Extract thumbnail for any file in Windows

This is an improved version of the (awesome) answer that was accepted. The following was changed:

• Use unsafe code instead of SetPixel() -> that improves performance drastically

• remove the whole isAlphaBitmap check. I have no idea why that was implemented in the first place, but I see no reason to determine between two images which contain the same content (I just return the Bitmap with transparency, regardless if that changed something)

  [Flags]
  public enum ThumbnailOptions
  {
      None = 0x00,
      BiggerSizeOk = 0x01,
      InMemoryOnly = 0x02,
      IconOnly = 0x04,
      ThumbnailOnly = 0x08,
      InCacheOnly = 0x10
  }
  
  public class WindowsThumbnailProvider
  {
      private const string IShellItem2Guid = "7E9FB0D3-919F-4307-AB2E-9B1860310C93";
  
      [DllImport("shell32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
      internal static extern int SHCreateItemFromParsingName(
          [MarshalAs(UnmanagedType.LPWStr)] string path,
          // The following parameter is not used - binding context.
          IntPtr pbc,
          ref Guid riid,
          [MarshalAs(UnmanagedType.Interface)] out IShellItem shellItem);
  
      [DllImport("gdi32.dll")]
      [return: MarshalAs(UnmanagedType.Bool)]
      internal static extern bool DeleteObject(IntPtr hObject);
  
      [DllImport("msvcrt.dll", CallingConvention = CallingConvention.Cdecl)]
      public static extern unsafe IntPtr memcpy(void* dst, void* src, UIntPtr count);
  
      public static Bitmap GetThumbnail(string fileName, int width, int height, ThumbnailOptions options)
      {
          var hBitmap = GetHBitmap(Path.GetFullPath(fileName), width, height, options);
  
          try
          {
              // return a System.Drawing.Bitmap from the hBitmap
              return GetBitmapFromHBitmap(hBitmap);
          }
          finally
          {
              // delete HBitmap to avoid memory leaks
              DeleteObject(hBitmap);
          }
      }
  
      public static Bitmap GetBitmapFromHBitmap(IntPtr nativeHBitmap)
      {
          Bitmap bmp = Image.FromHbitmap(nativeHBitmap);
  
          if (Image.GetPixelFormatSize(bmp.PixelFormat) < 32)
              return bmp;
  
          using (bmp)
              return CreateAlphaBitmap(bmp, PixelFormat.Format32bppArgb);
      }
  
      public static unsafe Bitmap CreateAlphaBitmap(Bitmap srcBitmap, PixelFormat targetPixelFormat)
      {
          var result = new Bitmap(srcBitmap.Width, srcBitmap.Height, targetPixelFormat);
  
          var bmpBounds = new Rectangle(0, 0, srcBitmap.Width, srcBitmap.Height);
          var srcData = srcBitmap.LockBits(bmpBounds, ImageLockMode.ReadOnly, srcBitmap.PixelFormat);
          var destData = result.LockBits(bmpBounds, ImageLockMode.ReadOnly, targetPixelFormat);
  
          var srcDataPtr = (byte*) srcData.Scan0;
          var destDataPtr = (byte*) destData.Scan0;
  
          try
          {
              for (int y = 0; y <= srcData.Height - 1; y++)
              {
                  for (int x = 0; x <= srcData.Width - 1; x++)
                  {
                      //this is really important because one stride may be positive and the other negative
                      var position = srcData.Stride * y + 4 * x;
                      var position2 = destData.Stride * y + 4 * x;
  
                      memcpy(destDataPtr + position2, srcDataPtr + position, (UIntPtr) 4);
                  }
              }
          }
          finally
          {
              srcBitmap.UnlockBits(srcData);
              result.UnlockBits(destData);
          }
  
          return result;
      }
  
      private static IntPtr GetHBitmap(string fileName, int width, int height, ThumbnailOptions options)
      {
          IShellItem nativeShellItem;
          Guid shellItem2Guid = new Guid(IShellItem2Guid);
          int retCode = SHCreateItemFromParsingName(fileName, IntPtr.Zero, ref shellItem2Guid, out nativeShellItem);
  
          if (retCode != 0)
              throw Marshal.GetExceptionForHR(retCode);
  
          NativeSize nativeSize = new NativeSize();
          nativeSize.Width = width;
          nativeSize.Height = height;
  
          IntPtr hBitmap;
          HResult hr = ((IShellItemImageFactory) nativeShellItem).GetImage(nativeSize, options, out hBitmap);
  
