WinForms Interop, Drag & Drop from WinForms -> WPF

Question

I'm trying to drag data from the Winforms portion of my application on a WPF controls that's contained inside an "ElementHost". And it crashes when I try doing so.

Trying the same thing but from Winforms to Winforms works fine. (See example code below)

I need help on making this work... have any clues what I'm doing wrong?

Thanks!

Example:
In the sample code below, I'm just trying to drag a custom MyContainerClass object created when initating the drag on the label control on a 1) System.Windows.Forms.TextBox (Winforms) and 2) System.Windows.TextBox (WPF, added to an ElementHost).

Case 1) works fine but case 2) is crashing when trying to retrieve the drop data using GetData(). GetDataPresent("WindowsFormsApplication1.MyContainerClass") returns "true" so In theory, I should be able to retrive my drop data of that type like in Winforms.

Here is the stack trace of the crash:

"Error HRESULT E_FAIL has been returned from a call to a COM component" with the following stack trace:
 at System.Runtime.InteropServices.Marshal.ThrowExceptionForHRInternal(Int32 errorCode, IntPtr errorInfo)
 at System.Windows.Forms.DataObject.GetDataIntoOleStructs(FORMATETC& formatetc, STGMEDIUM& medium)
 at System.Windows.Forms.DataObject.System.Runtime.InteropServices.ComTypes.IDataObject.GetDataHere(FORMATETC& formatetc, STGMEDIUM& medium)
 at System.Windows.Forms.DataObject.System.Runtime.InteropServices.ComTypes.IDataObject.GetData(FORMATETC& formatetc, STGMEDIUM& medium)
 at System.Windows.DataObject.OleConverter.GetDataInner(FORMATETC& formatetc, STGMEDIUM& medium)
 at System.Windows.DataObject.OleConverter.GetDataFromOleHGLOBAL(String format, DVASPECT aspect, Int32 index)
 at System.Windows.DataObject.OleConverter.GetDataFromBoundOleDataObject(String format, DVASPECT aspect, Int32 index)
 at System.Windows.DataObject.OleConverter.GetData(String format, Boolean autoConvert, DVASPECT aspect, Int32 index)
 at System.Windows.DataObject.OleConverter.GetData(String format, Boolean autoConvert)
 at System.Windows.DataObject.GetData(String format, Boolean autoConvert)
 at System.Windows.DataObject.GetData(String format)
 at WindowsFormsApplication1.Form1.textBox_PreviewDragEnter(Object sender, DragEventArgs e) in WindowsFormsApplication1\WindowsFormsApplication1\Form1.cs:line 48

Here is some code:

// -- Add an ElementHost to your form --
// -- Add a label to your form --

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        System.Windows.Controls.TextBox textBox = new System.Windows.Controls.TextBox();
        textBox.Text = "WPF TextBox";
        textBox.AllowDrop = true;
        elementHost2.Child = textBox;
        textBox.PreviewDragEnter += new System.Windows.DragEventHandler(textBox_PreviewDragEnter);

        System.Windows.Forms.TextBox wfTextBox = new System.Windows.Forms.TextBox();
        wfTextBox.Text = "Winforms TextBox";
        wfTextBox.AllowDrop = true;
        wfTextBox.DragEnter += new DragEventHandler(wfTextBox_DragEnter);
        Controls.Add(wfTextBox);
    }

    void wfTextBox_DragEnter(object sender, DragEventArgs e)
    {
        bool dataPresent = e.Data.GetDataPresent("WindowsFormsApplication1.MyContainerClass");

        // NO CRASH here!
        object data = e.Data.GetData("WindowsFormsApplication1.MyContainerClass");
    }

    void textBox_PreviewDragEnter(object sender, System.Windows.DragEventArgs e)
    {
        bool dataPresent = e.Data.GetDataPresent("WindowsFormsApplication1.MyContainerClass");

        // Crash appens here!!
        // {"Error HRESULT E_FAIL has been returned from a call to a COM component."}
        object data = e.Data.GetData("WindowsFormsApplication1.MyContainerClass");
    }

    private void label1_MouseDown(object sender, MouseEventArgs e)
    {
        label1.DoDragDrop(new MyContainerClass(label1.Text), DragDropEffects.Copy);
    }
}

public class MyContainerClass
{
    public object Data { get; set; }

    public MyContainerClass(object data)
    {
        Data = data;
    }
}

SuperOli SuperOli · Accepted Answer · 2009-08-17T14:37:03

@Pedery & jmayor: Thanks for the suggestions guys! (see my findings below)

After quite a few experimentation, trials and errors, and a bit of "Reflector'ing", I managed to figure out exactly why I was receiving the cryptic error message "Error HRESULT E_FAIL has been returned from a call to a COM component".

