;

    await DoMoreAsync();
    int? id7 = Task.CurrentId;
}

private async Task DoMoreAsync()
{
    StorageFolder folder = ApplicationData.Current.LocalFolder;
    StorageFile file = await folder.CreateFileAsync("test.txt",
        CreationCollisionOption.OpenIfExists);
    int? id5 = Task.CurrentId;

    await FileIO.AppendTextAsync(file, "test");
    int? id6 = Task.CurrentId;
}

All these ids are null. Why? This code does create tasks, doesn't it? Shouldn't they have an id?

Answer 1

Because you're using async / await, all your Task.CurrentId calls are happening on the UI thread.

The UI thread is not a Task, so it has a null CurrentId

---

If you create a Task, its CurrentId will be set:

int? idTask = await Task.Run( () => Task.CurrentId );