AccessViolationException after copying a file from inside a zip archive to the clipboard and handling its data

Question

I have an issue I am having trouble with and hope somebody here can help.

We use somewhat modified code from this OutlookDataObject project to handle files dropped from various mail clients like outlook as well as the general copy pasting of files via clipboard to process them and save them to the file system.

It works fine in most cases, however if a user opens a ZIP file with the Windows Explorer, copys a file from there and tries to add it we get an AccessViolationException at the following part of the GetData Method of the FileContentDataObject:

//marshal the unmanaged memory to to FILEGROUPDESCRIPTORW struct
object fileGroupDescriptorObject = Marshal.PtrToStructure(fileGroupDescriptorWPointer, typeof(NativeMethods.FILEGROUPDESCRIPTORW));

Here is a reduced version of the whole code in question with which you should be able to reproduce this issue:

// Starting Method
public void InsertFromClipboard()
{ 
    FileContentDataObjectBase dataObject = GetDataObject();
    if(dataObject!= null)
    {
        dataObject.SaveToFileSystem("C:/temp");
    }
}

private FileContentDataObjectBase GetDataObject()
{
    var dataObject = System.Windows.Forms.Clipboard.GetDataObject();

    return new FileContentDataObject(System.Windows.Forms.Clipboard.GetDataObject());
}

public class FileContentDataObject : FileContentDataObjectBase
{
    /// <summary>
    /// Initializes a new instance of the <see cref="OutlookDataObject"/> class.
    /// </summary>
    /// <param name="underlyingDataObject">The underlying data object to wrap.</param>
    public FileContentDataObject(System.Windows.Forms.IDataObject underlyingDataObject)
        : base(underlyingDataObject) { }

    public override void SaveToFileSystem(string path)
    {
        string[] filenames;

        //get the names and data streams of the files dropped
        if (this.GetFormats().Contains("FileGroupDescriptor"))
            filenames = (string[])this.GetData("FileGroupDescriptor", true);
        else if (this.GetFormats().Contains("FileGroupDescriptorW"))
            filenames = (string[])this.GetData("FileGroupDescriptorW", true);
        else
            return;

        MemoryStream[] filestreams = (MemoryStream[])this.GetData("FileContents");

        SaveToFileSystem(filenames, filestreams, path);
     }

     public new object GetData(string format, bool autoConvert)
     {
         switch (format)
         {
            case "FileGroupDescriptorW":
                //override the default handling of FileGroupDescriptorW which returns a
                //MemoryStream and instead return a string array of file names
                IntPtr fileGroupDescriptorWPointer = IntPtr.Zero;
                try
                {
                    //use the underlying IDataObject to get the FileGroupDescriptorW as a MemoryStream
                    MemoryStream fileGroupDescriptorStream = (MemoryStream)this.underlyingDataObject.GetData("FileGroupDescriptorW");
                    byte[] fileGroupDescriptorBytes = new byte[fileGroupDescriptorStream.Length];
                    fileGroupDescriptorStream.Read(fileGroupDescriptorBytes, 0, fileGroupDescriptorBytes.Length);
                    fileGroupDescriptorStream.Close();

                    //copy the file group descriptor into unmanaged memory
                    fileGroupDescriptorWPointer = Marshal.AllocHGlobal(fileGroupDescriptorBytes.Length);
                    Marshal.Copy(fileGroupDescriptorBytes, 0, fileGroupDescriptorWPointer, fileGroupDescriptorBytes.Length);

                    //marshal the unmanaged memory to to FILEGROUPDESCRIPTORW struct
                    object fileGroupDescriptorObject = Marshal.PtrToStructure(fileGroupDescriptorWPointer, typeof(NativeMethods.FILEGROUPDESCRIPTORW));
                    NativeMethods.FILEGROUPDESCRIPTORW fileGroupDescriptor = (NativeMethods.FILEGROUPDESCRIPTORW)fileGroupDescriptorObject;

                    //create a new array to store file names in of the number of items in the file group descriptor
                    string[] fileNames = new string[fileGroupDescriptor.cItems];

                    //get the pointer to the first file descriptor
                    IntPtr fileDescriptorPointer = (IntPtr)((int)fileGroupDescriptorWPointer + Marshal.SizeOf(fileGroupDescriptor.cItems));

