How do I drag-and-drop from an arbitrary source into my WPF application?

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I have a WPF application that has a mini-file-explorer. I want to be able to drag files from various sources (maybe Explorer or maybe an email attachment in an email program) and drop them on my file-explorer control. The type of the file does not matter because the semantics of the operation are always the same: copy the file into the current directory being displayed by my mini-file-explorer control.

How do I do this? I have read loads of stuff on the internet, but they all begin with the handling of the mouse-drag that initiates the drag-and-drop. I don't have any control over this because this occurs in one of a number of other applications.

Any help would be greatly appreciated.

What I have tried:

I have tried setting Drop="True" on my target control and providing a handler for the Drop event, but the control simply displays a no-entry icon when I drag a file onto it and the event does not fire.

Posted 24-Nov-16 23:22pm

Patrick Skelton

Updated 25-Nov-16 7:19am

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Richard Deeming · Answer 1 · 2016-11-25T07:19:00

Solution 1

You'll need to handle the DragDrop.DragOver attached event[^] and set the Effects property on the DragEventArgs parameter to an appropriate value based on the available data formats.

C#

private void control_DragOver(object sender, DragEventArgs e)
{
    e.Effects = DragDropEffects.None;

    if (e.Data.GetDataPresent(DataFormats.FileDrop))
    {
        e.Effects = DragDropEffects.Copy;
    }
}

private void control_Drop(object sender, DragEventArgs e)
{
    string[] droppedFiles = e.Data.GetData(DataFormats.FileDrop) as string[];
    if (droppedFiles != null && droppedFiles.Length != 0)
    {
        ...
    }
}

Drag and Drop Overview[^]

I haven't used it, but I've just found this WPF drag/drop library[^], which looks promising.

Posted 25-Nov-16 7:19am

Richard Deeming

Comments

Patrick Skelton 26-Nov-16 4:34am

Thank you for the information. I wouldn't have worked that out any time soon.

I still get a no-entry sign when I drag over the application. I have tried adding the event handlers to the window itself, but still it doesn't work, so I am obviously missing a key piece of the jigsaw somewhere.

Richard Deeming 28-Nov-16 7:16am

Does the DragOver event handler get hit? If so, does the GetDataPresent call return true?

Patrick Skelton 28-Nov-16 7:50am

No. I've added event handlers for DragOver and PreviewDragOver, to both the control and the containing window. None are ever hit.

I'll have to play with the library above, to try to work out what it is doing that I am not. I'd simply use it, but it seems like overkill for my simple requirement.

Richard Deeming 28-Nov-16 7:52am

And you've definitely set AllowDrop[^] to true?

Patrick Skelton 28-Nov-16 7:55am

Yes, again on both the control and the containing window.

Richard Deeming 28-Nov-16 8:10am

Strange. I've just tried a very simple example in LINQPad, and it works perfectly.

Are you using a non-standard AppDomain? There was an issue where the window wouldn't register itself as an OLE drop target, which needed some reflection to work around:
http://blog.functionalfun.net/2009/10/work-around-for-wpf-bug-drag-and-drop.html[^]

Patrick Skelton 28-Nov-16 9:22am

Ah, your comment about AppDomains got me thinking. It turns out to be the simple fact that I was running it in the debugger. I could kick myself for not trying it using a simple double-click on the .exe.

Thank you very much for your help.