com - Drag and drop large virtual files from C# to Windows Explorer

Question

I have a C# WPF application with a section that serves as an FTP client, listing files on a remote server and allowing the user to download them. I want the user to be able to drag and drop files from the file listing onto their own machine (i.e. into a Windows Explorer shell).

To accomplish this, I used the VirtualFileDataObject code from Delay's blog, using the Action<Stream> overload of SetData. This works great on smaller files.

My problem is: some of the files I'm dealing with are very large (2+ GB), and the way the VirtualFileDataObject class handles the stream involves reading the entire thing into memory, which can end up throwing a "not enough storage" error for those very large files.

The relevant section of the VirtualFileDataObject code is below. How can I rewrite this code to not require the entire stream to be in memory?

    public void SetData(short dataFormat, int index, Action<Stream> streamData) {
        _dataObjects.Add(
            new DataObject {
                FORMATETC = new FORMATETC {
                    cfFormat = dataFormat,
                    ptd = IntPtr.Zero,
                    dwAspect = DVASPECT.DVASPECT_CONTENT,
                    lindex = index,
                    tymed = TYMED.TYMED_ISTREAM
                },
                GetData = () => {
                    // Create IStream for data
                    var ptr = IntPtr.Zero;
                    var iStream = NativeMethods.CreateStreamOnHGlobal(IntPtr.Zero, true);
                    if (streamData != null) {
                        // Wrap in a .NET-friendly Stream and call provided code to fill it
                        using (var stream = new IStreamWrapper(iStream)) {
                            streamData(stream);
                        }
                    }
                    // Return an IntPtr for the IStream
                    ptr = Marshal.GetComInterfaceForObject(iStream, typeof(IStream));
                    Marshal.ReleaseComObject(iStream);
                    return new Tuple<IntPtr, int>(ptr, NativeMethods.S_OK);
                },
            });
    }

In particular, this section of GetData is the culprit:

// Wrap in a .NET-friendly Stream and call provided code to fill it
using (var stream = new IStreamWrapper(iStream)) {
    streamData(stream);
}

streamData is the Action<stream> I provide which writes the actual file data to the stream. My delegate is just opening a file and reading the bytes into the provided stream.

Is there a way to avoid this last step, perhaps somehow passing the file stream directly to be read from by the Explorer shell? I'm thinking something like replacing iStream with a pointer to the .NET filestream I've got...but I don't know enough about COM interop to even know the syntax for doing that. Any tips/direction would be appreciated!

Answer 1

After more Googling and stumbling around and trying one thing and another, I've got something that works, but I'm still open to better solutions. For now, when the drop operation happens I'm retrieving the file to a temporary location then using SHCreateStreamOnFileEx to open an IStream to that location. The revised part, the GetData lambda, is as follows:

GetData = () => {
    var filename = getFilename();

    IStream stream = null;
    NativeMethods.SHCreateStreamOnFileEx(filename, NativeMethods.STGM_FAILIFTHERE, NativeMethods.FILE_ATTRIBUTE_NORMAL, false, null, ref stream);
    var ptr = Marshal.GetComInterfaceForObject(stream, typeof(IStream));
    Marshal.ReleaseComObject(stream);
    return new Tuple<IntPtr, int>(ptr, NativeMethods.S_OK);
}

As I say, I don't know whether this is the best way to do it or if I could manage it more cleanly, but this seems to work.