                    //loop for the number of files acording to the file group descriptor
                    for (int fileDescriptorIndex = 0; fileDescriptorIndex < fileGroupDescriptor.cItems; fileDescriptorIndex++)
                    {
                        //marshal the pointer top the file descriptor as a FILEDESCRIPTORW struct and get the file name
                        NativeMethods.FILEDESCRIPTORW fileDescriptor = (NativeMethods.FILEDESCRIPTORW)Marshal.PtrToStructure(fileDescriptorPointer, typeof(NativeMethods.FILEDESCRIPTORW));
                        fileNames[fileDescriptorIndex] = fileDescriptor.cFileName;

                        //move the file descriptor pointer to the next file descriptor
                        fileDescriptorPointer = (IntPtr)((int)fileDescriptorPointer + Marshal.SizeOf(fileDescriptor));
                    }

                    //return the array of filenames
                    return fileNames;
                }
                finally
                {
                    //free unmanaged memory pointer
                    Marshal.FreeHGlobal(fileGroupDescriptorWPointer);
                }
        }

            //use underlying IDataObject to handle getting of data
            return this.underlyingDataObject.GetData(format, autoConvert);
        }
    }

    public abstract class FileContentDataObjectBase : System.Windows.Forms.IDataObject
    {
        #region NativeMethods

        protected class NativeMethods
        {
            [DllImport("kernel32.dll")]
            static extern IntPtr GlobalLock(IntPtr hMem);

            [DllImport("ole32.dll", PreserveSig = false)]
            internal static extern ILockBytes CreateILockBytesOnHGlobal(IntPtr hGlobal, bool fDeleteOnRelease);

            [DllImport("OLE32.DLL", CharSet = CharSet.Auto, PreserveSig = false)]
            internal static extern IntPtr GetHGlobalFromILockBytes(ILockBytes pLockBytes);

            [DllImport("OLE32.DLL", CharSet = CharSet.Unicode, PreserveSig = false)]
            internal static extern IStorage StgCreateDocfileOnILockBytes(ILockBytes plkbyt, uint grfMode, uint reserved);

            [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("0000000B-0000-0000-C000-000000000046")]
            internal interface IStorage
            {
                [return: MarshalAs(UnmanagedType.Interface)]
                IStream CreateStream([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, [In, MarshalAs(UnmanagedType.U4)] int grfMode, [In, MarshalAs(UnmanagedType.U4)] int reserved1, [In, MarshalAs(UnmanagedType.U4)] int reserved2);
                [return: MarshalAs(UnmanagedType.Interface)]
                IStream OpenStream([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, IntPtr reserved1, [In, MarshalAs(UnmanagedType.U4)] int grfMode, [In, MarshalAs(UnmanagedType.U4)] int reserved2);
                [return: MarshalAs(UnmanagedType.Interface)]
                IStorage CreateStorage([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, [In, MarshalAs(UnmanagedType.U4)] int grfMode, [In, MarshalAs(UnmanagedType.U4)] int reserved1, [In, MarshalAs(UnmanagedType.U4)] int reserved2);
                [return: MarshalAs(UnmanagedType.Interface)]
                IStorage OpenStorage([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, IntPtr pstgPriority, [In, MarshalAs(UnmanagedType.U4)] int grfMode, IntPtr snbExclude, [In, MarshalAs(UnmanagedType.U4)] int reserved);
                void CopyTo(int ciidExclude, [In, MarshalAs(UnmanagedType.LPArray)] Guid[] pIIDExclude, IntPtr snbExclude, [In, MarshalAs(UnmanagedType.Interface)] IStorage stgDest);
                void MoveElementTo([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, [In, MarshalAs(UnmanagedType.Interface)] IStorage stgDest, [In, MarshalAs(UnmanagedType.BStr)] string pwcsNewName, [In, MarshalAs(UnmanagedType.U4)] int grfFlags);
                void Commit(int grfCommitFlags);
                void Revert();
                void EnumElements([In, MarshalAs(UnmanagedType.U4)] int reserved1, IntPtr reserved2, [In, MarshalAs(UnmanagedType.U4)] int reserved3, [MarshalAs(UnmanagedType.Interface)] out object ppVal);
                void DestroyElement([In, MarshalAs(UnmanagedType.BStr)] string pwcsName);
                void RenameElement([In, MarshalAs(UnmanagedType.BStr)] string pwcsOldName, [In, MarshalAs(UnmanagedType.BStr)] string pwcsNewName);
                void SetElementTimes([In, MarshalAs(UnmanagedType.BStr)] string pwcsName, [In] System.Runtime.InteropServices.ComTypes.FILETIME pctime, [In] System.Runtime.InteropServices.ComTypes.FILETIME patime, [In] System.Runtime.InteropServices.ComTypes.FILETIME pmtime);
                void SetClass([In] ref Guid clsid);
                void SetStateBits(int grfStateBits, int grfMask);
                void Stat([Out]out System.Runtime.InteropServices.ComTypes.STATSTG pStatStg, int grfStatFlag);
            }

