using System.Windows;
using System.Windows.Controls;
 
namespace WX.Classes.Attached
{
  public static class DialogResultHelper
  {
    public static readonly DependencyProperty ResultProperty =
      DependencyProperty.RegisterAttached(
        "Result", typeof(bool?), typeof(DialogResultHelper),
        new PropertyMetadata(false, OnResultPropertyChanged));
 
    public static bool? GetResult(DependencyObject o)
    {
      return (bool?)o.GetValue(ResultProperty);
    }
 
    public static void SetResult(DependencyObject o, bool? value)
    {
      o.SetValue(ResultProperty, value);
    }
 
    static void OnResultPropertyChanged(DependencyObject o,
        DependencyPropertyChangedEventArgs ea)
    {
      var btn = o as Button;
      if (btn == null) return;
      btn.Click -= HandleResult;
      if (ea.NewValue == null) return;
      btn.Click += HandleResult;
    }
 
    static void HandleResult(object sender, RoutedEventArgs ea)
    {
      var btn = sender as Button;
      if (btn == null || GetResult(btn) == null) return;
      var window = Window.GetWindow(btn);
      if (window == null) return;
      window.DialogResult = GetResult(btn);
      window.Close();
    }
  }
}

<Window
  x:Class="wpftest2.Window1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:Attached="clr-namespace:WX.Classes.Attached;assembly=WX.Classes"
  TextOptions.TextFormattingMode="Display"
  UseLayoutRounding="True">
 
  <Grid>
    <Button
      VerticalAlignment="Center"
      HorizontalAlignment="Center"
      Padding="10"
      Content="OkCancel"
      IsDefault="True"
      Attached:DialogResultHelper.Result="True"/>
  </Grid>
</Window>