QuickImage – Rev 1

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using System;
using System.IO;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;
using System.Windows.Forms;

namespace QuickImage.Utilities.Extensions
{
    /// <summary>Helper methods for getting FileContents from DragDrop data.</summary>
    /// <see cref="!:https://www.codeproject.com/Articles/28209/Outlook-Drag-and-Drop-in-C"/>
    /// <remarks>https://stackoverflow.com/questions/49695870/how-can-i-retrieve-the-drag-drop-data-from-chrome-like-windows-explorer-does</remarks>
    public static class IDataObjectExtensionMethods
    {
        /// <summary>Gets the array of FileNames from the FileGroupDescriptors format.</summary>
        public static string[] GetFileContentNames(this System.Windows.Forms.IDataObject data)
        {
            var names = new string[data.GetFileContentCount()];

            if (names.Length != 0)
            {
                var bytes = data.GetFileGroupDescriptor().ToArray();
                IntPtr fgdPtr = IntPtr.Zero;
                try
                {
                    fgdPtr = Marshal.AllocHGlobal(bytes.Length);

                    int offset = Marshal.SizeOf(typeof(UInt32));
                    int size = Marshal.SizeOf(typeof(FILEDESCRIPTORW));

                    for (int i = 0; i < names.Length; i++)
                    {
                        var fd = (FILEDESCRIPTORW)Marshal.PtrToStructure(fgdPtr + offset + (i * size), typeof(FILEDESCRIPTORW));
                        names[i] = fd.cFileName;
                    }

                }
                finally
                {
                    if (fgdPtr != IntPtr.Zero) Marshal.FreeHGlobal(fgdPtr);

                }
            }

            return names;
        }

        /// <summary>Gets the number of files available in the FileGroupDescriptor format.</summary>
        public static int GetFileContentCount(this System.Windows.Forms.IDataObject data)
        {
            // File count is stored as an UInt32 in the FileGroupDescriptor format
            MemoryStream ms = data.GetFileGroupDescriptor();
            if (ms == null) return 0;

            using (var reader = new BinaryReader(ms))
            {
                return (int)reader.ReadUInt32(); // Assume this won't overflow!
            }
        }

        /// <summary>Gets the file content for the specified FileDescriptor index.</summary>
        /// <param name="index">The index of the file content to retrieve.</param>
        public static MemoryStream GetFileContent(this System.Windows.Forms.IDataObject data, int index)
        {
            // As this is indexed, "FileContent" is most likely null, so the COM IDataObject needs to be used
            var comData = (System.Runtime.InteropServices.ComTypes.IDataObject)data;

            var formatetc = new FORMATETC()
            {
                cfFormat = (short)DataFormats.GetFormat("FileContents").Id,
                dwAspect = DVASPECT.DVASPECT_CONTENT,
                lindex = index,
                ptd = IntPtr.Zero,
                tymed = TYMED.TYMED_ISTREAM | TYMED.TYMED_HGLOBAL
            };

            var medium = new STGMEDIUM();
            comData.GetData(ref formatetc, out medium);

            switch (medium.tymed)
            {
                case TYMED.TYMED_HGLOBAL:
                    return data.GetFileContentFromHGlobal(medium);

                case TYMED.TYMED_ISTREAM:
                    return data.GetFileContentFromIStream(medium);

                default:
                    throw new InvalidOperationException($"Cannot get FileContent for {medium.tymed} TYMED.");

            }
        }

        private static MemoryStream GetFileContentFromHGlobal(this System.Windows.Forms.IDataObject data, STGMEDIUM medium)
        {
            var innerDataField = data.GetType().GetField("innerData", BindingFlags.NonPublic | BindingFlags.Instance);
            var oldData = (System.Windows.Forms.IDataObject)innerDataField.GetValue(data);

            var getDataFromHGLOBLALMethod = oldData.GetType().GetMethod("GetDataFromHGLOBLAL", BindingFlags.NonPublic | BindingFlags.Instance);

            return (MemoryStream)getDataFromHGLOBLALMethod.Invoke(oldData, new object[] { "FileContents", medium.unionmember });
        }

        private static MemoryStream GetFileContentFromIStream(this System.Windows.Forms.IDataObject data, STGMEDIUM medium)
        {
            var iStream = (IStream)Marshal.GetObjectForIUnknown(medium.unionmember);
            Marshal.Release(medium.unionmember);

            var iStreamStat = new System.Runtime.InteropServices.ComTypes.STATSTG();
            iStream.Stat(out iStreamStat, 0);

            var content = new byte[(int)iStreamStat.cbSize];
            iStream.Read(content, content.Length, IntPtr.Zero);

            return new MemoryStream(content);
        }

        private static MemoryStream GetFileGroupDescriptor(this System.Windows.Forms.IDataObject data)
        {
            MemoryStream ms = null;
            if (data.GetDataPresent("FileGroupDescriptorW"))
            {
                ms = (MemoryStream)data.GetData("FileGroupDescriptorW", true);
            }

            return ms;
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        private struct FILEDESCRIPTORW
        {
            public UInt32 dwFlags;
            public Guid clsid;
            public System.Drawing.Size sizel;
            public System.Drawing.Point pointl;
            public UInt32 dwFileAttributes;
            public System.Runtime.InteropServices.ComTypes.FILETIME ftCreationTime;
            public System.Runtime.InteropServices.ComTypes.FILETIME ftLastAccessTime;
            public System.Runtime.InteropServices.ComTypes.FILETIME ftLastWriteTime;
            public UInt32 nFileSizeHigh;
            public UInt32 nFileSizeLow;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
            public String cFileName;
        }
    }
}

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