winform.rb

require 'mscorlib'
require 'System.Drawing, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
require 'System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'

include System::Drawing
include System::Windows::Forms

class MyForm < Form
  def initialize
    button = Button.new
    button.location = Point.new(150, 150)                                                        
    button.text = "Click"
    button.click {|sender, e| MessageBox.show 'Hello world!', self.text}
    self.controls << button

    self.text = "WinForm on Ruby"
    self.size = Size.new(400, 400)
  end
end

Application.run MyForm.new