イベントハンドラを yield return

yield.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

class MyForm : Form
{
  static void Main()
  {
    Application.Run( new MyForm() );
  }

  readonly List<Action<Graphics>> _entities = new List<Action<Graphics>>();
  IEnumerator<MouseEventHandler> _handler;

  public MyForm()
  {
    this.Paint += ( sender, e ) =>
    {
      foreach ( var draw in _entities ) draw( e.Graphics );
    };

    this.MouseClick += ( sender, e ) =>
    {
      if ( _handler != null ) {
        _handler.Current( this, e );
        this.MoveNext();
      }
    };

    _handler = this.DrawLines();
    this.MoveNext();
  }

  void MoveNext()
  {
    if ( !_handler.MoveNext() ) {
      _handler.Dispose();
      _handler = null;
    }
  }

  IEnumerator<MouseEventHandler> DrawLines()
  {
    while ( true ) {
      var p1 = new Point();
      var p2 = new Point();
      yield return ( sender, e ) => p1 = e.Location;
      yield return ( sender, e ) => p2 = e.Location;
      _entities.Add( g => g.DrawLine( new Pen( Brushes.Black ), p1, p2 ) );
      this.Invalidate();
    }
  }
}

2点をクリックすることで直線を描くことができるというお話。
いわゆる複数点間で何かイベント起こしたらどうこうするって時に役立ちそう。