            [ComImport, Guid("0000000A-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
            internal interface ILockBytes
            {
                void ReadAt([In, MarshalAs(UnmanagedType.U8)] long ulOffset, [Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] byte[] pv, [In, MarshalAs(UnmanagedType.U4)] int cb, [Out, MarshalAs(UnmanagedType.LPArray)] int[] pcbRead);
                void WriteAt([In, MarshalAs(UnmanagedType.U8)] long ulOffset, IntPtr pv, [In, MarshalAs(UnmanagedType.U4)] int cb, [Out, MarshalAs(UnmanagedType.LPArray)] int[] pcbWritten);
                void Flush();
                void SetSize([In, MarshalAs(UnmanagedType.U8)] long cb);
                void LockRegion([In, MarshalAs(UnmanagedType.U8)] long libOffset, [In, MarshalAs(UnmanagedType.U8)] long cb, [In, MarshalAs(UnmanagedType.U4)] int dwLockType);
                void UnlockRegion([In, MarshalAs(UnmanagedType.U8)] long libOffset, [In, MarshalAs(UnmanagedType.U8)] long cb, [In, MarshalAs(UnmanagedType.U4)] int dwLockType);
                void Stat([Out]out System.Runtime.InteropServices.ComTypes.STATSTG pstatstg, [In, MarshalAs(UnmanagedType.U4)] int grfStatFlag);
            }

            [StructLayout(LayoutKind.Sequential)]
            internal sealed class POINTL
            {
                public int x;
                public int y;
            }

            [StructLayout(LayoutKind.Sequential)]
            internal sealed class SIZEL
            {
                public int cx;
                public int cy;
            }

            [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
            internal sealed class FILEGROUPDESCRIPTORA
            {
                public uint cItems;
                public FILEDESCRIPTORA[] fgd;
            }

            [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
            internal sealed class FILEDESCRIPTORA
            {
                public uint dwFlags;
                public Guid clsid;
                public SIZEL sizel;
                public POINTL pointl;
                public uint dwFileAttributes;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftCreationTime;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftLastAccessTime;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftLastWriteTime;
                public uint nFileSizeHigh;
                public uint nFileSizeLow;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
                public string cFileName;
            }

            [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
            internal sealed class FILEGROUPDESCRIPTORW
            {
                public uint cItems;
                public FILEDESCRIPTORW[] fgd;
            }

            [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
            internal sealed class FILEDESCRIPTORW
            {
                public uint dwFlags;
                public Guid clsid;
                public SIZEL sizel;
                public POINTL pointl;
                public uint dwFileAttributes;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftCreationTime;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftLastAccessTime;
                public System.Runtime.InteropServices.ComTypes.FILETIME ftLastWriteTime;
                public uint nFileSizeHigh;
                public uint nFileSizeLow;
                [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
                public string cFileName;
            }
        }

        #endregion

        #region Property(s)

        /// <summary>
        /// Holds the <see cref="System.Windows.Forms.IDataObject"/> that this class is wrapping
        /// </summary>
        protected System.Windows.Forms.IDataObject underlyingDataObject;

        /// <summary>
        /// Holds the <see cref="System.Runtime.InteropServices.ComTypes.IDataObject"/> interface to the <see cref="System.Windows.Forms.IDataObject"/> that this class is wrapping.
        /// </summary>
        protected System.Runtime.InteropServices.ComTypes.IDataObject comUnderlyingDataObject;

        /// <summary>
        /// Holds the internal ole <see cref="System.Windows.Forms.IDataObject"/> to the <see cref="System.Windows.Forms.IDataObject"/> that this class is wrapping.
        /// </summary>
        protected System.Windows.Forms.IDataObject oleUnderlyingDataObject;

