C#获取文件格式图标关联应用程序图标

 class SystemIcon
    {
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
        public struct SHFILEINFO
        {
            public IntPtr hIcon;
            public int iIcon;
            public uint dwAttributes;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
            public string szDisplayName;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
            public string szTypeName;
        }
        [DllImport("Shell32.dll", EntryPoint = "SHGetFileInfo", SetLastError = true, CharSet = CharSet.Auto)]
        public static extern IntPtr SHGetFileInfo(string pszPath, uint dwFileAttributes, ref SHFILEINFO psfi, uint cbFileInfo, uint uFlags);

        [DllImport("User32.dll", EntryPoint = "DestroyIcon")]
        public static extern int DestroyIcon(IntPtr hIcon);
        #region API 参数的常量定义

        public enum FileInfoFlags : uint
        {
            SHGFI_ICON = 0x000000100,  //  get icon
            SHGFI_DISPLAYNAME = 0x000000200,  //  get display name
            SHGFI_TYPENAME = 0x000000400,  //  get type name
            SHGFI_ATTRIBUTES = 0x000000800,  //  get attributes
            SHGFI_ICONLOCATION = 0x000001000,  //  get icon location
            SHGFI_EXETYPE = 0x000002000,  //  return exe type
            SHGFI_SYSICONINDEX = 0x000004000,  //  get system icon index
            SHGFI_LINKOVERLAY = 0x000008000,  //  put a link overlay on icon
            SHGFI_SELECTED = 0x000010000,  //  show icon in selected state
            SHGFI_ATTR_SPECIFIED = 0x000020000,  //  get only specified attributes
            SHGFI_LARGEICON = 0x000000000,  //  get large icon
            SHGFI_SMALLICON = 0x000000001,  //  get small icon
            SHGFI_OPENICON = 0x000000002,  //  get open icon
            SHGFI_SHELLICONSIZE = 0x000000004,  //  get shell size icon
            SHGFI_PIDL = 0x000000008,  //  pszPath is a pidl
            SHGFI_USEFILEATTRIBUTES = 0x000000010,  //  use passed dwFileAttribute
            SHGFI_ADDOVERLAYS = 0x000000020,  //  apply the appropriate overlays
            SHGFI_OVERLAYINDEX = 0x000000040   //  Get the index of the overlay
        }
        public enum FileAttributeFlags : uint
        {
            FILE_ATTRIBUTE_READONLY = 0x00000001,
            FILE_ATTRIBUTE_HIDDEN = 0x00000002,
            FILE_ATTRIBUTE_SYSTEM = 0x00000004,
            FILE_ATTRIBUTE_DIRECTORY = 0x00000010,
            FILE_ATTRIBUTE_ARCHIVE = 0x00000020,
            FILE_ATTRIBUTE_DEVICE = 0x00000040,
            FILE_ATTRIBUTE_NORMAL = 0x00000080,
            FILE_ATTRIBUTE_TEMPORARY = 0x00000100,
            FILE_ATTRIBUTE_SPARSE_FILE = 0x00000200,
            FILE_ATTRIBUTE_REPARSE_POINT = 0x00000400,
            FILE_ATTRIBUTE_COMPRESSED = 0x00000800,
            FILE_ATTRIBUTE_OFFLINE = 0x00001000,
            FILE_ATTRIBUTE_NOT_CONTENT_INDEXED = 0x00002000,
            FILE_ATTRIBUTE_ENCRYPTED = 0x00004000
        }
        #endregion
        /// <summary>
        /// 获取文件类型的关联图标
        /// </summary>
        /// <param name="fileName">文件类型的扩展名或文件的绝对路径</param>
        /// <param name="isLargeIcon">是否返回大图标</param>
        /// <returns>获取到的图标</returns>
        public static Icon GetIcon(string fileName, bool isLargeIcon)
        {
            SHFILEINFO shfi = new SHFILEINFO();
            IntPtr hI;

            if (isLargeIcon)
                hI = SHGetFileInfo(fileName, 0, ref shfi, (uint)Marshal.SizeOf(shfi), (uint)FileInfoFlags.SHGFI_ICON | (uint)FileInfoFlags.SHGFI_USEFILEATTRIBUTES | (uint)FileInfoFlags.SHGFI_LARGEICON);
            else
                hI = SHGetFileInfo(fileName, 0, ref shfi, (uint)Marshal.SizeOf(shfi), (uint)FileInfoFlags.SHGFI_ICON | (uint)FileInfoFlags.SHGFI_USEFILEATTRIBUTES | (uint)FileInfoFlags.SHGFI_SMALLICON);

            Icon icon = Icon.FromHandle(shfi.hIcon).Clone() as Icon;

            DestroyIcon(shfi.hIcon); //释放资源
            return icon;
        }
        /// <summary>  
        /// 获取文件夹图标
        /// </summary>  
        /// <returns>图标</returns>  
        public static Icon GetDirectoryIcon(bool isLargeIcon)
        {
            SHFILEINFO _SHFILEINFO = new SHFILEINFO();
            IntPtr _IconIntPtr;
            if (isLargeIcon)
            {
                _IconIntPtr = SHGetFileInfo(@"", 0, ref _SHFILEINFO, (uint)Marshal.SizeOf(_SHFILEINFO), ((uint)FileInfoFlags.SHGFI_ICON | (uint)FileInfoFlags.SHGFI_LARGEICON));
            }
            else
            {
                _IconIntPtr = SHGetFileInfo(@"", 0, ref _SHFILEINFO, (uint)Marshal.SizeOf(_SHFILEINFO), ((uint)FileInfoFlags.SHGFI_ICON | (uint)FileInfoFlags.SHGFI_SMALLICON));
            }
            if (_IconIntPtr.Equals(IntPtr.Zero)) return null;
            Icon _Icon = System.Drawing.Icon.FromHandle(_SHFILEINFO.hIcon);
            return _Icon;
        }
    }