How do I build a WPF application where I can drag and drop a user control between windows?

Question

I'm building a simple Todo List application where I want to be able to have multiple lists floating around my desktop that I can label and manage tasks in.

The relevant UIElements in my app are:

Window1 (Window) TodoList (User Control) TodoStackCard (User Control)

Window1 looks like this:

<Window x:Class="TaskHole.App.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:t="clr-namespace:TaskHole.App.Controls"
    xmlns:tcc="clr-namespace:TaskHole.CustomControls"
    Title="Window1" Width="500" Height="500" Background="Transparent" WindowStyle="None" AllowsTransparency="True" >
    <Canvas Name="maincanvas" Width="500" Height="500" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
        <ResizeGrip SizeChanged="ResizeGrip_SizeChanged" />
        <t:TodoList Canvas.Top="0" Canvas.Left="0" MinWidth="30" Width="50" Height="500" x:Name="todoList" TaskHover="todoList_TaskHover" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
    </Canvas>
</Window>

TodoList looks like this:

<UserControl x:Class="TaskHole.App.Controls.TodoList"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:t="clr-namespace:TaskHole.App.Controls"
        xmlns:tcc="clr-namespace:TaskHole.CustomControls"
        Background="Transparent">
    <StackPanel VerticalAlignment="Bottom" HorizontalAlignment="Stretch" MinWidth="1" Grid.Row="2" Height="Auto" AllowDrop="True">
        <ItemsControl Name="todolist" ItemsSource="{Binding}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <VirtualizingStackPanel Name="stackPanel" VerticalAlignment="Bottom">
                    </VirtualizingStackPanel>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <t:TodoStackCard x:Name="card" TaskHover="card_TaskHover" Orientation="Vertical" VerticalContentAlignment="Top" />
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </StackPanel>
</UserControl>

I have multiple instances of these windows, and I want to be able to drag any of the controls between the windows. I have tried using a Thumb control and, while this works, it only allows me to drag a control around the containing canvas.

How do I mimic the behaviour of, say, Windows Explorer, where I can drag a file outside of the application and onto another application, all the while seeing a ghosted representation of the file under the cursor.

Can I accomplish this purely in C# and WPF? If so/if not, how?

Answer 1

You have to call DoDragDrop to initialize the Drag And Drop framework. Jaime Rodriguez provides a guide to Drag and Drop here

Answer 2

Just as an FYI, there's a big difference to "dragging controls" around, and doing what Explorer does, which is Drag and Drop, specifically with files. That's what you'll want to look up, how to do drag and drop from a WPF app to something else. You'll need something that creates a Data Object (IDataObject) or whatever they call that in WPF world, and then you need to call DoDragDrop (again, or whatever is analogous to this in WPF) to start the dragging. Doing what explorer does is also possible, put I suspect you need ot make some lower level calls to accomplish this. Take a look at http://www.codeproject.com/KB/wtl/wtl4mfc10.aspx to see the stuff you need ot look for. WPF may in fact wrap all this up, but if it doesn't these are some of the things you need to look into, especially IDragSourceHelper.