Make WPF window draggable, no matter what element is clicked

Question

Sure, apply the following MouseDown event of your Window

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

This will allow users to drag the Window when they click/drag on any control, EXCEPT for controls which eat the MouseDown event (e.Handled = true)

You can use PreviewMouseDown instead of MouseDown, but the drag event eats the Click event, so your window stops responding to left-mouse click events. If you REALLY wanted to be able to click and drag the form from any control, you could probably use PreviewMouseDown, start a timer to begin the drag operation, and cancel the operation if the MouseUp event fires within X milliseconds.

if the wpf form needs to be draggable no matter where it was clicked the easy work around is using a delegate to trigger the DragMove() method on either the windows onload event or the grid load event

private void Grid_Loaded(object sender, RoutedEventArgs 
{
      this.MouseDown += delegate{DragMove();};
}

Sometimes, we do not have access to Window, e.g. if we are using DevExpress, all that is available is a UIElement.

Step 1: Add attached property

The solution is to:

1. Hook into MouseMove events;
2. Search up the visual tree until we find the first parent Window;
3. Call .DragMove() on our newly discovered Window.

Code:

using System.Windows;
using System.Windows.Input;
using System.Windows.Media;

namespace DXApplication1.AttachedProperty
{
    public class EnableDragHelper
    {
        public static readonly DependencyProperty EnableDragProperty = DependencyProperty.RegisterAttached(
            "EnableDrag",
            typeof (bool),
            typeof (EnableDragHelper),
            new PropertyMetadata(default(bool), OnLoaded));

        private static void OnLoaded(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
        {
            var uiElement = dependencyObject as UIElement;
            if (uiElement == null || (dependencyPropertyChangedEventArgs.NewValue is bool) == false)
            {
                return;
            }
            if ((bool)dependencyPropertyChangedEventArgs.NewValue  == true)
            {
                uiElement.MouseMove += UIElementOnMouseMove;
            }
            else
            {
                uiElement.MouseMove -= UIElementOnMouseMove;
            }

        }

        private static void UIElementOnMouseMove(object sender, MouseEventArgs mouseEventArgs)
        {
            var uiElement = sender as UIElement;
            if (uiElement != null)
            {
                if (mouseEventArgs.LeftButton == MouseButtonState.Pressed)
                {
                    DependencyObject parent = uiElement;
                    int avoidInfiniteLoop = 0;
                    // Search up the visual tree to find the first parent window.
                    while ((parent is Window) == false)
                    {
                        parent = VisualTreeHelper.GetParent(parent);
                        avoidInfiniteLoop++;
                        if (avoidInfiniteLoop == 1000)
                        {
                            // Something is wrong - we could not find the parent window.
                            return;
                        }
                    }
                    var window = parent as Window;
                    window.DragMove();
                }
            }
        }

        public static void SetEnableDrag(DependencyObject element, bool value)
        {
            element.SetValue(EnableDragProperty, value);
        }

        public static bool GetEnableDrag(DependencyObject element)
        {
            return (bool)element.GetValue(EnableDragProperty);
        }
    }
}

Step 2: Add Attached Property to any element to let it drag the window

The user can drag the entire window by clicking on a specific element, if we add this attached property:

<Border local:EnableDragHelper.EnableDrag="True">
    <TextBlock Text="Click me to drag this entire window"/>
</Border>

---

Appendix A: Optional Advanced Example

In this example from DevExpress, we replace the title bar of a docking window with our own grey rectangle, then ensure that if the user clicks and drags said grey rectagle, the window will drag normally:

<dx:DXWindow x:Class="DXApplication1.MainWindow" Title="MainWindow" Height="464" Width="765" 
    xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:dxdo="http://schemas.devexpress.com/winfx/2008/xaml/docking" 
    xmlns:local="clr-namespace:DXApplication1.AttachedProperty"
    xmlns:dxdove="http://schemas.devexpress.com/winfx/2008/xaml/docking/visualelements"
    xmlns:themeKeys="http://schemas.devexpress.com/winfx/2008/xaml/docking/themekeys">

    <dxdo:DockLayoutManager FloatingMode="Desktop">
        <dxdo:DockLayoutManager.FloatGroups>
            <dxdo:FloatGroup FloatLocation="0, 0" FloatSize="179,204" MaxHeight="300" MaxWidth="400" 
                             local:TopmostFloatingGroupHelper.IsTopmostFloatingGroup="True"                             
                             >
                <dxdo:LayoutPanel ShowBorder="True" ShowMaximizeButton="False" ShowCaption="False" ShowCaptionImage="True" 
                                  ShowControlBox="True" ShowExpandButton="True" ShowInDocumentSelector="True" Caption="TradePad General" 
                                  AllowDock="False" AllowHide="False" AllowDrag="True" AllowClose="False"
                                  >
                    <Grid Margin="0">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                        </Grid.RowDefinitions>
                        <Border Grid.Row="0" MinHeight="15" Background="#FF515151" Margin="0 0 0 0"
                                                                  local:EnableDragHelper.EnableDrag="True">
                            <TextBlock Margin="4" Text="General" FontWeight="Bold"/>
                        </Border>
                        <TextBlock Margin="5" Grid.Row="1" Text="Hello, world!" />
                    </Grid>
                </dxdo:LayoutPanel>
            </dxdo:FloatGroup>
        </dxdo:DockLayoutManager.FloatGroups>
    </dxdo:DockLayoutManager>
</dx:DXWindow>

---

Disclaimer: I am not affiliated with DevExpress. This technique will work with any user element, including standard WPF or Telerik (another fine WPF library provider).

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

Is throwing an exception in some cases (i.e. if on the window you also have a clickable image that when clicked opens a message box. When you exit from message box you will get error) It is safer to use

private void Window_MouseDown(object sender, MouseButtonEventArgs e)
{
if (Mouse.LeftButton == MouseButtonState.Pressed)
            this.DragMove();
}

So you are sure that left button is pressed at that moment.

As already mentioned by @fjch1997 it's convenient to implement a behavior. Here it is, the core logic is the same as in the @loi.efy's answer:

public class DragMoveBehavior : Behavior<Window>
{
    protected override void OnAttached()
    {
        AssociatedObject.MouseMove += AssociatedObject_MouseMove;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
    }

    private void AssociatedObject_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed && sender is Window window)
        {
            // In maximum window state case, window will return normal state and
            // continue moving follow cursor
            if (window.WindowState == WindowState.Maximized)
            {
                window.WindowState = WindowState.Normal;

                // 3 or any where you want to set window location after
                // return from maximum state
                Application.Current.MainWindow.Top = 3;
            }

            window.DragMove();
        }
    }
}

Usage:

<Window ...
        xmlns:h="clr-namespace:A.Namespace.Of.DragMoveBehavior"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity">
    <i:Interaction.Behaviors>
        <h:DragMoveBehavior />
    </i:Interaction.Behaviors>
    ...
</Window>

This is all needed!

private void UiElement_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed)
        {
            if (this.WindowState == WindowState.Maximized) // In maximum window state case, window will return normal state and continue moving follow cursor
            {
                this.WindowState = WindowState.Normal;
                Application.Current.MainWindow.Top = 3;// 3 or any where you want to set window location affter return from maximum state
            }
            this.DragMove();
        }
    }