GroupJoin.xaml.cs

using System.Reactive;
using System.Reactive.Linq;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;

namespace LinqQueries;

/// <summary>
/// Interaction logic for GroupJoin.xaml
/// </summary>
public partial class GroupJoin : Window
{
    public GroupJoin()
    {
        InitializeComponent();

        IObservable<EventPattern<MouseEventArgs>> downs =
            Observable.FromEventPattern<MouseEventArgs>(
                background, nameof(background.MouseDown));
        IObservable<EventPattern<MouseEventArgs>> ups =
            Observable.FromEventPattern<MouseEventArgs>(
                background, nameof(background.MouseUp));
        IObservable<EventPattern<MouseEventArgs>> allMoves =
            Observable.FromEventPattern<MouseEventArgs>(
                background, nameof(background.MouseMove));

        var dragPointSets = from mouseDown in downs
                            join move in allMoves
                              on ups equals allMoves into m
                            select m.Select(e => e.EventArgs.GetPosition(background));

        dragPointSets.Subscribe(dragPoints =>
        {
            var currentLine = new Polyline { Stroke = Brushes.Black, StrokeThickness = 2 };
            background.Children.Add(currentLine);

            dragPoints.Subscribe(point =>
            {
                currentLine.Points.Add(point);
            });
        });
    }
}