MARKUS

            Telerik.Windows.Controls.StyleManager.ApplicationTheme = new Telerik.Windows.Controls.VisualStudio2013Theme();

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows;

using System.Windows.Media;

using System.Windows.Shapes;

namespace KCOM.Behaviors

{

    public class WindowBehavior : System.Windows.Interactivity.Behavior<System.Windows.Controls.Control>

    {

        protected override void OnAttached()

        {

            AssociatedObject.Loaded += AssociatedObject_Loaded;

            base.OnAttached();

        }

        protected override void OnDetaching()

        {

            AssociatedObject.Loaded -= AssociatedObject_Loaded;

            base.OnDetaching();

        }

        private void AssociatedObject_Loaded(object sender, RoutedEventArgs e)

        {

            var objectType = AssociatedObject.GetType();

            double objectWidth;

            double objectHeight;

            double ownerLeft = 0.0;

            double ownerTop = 0.0;

            double ownerWidth = 0.0;

            double ownerHeight = 0.0;

            if (AssociatedObject is System.Windows.Window || AssociatedObject is Telerik.Windows.Controls.RadWindow)

            {

                if (objectType.GetProperty("Owner") != null)

                {

                    var owner = AssociatedObject.GetType().GetProperty("Owner").GetValue(AssociatedObject);

                    if (owner is System.Windows.Controls.ContentControl)

                    {

                        var  ownerType = owner.GetType();

                        objectWidth = (double)objectType.GetProperty("Width").GetValue(AssociatedObject);

                        objectHeight = (double)objectType.GetProperty("Height").GetValue(AssociatedObject);

                        ownerLeft = (double)ownerType.GetProperty("Left").GetValue(owner);

                        ownerTop = (double)ownerType.GetProperty("Top").GetValue(owner);

                        ownerWidth = (double)ownerType.GetProperty("Width").GetValue(owner);

                        ownerHeight = (double)ownerType.GetProperty("Height").GetValue(owner);

                        if ((WindowState)ownerType.GetProperty("WindowState").GetValue(owner) != WindowState.Normal)

                        {

                            Point locationFromScreen = (owner as Visual).PointToScreen(new Point(0, 0));

                            ownerLeft = locationFromScreen.X;

                            ownerTop = locationFromScreen.Y;

                        }

                        objectType.GetProperty("Left").SetValue(AssociatedObject, ownerLeft + (ownerWidth - objectWidth) / 2);

                        objectType.GetProperty("Top").SetValue(AssociatedObject, ownerTop + (ownerHeight - objectHeight) / 2);

                    }

                    else

                    {

                        throw new Exception("Owner base Type is Not System.Windows.Controls.ContentControl");

                    }

                }

                else

                {

                    throw new Exception("Owner is Null");

                }

            }

            else

            {

                throw new Exception("this Behavior Only System.Windows.Window or Telerik.Windows.Controls.RadWindow");

            }

        }

        public static bool GetApplicationCenter(DependencyObject obj)

        {

            return (bool)obj.GetValue(ApplicationCenterProperty);

        }

        public static void SetApplicationCenter(DependencyObject obj, bool value)

        {

            obj.SetValue(ApplicationCenterProperty, value);

        }

        public static readonly DependencyProperty ApplicationCenterProperty = DependencyProperty.RegisterAttached("ApplicationCenter", typeof(bool), 

                                                                    typeof(WindowBehavior), new FrameworkPropertyMetadata(false, new PropertyChangedCallback(ApplicationCenterPropertyChanged)));

        private static void ApplicationCenterPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

        {

        }

    }

    public class ScreenBoundsConverter

    {

        private Matrix _transform;

        public ScreenBoundsConverter(Visual visual)

        {

            _transform =

              PresentationSource.FromVisual(visual).CompositionTarget.TransformFromDevice;

        }

        public Rect ConvertBounds(System.Drawing.Rectangle bounds)

        {

            var result = new Rect(bounds.X, bounds.Y, bounds.Width, bounds.Height);

            result.Transform(_transform);

            return result;

        }

    }

}

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Interactivity;

namespace KCOM

{

    public static class WindowHelper

    {

        public static void ShowCenter(this System.Windows.Controls.ContentControl window)

        {

            SetApplicationCenterBehavior(window);

            if (window is System.Windows.Window)

            {

                (window as System.Windows.Window).Show();

            }

            else if (window is Telerik.Windows.Controls.RadWindow)

            {

                (window as System.Windows.Window).Show();

            }

            else

            {

                throw new Exception("window Type is System.Windows.Window or  Telerik.Windows.Controls.RadWindow");

            }

        }

        public static bool? ShowDialogCenter(this System.Windows.Window window)

        {

            if (window is System.Windows.Window)

            {

                SetApplicationCenterBehavior(window);

                return window.ShowDialog();

            }

            else

            {

                throw new Exception("window Type is System.Windows.Window");

            }

        }

        public static void ShowDialogCenter(this Telerik.Windows.Controls.RadWindow window)

        {

            if (window is Telerik.Windows.Controls.RadWindow)

            {

                SetApplicationCenterBehavior(window);

                window.ShowDialog();

            }

            else

            {

                throw new Exception("window Type is Telerik.Windows.Controls.RadWindow");

            }

        }

        private static void SetApplicationCenterBehavior(System.Windows.Controls.ContentControl window)

        {

            if (window.GetType().GetProperty("Owner").GetValue(window) == null)

            {

                window.GetType().GetProperty("Owner").SetValue(window, App.Current.Windows.Cast<System.Windows.Window>().FirstOrDefault());

            }

            if (Interaction.GetBehaviors(window).Count(f => f is KCOM.Behaviors.WindowBehavior) == 0)

            {

                KCOM.Behaviors.WindowBehavior windowBehavior = new Behaviors.WindowBehavior();

                windowBehavior.SetValue(KCOM.Behaviors.WindowBehavior.ApplicationCenterProperty, true);

                //windowBehavior.SetBinding(KCOM.Behaviors.WindowBehavior.ApplicationCenterProperty, new Binding()

                //{

                //    ElementName = "_uc",

                //    Path = new PropertyPath("SelectedItems"),

                //    Mode = BindingMode.TwoWay

                //});

                Interaction.GetBehaviors(window).Add(windowBehavior);

            }

        }

    }

}

    <Compile Include="Behaviors\WindowBehavior.cs" />

    <Compile Include="Extensions\WindowHelper.cs" />