DeskCore/ShellIcon.cs

using System.Runtime.InteropServices;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace DeskCore;

/// <summary>Holt das Icon (bzw. Vorschaubild) einer Datei so, wie der Explorer es anzeigt.</summary>
public static class ShellIcon
{
    public static ImageSource? Get(string path, int size = 64)
    {
        IntPtr hbm = IntPtr.Zero;
        try
        {
            SHCreateItemFromParsingName(path, IntPtr.Zero, typeof(IShellItemImageFactory).GUID, out var factory);
            int hr = factory.GetImage(new SIZE { cx = size, cy = size }, 0, out hbm);
            Marshal.ReleaseComObject(factory);
            if (hr != 0 || hbm == IntPtr.Zero) return null;
            return FromHBitmap(hbm);
        }
        catch
        {
            return null;
        }
        finally
        {
            if (hbm != IntPtr.Zero) DeleteObject(hbm);
        }
    }

    // CreateBitmapSourceFromHBitmap verliert den Alphakanal, daher Pixel selbst kopieren.
    private static BitmapSource? FromHBitmap(IntPtr hbm)
    {
        GetObject(hbm, Marshal.SizeOf<BITMAP>(), out BITMAP bm);
        int w = bm.bmWidth, h = Math.Abs(bm.bmHeight);
        if (w <= 0 || h <= 0) return null;

        var bmi = new BITMAPINFOHEADER
        {
            biSize = Marshal.SizeOf<BITMAPINFOHEADER>(),
            biWidth = w,
            biHeight = -h, // top-down
            biPlanes = 1,
            biBitCount = 32,
        };
        var bits = new byte[w * h * 4];
        IntPtr hdc = GetDC(IntPtr.Zero);
        try
        {
            if (GetDIBits(hdc, hbm, 0, (uint)h, bits, ref bmi, 0) == 0) return null;
        }
        finally
        {
            ReleaseDC(IntPtr.Zero, hdc);
        }

        bool hasAlpha = false;
        for (int i = 3; i < bits.Length; i += 4)
            if (bits[i] != 0) { hasAlpha = true; break; }

        var src = BitmapSource.Create(w, h, 96, 96, hasAlpha ? PixelFormats.Pbgra32 : PixelFormats.Bgr32, null, bits, w * 4);
        src.Freeze();
        return src;
    }

    [ComImport, Guid("bcc18b79-ba16-442f-80c4-8a59c30c463b"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IShellItemImageFactory
    {
        [PreserveSig] int GetImage(SIZE size, int flags, out IntPtr phbm);
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct SIZE { public int cx, cy; }

    [StructLayout(LayoutKind.Sequential)]
    private struct BITMAP
    {
        public int bmType, bmWidth, bmHeight, bmWidthBytes;
        public ushort bmPlanes, bmBitsPixel;
        public IntPtr bmBits;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct BITMAPINFOHEADER
    {
        public int biSize, biWidth, biHeight;
        public ushort biPlanes, biBitCount;
        public int biCompression, biSizeImage, biXPelsPerMeter, biYPelsPerMeter, biClrUsed, biClrImportant;
    }

    [DllImport("shell32.dll", CharSet = CharSet.Unicode, PreserveSig = false)]
    private static extern void SHCreateItemFromParsingName(string path, IntPtr pbc,
        [MarshalAs(UnmanagedType.LPStruct)] Guid riid, out IShellItemImageFactory ppv);

    [DllImport("gdi32.dll")] private static extern bool DeleteObject(IntPtr h);
    [DllImport("gdi32.dll")] private static extern int GetObject(IntPtr h, int size, out BITMAP bm);
    [DllImport("gdi32.dll")] private static extern int GetDIBits(IntPtr hdc, IntPtr hbm, uint start, uint lines, byte[] bits, ref BITMAPINFOHEADER bmi, uint usage);
    [DllImport("user32.dll")] private static extern IntPtr GetDC(IntPtr hwnd);
    [DllImport("user32.dll")] private static extern int ReleaseDC(IntPtr hwnd, IntPtr hdc);
}