Interop.cs

using System;
using System.Runtime.InteropServices;

namespace ExtractAssociatedIcon
{
    public static class Interop
    {
        [DllImport("shell32.dll")]
        public static extern IntPtr SHGetFileInfo(string path, uint fileAttributes, ref SHFILEINFOW sfi, uint cbFileInfo, SHGFI flags);

        [DllImport("shell32.dll")]
        private static extern int SHGetImageList(SHIL iImageList, ref Guid iid, [MarshalAs(UnmanagedType.IUnknown, IidParameterIndex = 1)] out object imageList);

        public static IImageList SHGetImageList(SHIL iImageList)
        {
            var iid = typeof(IImageList).GUID;
            object imageList;
            int hr = SHGetImageList(iImageList, ref iid, out imageList);
            return hr >= 0
                ? (IImageList)imageList
                : null;
        }

        [Guid("46EB5926-582E-4017-9FDF-E8998DAA0950")]
        [ComImport]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        public interface IImageList
        {
            int Add(IntPtr hbmImage, IntPtr hbmMask);

            int ReplaceIcon(int i, SafeIconHandle hIcon);

            void SetOverlayImage(int iImage, int iOverlay);

            void Replace(int i, IntPtr hbmImage, IntPtr hbmMask);

            int AddMasked(IntPtr hbmImage, COLORREF crMask);

            void Draw(ref IMAGELISTDRAWPARAMS imldp);

            void Remove(int i);

            SafeIconHandle GetIcon(int i, uint flags);

            IMAGEINFO GetImageInfo(int i);

            void Copy(int iDst, [MarshalAs(UnmanagedType.IUnknown)] object punkSrc, int iSrc, uint uFlags);

            [return: MarshalAs(UnmanagedType.IUnknown, IidParameterIndex = 5)]
            void Merge(int i1, [MarshalAs(UnmanagedType.IUnknown)] object punk2, int i2, int dx, int dy, ref Guid riid);

            [return: MarshalAs(UnmanagedType.IUnknown, IidParameterIndex = 0)]

            object Clone(ref Guid riid);

            RECT GetImageRect(int i);

            int GetIconSize(out int cx);

            void SetIconSize(int cx, int cy);

            int GetImageCount();

            void SetImageCount(uint uNewCount);

            COLORREF SetBkColor(COLORREF clrBk);

            COLORREF GetBkColor();

            void BeginDrag(int iTrack, int dxHotspot, int dyHotspot);

            void EndDrag();

            void DragEnter(IntPtr IntPtrLock, int x, int y);

            void DragLeave(IntPtr IntPtrLock);

            void DragMove(int x, int y);

            void SetDragCursorImage([MarshalAs(UnmanagedType.IUnknown)] object punk, int iDrag, int dxHotspot, int dyHotspot);

            void DragShowNolock(bool fShow);

            [return: MarshalAs(UnmanagedType.IUnknown, IidParameterIndex = 2)]
            object GetDragImage(out POINT ppt, out POINT pptHotspot, ref Guid riid);

            uint GetItemFlags(int i);

            int GetOverlayImage(int iOverlay);
        };