          Marshal.ReleaseComObject(nativeShellItem);
  
          if (hr == HResult.Ok) return hBitmap;
  
          throw Marshal.GetExceptionForHR((int) hr);
      }
  
      [ComImport]
      [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
      [Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe")]
      internal interface IShellItem
      {
          void BindToHandler(IntPtr pbc,
              [MarshalAs(UnmanagedType.LPStruct)] Guid bhid,
              [MarshalAs(UnmanagedType.LPStruct)] Guid riid,
              out IntPtr ppv);
  
          void GetParent(out IShellItem ppsi);
          void GetDisplayName(SIGDN sigdnName, out IntPtr ppszName);
          void GetAttributes(uint sfgaoMask, out uint psfgaoAttribs);
          void Compare(IShellItem psi, uint hint, out int piOrder);
      }
  
      internal enum SIGDN : uint
      {
          NORMALDISPLAY = 0,
          PARENTRELATIVEPARSING = 0x80018001,
          PARENTRELATIVEFORADDRESSBAR = 0x8001c001,
          DESKTOPABSOLUTEPARSING = 0x80028000,
          PARENTRELATIVEEDITING = 0x80031001,
          DESKTOPABSOLUTEEDITING = 0x8004c000,
          FILESYSPATH = 0x80058000,
          URL = 0x80068000
      }
  
      internal enum HResult
      {
          Ok = 0x0000,
          False = 0x0001,
          InvalidArguments = unchecked((int) 0x80070057),
          OutOfMemory = unchecked((int) 0x8007000E),
          NoInterface = unchecked((int) 0x80004002),
          Fail = unchecked((int) 0x80004005),
          ElementNotFound = unchecked((int) 0x80070490),
          TypeElementNotFound = unchecked((int) 0x8002802B),
          NoObject = unchecked((int) 0x800401E5),
          Win32ErrorCanceled = 1223,
          Canceled = unchecked((int) 0x800704C7),
          ResourceInUse = unchecked((int) 0x800700AA),
          AccessDenied = unchecked((int) 0x80030005)
      }
  
      [ComImport]
      [Guid("bcc18b79-ba16-442f-80c4-8a59c30c463b")]
      [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
      internal interface IShellItemImageFactory
      {
          [PreserveSig]
          HResult GetImage(
              [In, MarshalAs(UnmanagedType.Struct)] NativeSize size,
              [In] ThumbnailOptions flags,
              [Out] out IntPtr phbm);
      }
  
      [StructLayout(LayoutKind.Sequential)]
      internal struct NativeSize
      {
          private int width;
          private int height;
  
          public int Width
          {
              set { width = value; }
          }
  
          public int Height
          {
              set { height = value; }
          }
      }
  
      [StructLayout(LayoutKind.Sequential)]
      public struct RGBQUAD
      {
          public byte rgbBlue;
          public byte rgbGreen;
          public byte rgbRed;
          public byte rgbReserved;
      }
  }
  

Some time ago I wrote a ThumbnailProvider that loads a thumbnail from the Win API. Supports transparent images. This is the implementation:

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;
using System.IO;

    namespace ThumbnailGenerator
    {
        [Flags]
        public enum ThumbnailOptions
        {
            None = 0x00,
            BiggerSizeOk = 0x01,
            InMemoryOnly = 0x02,
            IconOnly = 0x04,
            ThumbnailOnly = 0x08,
            InCacheOnly = 0x10,
        }

        public class WindowsThumbnailProvider
        {
            private const string IShellItem2Guid = "7E9FB0D3-919F-4307-AB2E-9B1860310C93";

            [DllImport("shell32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
            internal static extern int SHCreateItemFromParsingName(
                [MarshalAs(UnmanagedType.LPWStr)] string path,
                // The following parameter is not used - binding context.
                IntPtr pbc,
                ref Guid riid,
                [MarshalAs(UnmanagedType.Interface)] out IShellItem shellItem);

            [DllImport("gdi32.dll")]
            [return: MarshalAs(UnmanagedType.Bool)]
            internal static extern bool DeleteObject(IntPtr hObject);

            [ComImport]
            [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
            [Guid("43826d1e-e718-42ee-bc55-a1e261c37bfe")]
            internal interface IShellItem
            {
                void BindToHandler(IntPtr pbc,
                    [MarshalAs(UnmanagedType.LPStruct)]Guid bhid,
                    [MarshalAs(UnmanagedType.LPStruct)]Guid riid,
                    out IntPtr ppv);

                void GetParent(out IShellItem ppsi);
                void GetDisplayName(SIGDN sigdnName, out IntPtr ppszName);
                void GetAttributes(uint sfgaoMask, out uint psfgaoAttribs);
                void Compare(IShellItem psi, uint hint, out int piOrder);
            };

            internal enum SIGDN : uint
            {
                NORMALDISPLAY = 0,
                PARENTRELATIVEPARSING = 0x80018001,
                PARENTRELATIVEFORADDRESSBAR = 0x8001c001,
                DESKTOPABSOLUTEPARSING = 0x80028000,
                PARENTRELATIVEEDITING = 0x80031001,
                DESKTOPABSOLUTEEDITING = 0x8004c000,
                FILESYSPATH = 0x80058000,
                URL = 0x80068000
            }

            internal enum HResult
            {
                Ok = 0x0000,
                False = 0x0001,
                InvalidArguments = unchecked((int)0x80070057),
                OutOfMemory = unchecked((int)0x8007000E),
                NoInterface = unchecked((int)0x80004002),
                Fail = unchecked((int)0x80004005),
                ElementNotFound = unchecked((int)0x80070490),
                TypeElementNotFound = unchecked((int)0x8002802B),
                NoObject = unchecked((int)0x800401E5),
                Win32ErrorCanceled = 1223,
                Canceled = unchecked((int)0x800704C7),
                ResourceInUse = unchecked((int)0x800700AA),
                AccessDenied = unchecked((int)0x80030005)
            }

