How do I drag and drop files into an application?

Question

Some sample code:

  public partial class Form1 : Form {
    public Form1() {
      InitializeComponent();
      this.AllowDrop = true;
      this.DragEnter += new DragEventHandler(Form1_DragEnter);
      this.DragDrop += new DragEventHandler(Form1_DragDrop);
    }

    void Form1_DragEnter(object sender, DragEventArgs e) {
      if (e.Data.GetDataPresent(DataFormats.FileDrop)) e.Effect = DragDropEffects.Copy;
    }

    void Form1_DragDrop(object sender, DragEventArgs e) {
      string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
      foreach (string file in files) Console.WriteLine(file);
    }
  }

Be aware of windows vista/windows 7 security rights - if you are running Visual Studio as administrator, you will not be able to drag files from a non-administrator explorer window into your program when you run it from within visual studio. The drag related events will not even fire! I hope this helps somebody else out there not waste hours of their life...

In Windows Forms, set the control's AllowDrop property, then listen for DragEnter event and DragDrop event.

When the DragEnter event fires, set the argument's AllowedEffect to something other than none (e.g. e.Effect = DragDropEffects.Move).

When the DragDrop event fires, you'll get a list of strings. Each string is the full path to the file being dropped.

You need to be aware of a gotcha. Any class that you pass around as the DataObject in the drag/drop operation has to be Serializable. So if you try and pass an object, and it is not working, ensure it can be serialized as that is almost certainly the problem. This has caught me out a couple of times!

Yet another gotcha:

The framework code that calls the Drag-events swallow all exceptions. You might think your event code is running smoothly, while it is gushing exceptions all over the place. You can't see them because the framework steals them.

That's why I always put a try/catch in these event handlers, just so I know if they throw any exceptions. I usually put a Debugger.Break(); in the catch part.

Before release, after testing, if everything seems to behave, I remove or replace these with real exception handling.

Another common gotcha is thinking you can ignore the Form DragOver (or DragEnter) events. I typically use the Form's DragOver event to set the AllowedEffect, and then a specific control's DragDrop event to handle the dropped data.