Very Simple WPF Drag and Drop Sample without Win32 Calls

• Download test - 9.63 KB

Introduction

I learned how to implement drag and drop in WPF from Josh Smith's article Drag and Drop Items in a WPF ListView. Josh's great article and attached code describe many WPF features related to drag and drop functionality. I felt, however, that most of the time people might want to use just a plain, vanilla drag and drop without most of those features. For such people, this article and the related code can be more useful.

On top of that, Josh uses the MouseUtilities class developed by Dan Crevier that calls some Win32 functions. I got around this problem by using the event's GetPosition method. So, one should be able to use the functionality presented here in a Web browser application (XBAP) without any problems.

Later note - added on 03/16/08: Actually this is quite true, one cannot run an XBAP application with DragDrop.DoDragDrop() function under partial trust. To see an example of Drag and Drop implementation for XBAP, take a look at Implementing Drag Drop Operations for Browser Based WPF Applications (XBAP). Also for multiselect drag and drop, see MultiSelect Drag and Drop in WPF.

Using the Code

To use the code, just download it, open the project using Visual Studio 2008, compile and run.

Use drag and drop to re-order the items in the list.

Brief Code Description

There are basically three files containing the code:

1. Shape.cs
2. Window1.xaml
3. Window.xaml.cs

Shape.cs contains the definition for very simple objects that are used as items behind the ListView. Each item has a name and number of sides as properties.

Window1.xaml contains XAML representing a very simple Window with ListView control in it. ListView's ItemsSource property is set to point to a collection of Shape objects.

Window1.xaml.cs contains all the C# plumbing and the functions that actually do drag and drop:

• ListView1_PreviewMouseLeftButtonDown - originates the drag operation
• ListView1_Drop - completes the drop operation

Using Event Argument GetPosition Function to Determine Current Mouse Position

Both DragEventArgs and MouseButtonEventArgs classes have the function:

Point GetPosition(IInputElement)

This function returns the current mouse position with respect to the Visual element (passed to the function as the argument). Since DragEventArgs and MouseButtonEventArgs do not have a common superclass or interface that defines the GetPosition function, I wrote the code that takes GetPosition function as a delegate in order to be able to re-use it for both classes. Here is the functionality that returns the index of the current item in the list:

int GetCurrentIndex(GetPositionDelegate getPosition)
{
    int index = -1;
    for (int i = 0; i < this.ListView1.Items.Count; ++i)
    {
        ListViewItem item = GetListViewItem(i);
        if (this.IsMouseOverTarget(item, getPosition))
        {
            index = i;
            break;
        }
    }
    return index;
}

bool IsMouseOverTarget( Visual target, GetPositionDelegate getPosition)
{
    Rect bounds = VisualTreeHelper.GetDescendantBounds( target );
    Point mousePos = getPosition((IInputElement) target);
    return bounds.Contains( mousePos );
}

GetPositionDelegate is defined as:

delegate Point GetPositionDelegate(IInputElement element);

Here is how the function GetCurrentIndex is called: from ListView1_Drop function - responsible for dropping operation (this function uses DragEventArgs):

void ListView1_Drop(object sender, DragEventArgs e)
{
     ...
     int index = this.GetCurrentIndex(e.GetPosition);
     ...
}

From ListView1_PreviewMouseLeftButtonDown function - responsible for starting drag operation (this function uses MouseButtonEventArgs):

void ListView1_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    ...
    oldIndex = this.GetCurrentIndex(e.GetPosition);
    ...
}

History

• 3^rd March, 2008: Initial post
• 16^th March, 2008: Added references to other drag and drop articles