Simple method to retrieve the 16x16 shell file icon using SHFILEINFO structure.

ShellFileIcon.cs

internal const uint FILE_ATTRIBUTE_NORMAL = 0x80;

[Flags]
internal enum SHGetFileInfoFlags
{
    SHGFI_LARGEICON = 0x0,
    SHGFI_SMALLICON = 0x1,
    SHGFI_OPENICON = 0x2,
    SHGFI_SHELLICONSIZE = 0x4,
    SHGFI_PIDL = 0x8,
    SHGFI_USEFILEATTRIBUTES = 0x10,
    SHGFI_ADDOVERLAYS = 0x20,
    SHGFI_OVERLAYINDEX = 0x40,
    SHGFI_ICON = 0x100,
    SHGFI_DISPLAYNAME = 0x200,
    SHGFI_TYPENAME = 0x400,
    SHGFI_ATTRIBUTES = 0x800,
    SHGFI_ICONLOCATION = 1000,
    SHGFI_EXETYPE = 0x2000,
    SHGFI_SYSICONINDEX = 0x4000,
    SHGFI_LINKOVERLAY = 0x8000,
    SHGFI_SELECTED = 0x10000,
    SHGFI_ATTR_SPECIFIED = 0x20000
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
internal struct SHFILEINFO
{
    private const int MAX_PATH = 256, FTYPE_SIZE = 80;

    internal IntPtr hIcon;
    internal int iIcon;
    internal uint dwAttributes;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_PATH)]
    internal string szDisplayName;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = FTYPE_SIZE)]
    internal string szTypeName;
}

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool DestroyIcon(IntPtr hIcon);

[DllImport("shell32.dll", CharSet = CharSet.Unicode)]
internal static extern IntPtr SHGetFileInfo(string pszPath, uint dwFileAttributes,
                                            ref SHFILEINFO psfi, uint cbFileInfo,
                                            SHGetFileInfoFlags uFlags);

public static Icon GetShellFileIcon(string filename, bool linkOverlay)
{
    var sfi = new SHFILEINFO();
    var flags = SHGetFileInfoFlags.SHGFI_ICON |
                SHGetFileInfoFlags.SHGFI_SMALLICON |
                SHGetFileInfoFlags.SHGFI_USEFILEATTRIBUTES;

    if (linkOverlay)
        flags |= SHGetFileInfoFlags.SHGFI_LINKOVERLAY;

    Icon ico;

    try
    {
        SHGetFileInfo(filename, FILE_ATTRIBUTE_NORMAL, ref sfi, (uint)Marshal.SizeOf(sfi), flags);
        ico = (Icon)Icon.FromHandle(sfi.hIcon).Clone();
    }
    finally
    {
        DestroyIcon(sfi.hIcon);
    }

    return ico;
}