Mike-Ward.Net

Drag and Drop in WPF

28 Jun 2013

Well in this case, just Drop.

There are plenty of examples on the Web on how to do Drag and Drop in Windows but often they leave out a few essentials. To start, you tell windows that you’re a drop target by setting the AllowDrop property and assigning handlers for the Drop and DragOver events.

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"\  
        AllowDrop="True"  
        Drop="OnDrop"  
        DragOver="OnDragOver"  
        ...  
        >  

Many tutorials have you add a handler to the DragEnter event, but this not correct. DragEnter is useful if you need to allocate a resource, like a brush, to be used as custom indicator. In most circumstances, it’s the DragOver event that requires a handler.

The handlers themselves are straight-forward. Here’s one that handles file drops.

private void OnDragOver(object sender, DragEventArgs ea)  
{  
    ea.Handled = true;  
    ea.Effects = DragDropEffects.None;  
    if (ea.Data.GetDataPresent(DataFormats.FileDrop, true))  
    {  
        var filenames = ea.Data.GetData(DataFormats.FileDrop, true) as string[];  
        if (filenames != null && filenames.Length == 1 && IsValidImageExtension(filenames[0]))  
        {  
            ea.Effects = DragDropEffects.Copy;  
        }  
    }  
}  
  
private void OnDrop(object sender, DragEventArgs ea)  
{  
    if (ea.Data.GetDataPresent(DataFormats.FileDrop, true))  
    {  
        var filenames = ea.Data.GetData(DataFormats.FileDrop, true) as string[];  
        if (filenames != null && filenames.Length == 1 && IsValidImageExtension(filenames[0]))  
        {  
            // My program logic here...  
            ea.Handled = true;  
        }  
    }  
}  
  
private static bool IsValidImageExtension(string filename)  
{  
    var extension = Path.GetExtension(filename);  
    var extensions = new[] { ".png", ".jpg", ".jpeg", ".gif" };  
    return extensions.Any(e => extension.Equals(e, StringComparison.OrdinalIgnoreCase));  
}  

Here I’m checking that the file extension is a PNG, JPG or GIF. I’m also only interested in handling one file. Your program will have different requirements.

The one “gotcha” that most examples omit is setting the ea.Handled property to True (some of Microsoft’s examples miss this). Not doing so usually results in the drop indicator appearing not to work (DragDropEffects.Copy in this case). Worse, if you have multiple drop handlers, they may be inadvertently called (Not that I’ve ever done that).

Pro tip: Always write about something immediately after you’ve learned it but make sure you sound like you’ve been doing it that way for years :)

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Drag and Drop in WPF

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