C# WinForms: Identify type of drag-drop action event

Question

The use case: users need the ability to drag & drop email items from Outlook onto a form in my WinForms (.Net 4) application. The application saves these items in .msg format and stores them in a predetermined location.

The problem: my code is not robust against drag-drop from other sources (e.g. dragging a jpeg from IE onto the form triggers the same event). This is because I cannot determine whether the dragged item is an Outlook object, or what source the dragged item(s) came from.

Is there a workaround such that I can only accept drag-drop items of a particular type? Here is my code in the DragDrop event handler:

Outlook.Application outlook = new Outlook.Application();
Outlook.Selection sel = outlook.ActiveExplorer().Selection;

try
{    
    foreach (object item in sel)
    {
        if (item is Outlook.MailItem)
        {
            var mail = item as Outlook.MailItem;

            CopyMailItemToLocalTempDir(mail);

            txtComment.Text = "Email from " + mail.SenderName + " Regarding " + mail.Subject;
        }
    }
}
finally
{
    // This is hokey but it prevents Outlook from remembering previously selected items
    // - refer http://stackoverflow.com/questions/14090420/interop-outlook-doesnt-clear-selected-mails-at-drag-and-drop
    var folder = outlook.ActiveExplorer().CurrentFolder;
    outlook.ActiveExplorer().CurrentFolder = outlook.GetNamespace("MAPI").GetDefaultFolder(Outlook.OlDefaultFolders.olFolderContacts);
    Thread.Sleep(50);
    outlook.ActiveExplorer().CurrentFolder = folder;

    Marshal.ReleaseComObject(sel);
    Marshal.ReleaseComObject(outlook);
    sel = null;
    outlook = null;
}

Some details on the DragEventArgs object (e) when dragging from Outlook:

e.Data.GetFormats() returns: 
{string[15]}
    [0]: "RenPrivateSourceFolder"
    [1]: "RenPrivateLatestMessages"
    [2]: "RenPrivateMessages"
    [3]: "RenPrivateItem"
    [4]: "FileGroupDescriptor"
    [5]: "FileGroupDescriptorW"
    [6]: "FileDrop"
    [7]: "FileNameW"
    [8]: "FileName"
    [9]: "FileContents"
    [10]: "Object Descriptor"
    [11]: "System.String"
    [12]: "UnicodeText"
    [13]: "Text"
    [14]: "CSV"

e.Data.GetData("Text") returns: 
"From\tSubject\tReceived\tSize\tCategories\t\r\nJoe Sender\tThis is the email subject\t10:51 AM\t3 KB\t\t"

Answer 1

Is there a workaround such that I can only accept drag-drop items of a particular type?

Yes, there is 2 way

(When working with single files):

if (e.Data.GetData(typeof(...)) is ...)
    //process it

and

if (e.Data.GetDataPresent(typeof(...)))
    //process it

About robustness

Since you have not specified the type to be Outlook.MailItem (or maybe Image) but the source: I assume your program fails on other parts of the code (due to bad design). E.g. ActiveExplorer().Selection in 2nd line or in the finally {} part. This is easily manageable by prefiltering/try-catching in place (before involving outlook) or in the DragEnter event (better when the type is strictly some type of outlook).

To master drag & drop stuff, check out this.

For multiple file handling

or for special types like outlook messages, you already have a good solution with plenty of explanations in the article you mentioned.

I also found an answer to your non-general (outlook only) use case (wrote in 2012) that exhibits OutlookDataObject class and probably based on the article you mentioned (wrote in 2008).

Answer 2

i have here the source code of solution that allow to drop only outlook items. here are the event handlers:

private void Form1_DragEnter(object sender, DragEventArgs e)
    {
        //display formats available
        this.label1.Text = "Formats:\n";
        foreach (string format in e.Data.GetFormats())
        {
            this.label1.Text += "    " + format + "\n";
        }

        //ensure FileGroupDescriptor is present before allowing drop
        if (e.Data.GetDataPresent("FileGroupDescriptor"))
        {
            e.Effect = DragDropEffects.All;
        }
    }

    private void Form1_DragDrop(object sender, DragEventArgs e)
    {
        //wrap standard IDataObject in OutlookDataObject
        OutlookDataObject dataObject = new OutlookDataObject(e.Data);

        //get the names and data streams of the files dropped
        string[] filenames = (string[])dataObject.GetData("FileGroupDescriptorW");
        MemoryStream[] filestreams = (MemoryStream[])dataObject.GetData("FileContents");

        this.label1.Text += "Files:\n";
        for (int fileIndex = 0; fileIndex < filenames.Length; fileIndex++)
        {
            //use the fileindex to get the name and data stream
            string filename = filenames[fileIndex];
            MemoryStream filestream = filestreams[fileIndex];
            this.label1.Text += "    " + filename + "\n";

            //save the file stream using its name to the application path
            FileStream outputStream = File.Create(filename);
            filestream.WriteTo(outputStream);
            outputStream.Close();
        }
    }