        [Flags]
        public enum SFGAO : uint
        {
            CanCopy         = 0x00000001, // Objects can be copied    (0x1)
            CanMove         = 0x00000002, // Objects can be moved     (0x2)
            CanLink         = 0x00000004, // Objects can be linked    (0x4)
            CanRename       = 0x00000010, // Objects can be renamed
            CanDelete       = 0x00000020, // Objects can be deleted
            HasPropSheet    = 0x00000040, // Objects have property sheets
            DropTarget      = 0x00000100, // Objects are drop target
            CapabilityMask  = 0x00000177,
            Link            = 0x00010000, // Shortcut (link)
            Share           = 0x00020000, // shared
            ReadOnly        = 0x00040000, // read-only
            Ghosted         = 0x00080000, // ghosted icon
            Hidden          = 0x00080000, // hidden object
            DisplayAttrMask = 0x000F0000,
            FileSysAncestor = 0x10000000, // It contains file system folder
            Folder          = 0x20000000, // It's a folder.
            FileSystem      = 0x40000000, // is a file system thing (file/folder/root)
            HasSubfolder    = 0x80000000, // Expandable in the map pane
            ContentsMask    = 0x80000000,
            Validate        = 0x01000000, // invalidate cached information
            Removable       = 0x02000000, // is this removeable media?
            Compressed      = 0x04000000, // Object is compressed (use alt color)
            Browsable       = 0x08000000, // is in-place browsable
            NonEnumerated   = 0x00100000, // is a non-enumerated object
            NewContent      = 0x00200000, // should show bold in explorer tree
            CanMoniker      = 0x00400000, // can create monikers for its objects
        }

        [Flags]
        public enum SHGFI : uint
        {
            /// <summary>get icon</summary>
            Icon              = 0x000000100,
            /// <summary>get display name</summary>
            DisplayName       = 0x000000200,
            /// <summary>get type name</summary>
            TypeName          = 0x000000400,
            /// <summary>get attributes</summary>
            Attributes        = 0x000000800,
            /// <summary>get icon location</summary>
            IconLocation      = 0x000001000,
            /// <summary>return exe type</summary>
            ExeType           = 0x000002000,
            /// <summary>get system icon index</summary>
            SysIconIndex      = 0x000004000,
            /// <summary>put a link overlay on icon</summary>
            LinkOverlay       = 0x000008000,
            /// <summary>show icon in selected state</summary>
            Selected          = 0x000010000,
            /// <summary>get only specified attributes</summary>
            AttrSpecified     = 0x000020000,
            /// <summary></summary>
            LargeIcon         = 0x000000000,
            /// <summary>get small icon</summary>
            SmallIcon         = 0x000000001,
            /// <summary>get open icon</summary>
            OpenIcon          = 0x000000002,
            /// <summary>get shell size icon</summary>
            ShellIconSize     = 0x000000004,
            /// <summary>pszPath is a pidl</summary>
            Pidl              = 0x000000008,
            /// <summary>use passed dwFileAttribute</summary>
            UseFileAttributes = 0x000000010,
            /// <summary>apply the appropriate overlays</summary>
            AddOverlays       = 0x000000020,
            /// <summary>Get the index of the overlay in the upper 8 bits of the iIcon</summary>
            OverlayIndex      = 0x000000040,
        }

        public enum SHIL
        {
            /// <summary>normally 32x32</summary>
            Large      = 0,
            /// <summary>normally 16x16</summary>
            Small = 1,
            /// <summary>normally 48x48?</summary>
            ExtraLarge = 2,
            /// <summary>like SHIL_SMALL, but tracks system small icon metric correctly</summary>
            SysSmall = 3,
            /// <summary>normally 256x256</summary>
            Jumbo = 4,
        }

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public struct SHFILEINFOW
        {
            public IntPtr hIcon;
            public int iIcon;
            public SFGAO attributes;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
            public string displayName;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
            public string typeName;
        };

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct POINT
        {
            public int left;
            public int top;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct COLORREF
        {
            public uint value;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct IMAGEINFO
        {
            IntPtr hbmImage;
            IntPtr hbmMask;
            int Unused1;
            int Unused2;
            RECT rcImage;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct IMAGELISTDRAWPARAMS
        {
            uint cbSize;
            IntPtr himl;
            int i;
            IntPtr hdcDst;
            int x;
            int y;
            int cx;
            int cy;
            int xBitmap;        // x offest from the upperleft of bitmap
            int yBitmap;        // y offset from the upperleft of bitmap
            COLORREF rgbBk;
            COLORREF rgbFg;
            uint fStyle;
            uint dwRop;
            uint fState;
            uint Frame;
            COLORREF crEffect;
        }
    }
}