        /// <summary>
        /// Holds the <see cref="MethodInfo"/> of the "GetDataFromHGLOBLAL" method of the internal ole <see cref="System.Windows.Forms.IDataObject"/>.
        /// </summary>
        protected MethodInfo getDataFromHGLOBLALMethod;

        #endregion

        #region Constructor(s)

        /// <summary>
        /// Initializes a new instance of the <see cref="OutlookDataObject"/> class.
        /// </summary>
        /// <param name="underlyingDataObject">The underlying data object to wrap.</param>
        public FileContentDataObjectBase(System.Windows.Forms.IDataObject underlyingDataObject)
        {
            //get the underlying dataobject and its ComType IDataObject interface to it
            this.underlyingDataObject = underlyingDataObject;
            this.comUnderlyingDataObject = (System.Runtime.InteropServices.ComTypes.IDataObject)this.underlyingDataObject;

            //get the internal ole dataobject and its GetDataFromHGLOBLAL so it can be called later
            FieldInfo innerDataField = this.underlyingDataObject.GetType().GetField("innerData", BindingFlags.NonPublic | BindingFlags.Instance);
            this.oleUnderlyingDataObject = (System.Windows.Forms.IDataObject)innerDataField.GetValue(this.underlyingDataObject);
            this.getDataFromHGLOBLALMethod = this.oleUnderlyingDataObject.GetType().GetMethod("GetDataFromHGLOBLAL", BindingFlags.NonPublic | BindingFlags.Instance);
        }

        #endregion

        #region IDataObject Members

        /// <summary>
        /// Retrieves the data associated with the specified class type format.
        /// </summary>
        /// <param name="format">A <see cref="T:System.Type"></see> representing the format of the data to retrieve. See <see cref="T:System.Windows.Forms.DataFormats"></see> for predefined formats.</param>
        /// <returns>
        /// The data associated with the specified format, or null.
        /// </returns>
        public object GetData(Type format)
        {
            return this.GetData(format.FullName);
        }

        /// <summary>
        /// Retrieves the data associated with the specified data format.
        /// </summary>
        /// <param name="format">The format of the data to retrieve. See <see cref="T:System.Windows.Forms.DataFormats"></see> for predefined formats.</param>
        /// <returns>
        /// The data associated with the specified format, or null.
        /// </returns>
        public object GetData(string format)
        {
            return this.GetData(format, true);
        }

        /// <summary>
        /// Retrieves the data associated with the specified data format, using a Boolean to determine whether to convert the data to the format.
        /// </summary>
        /// <param name="format">The format of the data to retrieve. See <see cref="T:System.Windows.Forms.DataFormats"></see> for predefined formats.</param>
        /// <param name="autoConvert">true to convert the data to the specified format; otherwise, false.</param>
        /// <returns>
        /// The data associated with the specified format, or null.
        /// </returns>
        public object GetData(string format, bool autoConvert)
        {
            return this.underlyingDataObject.GetData(format, autoConvert);
        }

        /// <summary>
        /// Retrieves the data associated with the specified data format at the specified index.
        /// </summary>
        /// <param name="format">The format of the data to retrieve. See <see cref="T:System.Windows.Forms.DataFormats"></see> for predefined formats.</param>
        /// <param name="index">The index of the data to retrieve.</param>
        /// <returns>
        /// A <see cref="MemoryStream"/> containing the raw data for the specified data format at the specified index.
        /// </returns>
        public MemoryStream GetData(string format, int index)
        {
            //create a FORMATETC struct to request the data with
            FORMATETC formatetc = new FORMATETC();
            formatetc.cfFormat = (short)DataFormats.GetFormat(format).Id;
            formatetc.dwAspect = DVASPECT.DVASPECT_CONTENT;
            formatetc.lindex = index;
            formatetc.ptd = new IntPtr(0);
            formatetc.tymed = TYMED.TYMED_ISTREAM | TYMED.TYMED_ISTORAGE | TYMED.TYMED_HGLOBAL;

            //create STGMEDIUM to output request results into
            STGMEDIUM medium = new STGMEDIUM();

            //using the Com IDataObject interface get the data using the defined FORMATETC
            this.comUnderlyingDataObject.GetData(ref formatetc, out medium);

