RxExample.cs

namespace TheApp
{
    public partial class MainWindow : Window
    {        
        IObservable<MouseEventArgs> foo;

        public MainWindow()
        {
            var foo = Observable.FromEvent<MouseEventArgs>(this, "PreviewMouseMove")
                .Where(x => { var pos = x.EventArgs.GetPosition(this); return (pos.X < 10 && pos.Y < 10); })
                .Subscribe((v) => MessageBox.Show("You're in the corner!"), () => { ; });
        }
    }
}