            [ComImportAttribute()]
            [GuidAttribute("bcc18b79-ba16-442f-80c4-8a59c30c463b")]
            [InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]
            internal interface IShellItemImageFactory
            {
                [PreserveSig]
                HResult GetImage(
                [In, MarshalAs(UnmanagedType.Struct)] NativeSize size,
                [In] ThumbnailOptions flags,
                [Out] out IntPtr phbm);
            }

            [StructLayout(LayoutKind.Sequential)]
            internal struct NativeSize
            {
                private int width;
                private int height;

                public int Width { set { width = value; } }
                public int Height { set { height = value; } }
            };

            [StructLayout(LayoutKind.Sequential)]
            public struct RGBQUAD
            {
                public byte rgbBlue;
                public byte rgbGreen;
                public byte rgbRed;
                public byte rgbReserved;
            }

            public static Bitmap GetThumbnail(string fileName, int width, int height, ThumbnailOptions options)
            {
                IntPtr hBitmap = GetHBitmap(Path.GetFullPath(fileName), width, height, options);

                try
                {
                    // return a System.Drawing.Bitmap from the hBitmap
                    return GetBitmapFromHBitmap(hBitmap);
                }
                finally
                {
                    // delete HBitmap to avoid memory leaks
                    DeleteObject(hBitmap);
                }
            }

            public static Bitmap GetBitmapFromHBitmap(IntPtr nativeHBitmap)
            {
                Bitmap bmp = Bitmap.FromHbitmap(nativeHBitmap);

                if (Bitmap.GetPixelFormatSize(bmp.PixelFormat) < 32)
                    return bmp;

                return CreateAlphaBitmap(bmp, PixelFormat.Format32bppArgb);
            }

            public static Bitmap CreateAlphaBitmap(Bitmap srcBitmap, PixelFormat targetPixelFormat)
            {
                Bitmap result = new Bitmap(srcBitmap.Width, srcBitmap.Height, targetPixelFormat);

                Rectangle bmpBounds = new Rectangle(0, 0, srcBitmap.Width, srcBitmap.Height);

                BitmapData srcData = srcBitmap.LockBits(bmpBounds, ImageLockMode.ReadOnly, srcBitmap.PixelFormat);

                bool isAlplaBitmap = false;

                try
                {
                    for (int y = 0; y <= srcData.Height - 1; y++)
                    {
                        for (int x = 0; x <= srcData.Width - 1; x++)
                        {
                            Color pixelColor = Color.FromArgb(
                                Marshal.ReadInt32(srcData.Scan0, (srcData.Stride * y) + (4 * x)));

                            if (pixelColor.A > 0 & pixelColor.A < 255)
                            {
                                isAlplaBitmap = true;
                            }

                            result.SetPixel(x, y, pixelColor);
                        }
                    }
                }
                finally
                {
                    srcBitmap.UnlockBits(srcData);
                }

                if (isAlplaBitmap)
                {
                    return result;
                }
                else
                {
                    return srcBitmap;
                }
            }

            private static IntPtr GetHBitmap(string fileName, int width, int height, ThumbnailOptions options)
            {
                IShellItem nativeShellItem;
                Guid shellItem2Guid = new Guid(IShellItem2Guid);
                int retCode = SHCreateItemFromParsingName(fileName, IntPtr.Zero, ref shellItem2Guid, out nativeShellItem);

                if (retCode != 0)
                    throw Marshal.GetExceptionForHR(retCode);

                NativeSize nativeSize = new NativeSize();
                nativeSize.Width = width;
                nativeSize.Height = height;

                IntPtr hBitmap;
                HResult hr = ((IShellItemImageFactory)nativeShellItem).GetImage(nativeSize, options, out hBitmap);

                Marshal.ReleaseComObject(nativeShellItem);

                if (hr == HResult.Ok) return hBitmap;

                throw Marshal.GetExceptionForHR((int)hr);
            }
        }
    }

Then you can use it in the following way:

int THUMB_SIZE = 256;
Bitmap thumbnail = WindowsThumbnailProvider.GetThumbnail(
   fileName, THUMB_SIZE, THUMB_SIZE, ThumbnailOptions.None);

Remember that you need to Dispose() the bitmap after using it.