            //retrieve the data depending on the returned store type
            switch (medium.tymed)
            {
                case TYMED.TYMED_ISTORAGE:
                    //to handle a IStorage it needs to be written into a second unmanaged
                    //memory mapped storage and then the data can be read from memory into
                    //a managed byte and returned as a MemoryStream

                    NativeMethods.IStorage iStorage = null;
                    NativeMethods.IStorage iStorage2 = null;
                    NativeMethods.ILockBytes iLockBytes = null;
                    System.Runtime.InteropServices.ComTypes.STATSTG iLockBytesStat;
                    try
                    {
                        //marshal the returned pointer to a IStorage object
                        iStorage = (NativeMethods.IStorage)Marshal.GetObjectForIUnknown(medium.unionmember);
                        Marshal.Release(medium.unionmember);

                        //create a ILockBytes (unmanaged byte array) and then create a IStorage using the byte array as a backing store
                        iLockBytes = NativeMethods.CreateILockBytesOnHGlobal(IntPtr.Zero, true);
                        iStorage2 = NativeMethods.StgCreateDocfileOnILockBytes(iLockBytes, 0x00001012, 0);

                        //copy the returned IStorage into the new IStorage
                        iStorage.CopyTo(0, null, IntPtr.Zero, iStorage2);
                        iLockBytes.Flush();
                        iStorage2.Commit(0);

                        //get the STATSTG of the ILockBytes to determine how many bytes were written to it
                        iLockBytesStat = new System.Runtime.InteropServices.ComTypes.STATSTG();
                        iLockBytes.Stat(out iLockBytesStat, 1);
                        int iLockBytesSize = (int)iLockBytesStat.cbSize;

                        //read the data from the ILockBytes (unmanaged byte array) into a managed byte array
                        byte[] iLockBytesContent = new byte[iLockBytesSize];
                        iLockBytes.ReadAt(0, iLockBytesContent, iLockBytesContent.Length, null);

                        //wrapped the managed byte array into a memory stream and return it
                        return new MemoryStream(iLockBytesContent);
                    }
                    finally
                    {
                        //release all unmanaged objects
                        Marshal.ReleaseComObject(iStorage2);
                        Marshal.ReleaseComObject(iLockBytes);
                        Marshal.ReleaseComObject(iStorage);
                    }

                case TYMED.TYMED_ISTREAM:
                    //to handle a IStream it needs to be read into a managed byte and
                    //returned as a MemoryStream

                    IStream iStream = null;
                    System.Runtime.InteropServices.ComTypes.STATSTG iStreamStat;
                    try
                    {
                        //marshal the returned pointer to a IStream object
                        iStream = (IStream)Marshal.GetObjectForIUnknown(medium.unionmember);
                        Marshal.Release(medium.unionmember);

                        //get the STATSTG of the IStream to determine how many bytes are in it
                        iStreamStat = new System.Runtime.InteropServices.ComTypes.STATSTG();
                        iStream.Stat(out iStreamStat, 0);
                        int iStreamSize = (int)iStreamStat.cbSize;

                        //read the data from the IStream into a managed byte array
                        byte[] iStreamContent = new byte[iStreamSize];
                        iStream.Read(iStreamContent, iStreamContent.Length, IntPtr.Zero);

                        //wrapped the managed byte array into a memory stream and return it
                        return new MemoryStream(iStreamContent);
                    }
                    finally
                    {
                        //release all unmanaged objects
                        Marshal.ReleaseComObject(iStream);
                    }

                case TYMED.TYMED_HGLOBAL:
                    //to handle a HGlobal the exisitng "GetDataFromHGLOBLAL" method is invoked via
                    //reflection

                    return (MemoryStream)this.getDataFromHGLOBLALMethod.Invoke(this.oleUnderlyingDataObject, new object[] { DataFormats.GetFormat((short)formatetc.cfFormat).Name, medium.unionmember });
            }

            return null;
        }

        /// <summary>
        /// Returns a list of all formats that data stored in this instance is associated with or can be converted to.
        /// </summary>
        /// <returns>
        /// An array of the names that represents a list of all formats that are supported by the data stored in this object.
        /// </returns>
        public string[] GetFormats()
        {
            return this.underlyingDataObject.GetFormats();
        }

