Reference SystemParameters

<Window x:Class="Windows.CenterScreen"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="CenterScreen" Height="400" Width="600"
    >
  <Button Margin="18" Click="cmdCenter_Click">Center Me</Button>
</Window>
//File:Window.xaml.cs
using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.ComponentModel;

namespace Windows
{
    public partial class CenterScreen : System.Windows.Window
    {
        public CenterScreen()
        {
            InitializeComponent();
        }

        private void cmdCenter_Click(object sender, RoutedEventArgs e)
        {            
            double height = SystemParameters.WorkArea.Height;
            double width = SystemParameters.WorkArea.Width;            
            this.Top = (height - this.Height) / 2;            
            this.Left = (width - this.Width) / 2;            
         
        }
    }
}