Simple Drag & Drop with Reactive Extensions

RxDragDrop.cs

using System;
using System.Reactive.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace RxDragAndDrop {

    public partial class MainWindow : Window {

        public MainWindow() {
            InitializeComponent();

            // Stream of mousedown events with special condition (must drag on the ball image)
            var mouseDownOnBall = from mouseEvt in Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseDown")
                                  let pos = mouseEvt.EventArgs.GetPosition(image)
                                  where pos.X >= 0 && pos.Y >= 0 &&
                                        pos.X <= image.Width && pos.Y <= image.Height
                                  select mouseEvt;

            // Stream mouseup events
            var mouseUp = Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseUp");
            
            // Stream of mouse postion on mousemove
            var mouseMovePos = from md in Observable.FromEventPattern<MouseEventArgs>(this, "MouseMove")
                               select md.EventArgs.GetPosition(this);

            // deltas of postions (before/after), zip the mousemove with itself to compute the delta
            var mouseDeltas = mouseMovePos.Zip(mouseMovePos.Skip(1), (p, c) => new { X = c.X - p.X, Y = c.Y - p.Y });
            
            // pushes the deltas of mouse positions only when the mouse is down on the ball image
            var deltasOnDrag = from start in mouseDownOnBall
                               from moves in mouseDeltas.TakeUntil(mouseUp)
                               select moves;

            // Subscription and modify the postion of the ball image
            deltasOnDrag.Subscribe(val => {
                Canvas.SetTop(image, Canvas.GetTop(image) + val.Y);
                Canvas.SetLeft(image, Canvas.GetLeft(image) + val.X);
            });

        }
    }
}

WindowUI.xaml

<Window x:Class="RxDragAndDrop.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="450" Width="539">
        
    <Canvas>
        <TextBlock Name="textBlock" Text="!Rx!" />
        <Image Name="image" Source="ball.png" Canvas.Top="30" Canvas.Left="0" Height="64" Width="64" />
    </Canvas>
    
</Window>