        /// <summary>
        /// Gets a list of all formats that data stored in this instance is associated with or can be converted to, using a Boolean value to determine whether to retrieve all formats that the data can be converted to or only native data formats.
        /// </summary>
        /// <param name="autoConvert">true to retrieve all formats that data stored in this instance is associated with or can be converted to; false to retrieve only native data formats.</param>
        /// <returns>
        /// An array of the names that represents a list of all formats that are supported by the data stored in this object.
        /// </returns>
        public string[] GetFormats(bool autoConvert)
        {
            return this.underlyingDataObject.GetFormats(autoConvert);
        }

        #endregion

        #region methods

        public static void SaveToFileSystem(string[] filenames, MemoryStream[] filestreams, string path)
        {

            for (int fileIndex = 0; fileIndex < filenames.Length; fileIndex++)
            {
                try
                {
                    //use the fileindex to get the name and data stream
                    string filename = filenames[fileIndex];
                    MemoryStream filestream = filestreams[fileIndex];

                    //save the file stream using its name to the application path
                    FileStream outputStream = File.Create(Path.Combine(path, GetStrippedFileName(filename)));
                    filestream.WriteTo(outputStream);
                    outputStream.Close();
                }
                catch (IOException ex)
                {
                    throw new Exception(ex.Message, ex);
                }
                catch (SecurityException ex)
                {
                    throw new Exception(ex.Message, ex);
                }
            }

        }

        public static void SaveToFileSystem(FileContentDataObjectBase dataObject, string path)
        {
            dataObject.SaveToFileSystem(path);
        }

        public abstract void SaveToFileSystem(string path);

        private static string GetStrippedFileName(string input)
        {
            foreach (var chr in Path.GetInvalidFileNameChars())
                input = input.Replace(chr.ToString(), String.Empty);
            return input;
        }

        #endregion

}

I would like to figure out why it happens and how I can fix the issue.

Any help or insights would be welcome.

Thanks in advance.

Answer 1

After searching for quite a while I finally found the problem thanks to this helpful comment.

As it turns out the FILEGROUPDESCRIPTORA and FILEGROUPDESCRIPTORW declarations in the original code we used are wrong which can cause an AccessViolation Exception in some cases.

So I did change the declarations like the commentor described from...

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public sealed class FILEGROUPDESCRIPTORA
{
    public uint cItems;
    public FILEDESCRIPTORA[] fgd;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public sealed class FILEGROUPDESCRIPTORW
{
    public uint cItems;
    public FILEDESCRIPTORW[] fgd;
}

To...

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public sealed class FILEGROUPDESCRIPTORA
{
    public uint cItems;
    public FILEDESCRIPTORA fgd;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public sealed class FILEGROUPDESCRIPTORW
{
    public uint cItems;
    public FILEDESCRIPTORW fgd;
}

Afterwards the AccessViolation Exceptions were gone and I only had to modify the original code I linked in my question a little more to make it work.

I had to add some additional checks during the handling of the Clipboard files 'FileContents' format to make sure the correct of the two FILEGROUPDESCRIPTORs was used each time, but now it works perfectly again.

Answer 2

There could still be an error in your code. That first line below in the handler for 'FILEGROUPDESCRIPTORW' can/will fail on a Win8 machine.

replace

object fileGroupDescriptorObject = Marshal.PtrToStructure(fileGroupDescriptorWPointer, typeof(NativeMethods.FILEGROUPDESCRIPTORW));
NativeMethods.FILEGROUPDESCRIPTORW fgd = (NativeMethods.FILEGROUPDESCRIPTORW)fileGroupDescriptorObject;
files = new NativeMethods.FILEDESCRIPTOR[fgd.cItems];
pdata = (IntPtr)((int)pdata + Marshal.SizeOf(pdata));
for (int index = 0; index < fgd.cItems; index++)   

with

int ITEMCOUNT = Marshal.ReadInt32(pdata);                
files = new NativeMethods.FILEDESCRIPTOR[ITEMCOUNT];
// Set our pointer offset to the beginning of the FILEDESCRIPTOR* array
pdata = (IntPtr)((long)pdata + Marshal.SizeOf(pdata));
// Walk the array, converting each FILEDESCRIPTOR* to a FILEDESCRIPTOR
for (int index = 0; index < ITEMCOUNT; index++)

For more details, see:

Drag-and-Drop multiple Attached File From Outlook to C# Window Form