It was due to the fact that when dragging data WPF <-> Winforms in a same app, that data has to be Serializable!

I've checked how difficult it would be to transform all of our classes to "Serializable" and I would have a been a real pain for a couple of reasons... one, we would need to practically make all of classes serializable and two, some of these classes have references to Controls! And Controls aren't serializable. So a major refactoring would have been needed.

So... since we wanted to pass any object of any class to drag from/to WPF inside the same application, I decided to create a wrapper class, with the Serializable attribute and implementing ISerializable. I would have 1 contructor with 1 parameter of type "object" which would be the actual drag data. That wrapper, when serializing/de-serializing, would serialize not the object itself... but rather the IntPtr to the object (which we can do since we only want that functionnality inside our 1 instance only application.) See code sample below:

[Serializable]
public class DataContainer : ISerializable
{
public object Data { get; set; }

public DataContainer(object data)
{
    Data = data;
}

// Deserialization constructor
protected DataContainer(SerializationInfo info, StreamingContext context)
{
    IntPtr address = (IntPtr)info.GetValue("dataAddress", typeof(IntPtr));
    GCHandle handle = GCHandle.FromIntPtr(address);
    Data = handle.Target;
    handle.Free();
}

#region ISerializable Members

public void GetObjectData(SerializationInfo info, StreamingContext context)
{
    GCHandle handle = GCHandle.Alloc(Data);
    IntPtr address = GCHandle.ToIntPtr(handle);
    info.AddValue("dataAddress", address);
}

#endregion
}

To keep the IDataObject functionnality, I created the following DataObject wrapper:

public class DataObject : IDataObject
{
System.Collections.Hashtable _Data = new System.Collections.Hashtable();

public DataObject() { }

public DataObject(object data)
{
    SetData(data);
}

public DataObject(string format, object data)
{
    SetData(format, data);
}

#region IDataObject Members

public object GetData(Type format)
{
    return _Data[format.FullName];
}

public bool GetDataPresent(Type format)
{
    return _Data.ContainsKey(format.FullName);
}

public string[] GetFormats()
{
    string[] strArray = new string[_Data.Keys.Count];
    _Data.Keys.CopyTo(strArray, 0);
    return strArray;
}

public string[] GetFormats(bool autoConvert)
{
    return GetFormats();
}

private void SetData(object data, string format)
{
    object obj = new DataContainer(data);

    if (string.IsNullOrEmpty(format))
    {
        // Create a dummy DataObject object to retrieve all possible formats.
        // Ex.: For a System.String type, GetFormats returns 3 formats:
        // "System.String", "UnicodeText" and "Text"
        System.Windows.Forms.DataObject dataObject = new System.Windows.Forms.DataObject(data);
        foreach (string fmt in dataObject.GetFormats())
        {
            _Data[fmt] = obj;
        }
    }
    else
    {
        _Data[format] = obj;
    }
}

public void SetData(object data)
{
    SetData(data, null);
}

#endregion
}

And we are using the above classes like this:

myControl.DoDragDrop(new MyNamespace.DataObject(myNonSerializableObject));

// in the drop event for example
e.Data.GetData(typeof(myNonSerializableClass));

I know I know... it's not very pretty... but it's doing what we wanted. We also created a dragdrop helper class which masks the DataObject creation and has templated GetData functions to retrieve the data without any cast... a bit like:

myNonSerializableClass newObj = DragDropHelper.GetData<myNonSerializableClass>(e.Data);

So thanks again for the replies! You guys gave me good ideas where to look at for possible solutions!

-Oli

WinForms Interop, Drag & Drop from WinForms -> WPF

4 Answers