MainWindow.xaml.cs

using WPF.ViewModel;
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace WPF.View
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainViewModel();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            var myHeaderGrid = (Grid)this.FindName("Header");
            if (myHeaderGrid != null)
            {
                myHeaderGrid.MouseLeftButtonDown += OnMouseDown;
            }
        }

        private void OnMouseDown(object sender, MouseButtonEventArgs e)
        {
            if (e.ChangedButton == MouseButton.Left)
            {
                DragMove();
            }
        }
    }
}