C#选择文件返回缩略图

传入文件路径，返回临时文件中缩略图的路径，jpg,pdf,office,rar都行

string path = ThumbnailHelper.GetInstance().GetJPGThumbnail("D:\\1.jpg");

ThumbnailHelper

public class Win32Helper
{
    internal 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);
    };
    public enum ThumbnailOptions : uint
    {
        None = ,
        ReturnOnlyIfCached = ,
        ResizeThumbnail = ,
        UseCurrentScale =
    }
    internal enum SIGDN : uint
    {
        NORMALDISPLAY = ,
        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 = ,
        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 class ThumbnailHelper
{
    #region instance
    private static object ooo = new object();
    private static ThumbnailHelper _ThumbnailHelper;
    private ThumbnailHelper() { }
    public static ThumbnailHelper GetInstance()
    {
        if (_ThumbnailHelper == null)
        {
            lock (ooo)
            {
                if (_ThumbnailHelper == null)
                    _ThumbnailHelper = new ThumbnailHelper();
            }
        }
        return _ThumbnailHelper;
    }
    #endregion

    #region public methods

    public string GetJPGThumbnail(string filename, int width = , int height = , Win32Helper.ThumbnailOptions options = Win32Helper.ThumbnailOptions.None)
    {
        if (!File.Exists(filename))
            return string.Empty;
        Bitmap bit = GetBitmapThumbnail(filename, width, height, options);
        if (bit == null)
            return string.Empty;
        return GetThumbnailFilePath(bit);
    }
    #endregion

    #region private methods
    private Bitmap GetBitmapThumbnail(string fileName, int width = , int height = , Win32Helper.ThumbnailOptions options = Win32Helper.ThumbnailOptions.None)
    {
        IntPtr hBitmap = GetHBitmap(System.IO.Path.GetFullPath(fileName), width, height, options);

        try
        {
            Bitmap bmp = Bitmap.FromHbitmap(hBitmap);

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

            return CreateAlphaBitmap(bmp, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
        }
        finally
        {
            // delete HBitmap to avoid memory leaks
            Win32Helper.DeleteObject(hBitmap);
        }
    }
    private Bitmap CreateAlphaBitmap(Bitmap srcBitmap, System.Drawing.Imaging.PixelFormat targetPixelFormat)
    {
        Bitmap result = new Bitmap(srcBitmap.Width, srcBitmap.Height, targetPixelFormat);

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

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

        bool isAlplaBitmap = false;

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

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

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

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

    private IntPtr GetHBitmap(string fileName, int width, int height, Win32Helper.ThumbnailOptions options)
    {
        Win32Helper.IShellItem nativeShellItem;
        Guid shellItem2Guid = new Guid(Win32Helper.IShellItem2Guid);
        int retCode = Win32Helper.SHCreateItemFromParsingName(fileName, IntPtr.Zero, ref shellItem2Guid, out nativeShellItem);

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

        Win32Helper.NativeSize nativeSize = new Win32Helper.NativeSize();
        nativeSize.Width = width;
        nativeSize.Height = height;

        IntPtr hBitmap;
        Win32Helper.HResult hr = ((Win32Helper.IShellItemImageFactory)nativeShellItem).GetImage(nativeSize, options, out hBitmap);

        Marshal.ReleaseComObject(nativeShellItem);

        if (hr == Win32Helper.HResult.Ok) return hBitmap;

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

    /// <summary>
    /// 获取临时文件
    /// </summary>
    /// <returns></returns>
    private FileStream GetTemporaryFilePath(ref string filePath)
    {
        string path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), System.IO.Path.GetRandomFileName());
        var index = path.IndexOf('.');
        string temp = path.Substring(, index) + ".li";

        var fileStream = File.Create(temp);
        filePath = temp;
        return fileStream;
    }
    /// <summary>
    /// 参数
    /// </summary>
    /// <param name="mimeType"></param>
    /// <returns></returns>
    private ImageCodecInfo GetEncoderInfo(String mimeType)
    {
        int j;
        ImageCodecInfo[] encoders;
        encoders = ImageCodecInfo.GetImageEncoders();
        for (j = ; j < encoders.Length; ++j)
        {
            if (encoders[j].MimeType == mimeType)
                return encoders[j];
        }
        return null;
    }
    const int ExpectHeight = ;
    const int ExpectWidth = ;
    /// <summary>
    /// 得到图片缩放后的大小  图片大小过小不考虑缩放了
    /// </summary>
    /// <param name="originSize">原始大小</param>
    /// <returns>改变后大小</returns>
    private System.Drawing.Size ScalePhoto(System.Drawing.Size originSize, bool can)
    {
        if (originSize.Height * originSize.Width < ExpectHeight * ExpectWidth)
        {
            can = false;
        }
        if (can)
        {
            bool isportrait = false;

            if (originSize.Width <= originSize.Height)
            {
                isportrait = true;
            }

            if (isportrait)
            {
                double ratio = (double)ExpectHeight / (double)originSize.Height;
                return new System.Drawing.Size((int)(originSize.Width * ratio), (int)(originSize.Height * ratio));
            }
            else
            {
                double ratio = (double)ExpectWidth / (double)originSize.Width;
                return new System.Drawing.Size((int)(originSize.Width * ratio), (int)(originSize.Height * ratio));
            }
        }
        else
        {
            return new System.Drawing.Size(originSize.Width, originSize.Height);
        }

    }
    /// <summary>
    /// 获取缩略图文件
    /// </summary>
    /// <param name="BitmapIcon">缩略图</param>
    /// <returns>路径</returns>
    private string GetThumbnailFilePath(Bitmap bitmap)
    {
        var filePath = "";
        var fileStream = GetTemporaryFilePath(ref filePath);

        //bitmap.Save(filePath);

        ImageCodecInfo myImageCodecInfo = GetEncoderInfo("image/jpeg"); //image code info 图形图像解码压缩
        System.Drawing.Imaging.Encoder myEncoder = System.Drawing.Imaging.Encoder.Quality;
        EncoderParameter myEncoderParameter = new EncoderParameter(myEncoder, 100L);
        EncoderParameters encoderParameters = new EncoderParameters { Param = new EncoderParameter[] { myEncoderParameter } };
        var sizeScaled = ScalePhoto(bitmap.Size, true);
        //去黑色背景
        var finalBitmap = ClearBlackBackground(bitmap);
        finalBitmap.Save(fileStream, myImageCodecInfo, encoderParameters);
        fileStream.Dispose();
        return filePath;
    }

    private Bitmap ClearBlackBackground(Bitmap originBitmap)
    {
        using (var tempBitmap = new Bitmap(originBitmap.Width, originBitmap.Height))
        {
            tempBitmap.SetResolution(originBitmap.HorizontalResolution, originBitmap.VerticalResolution);

            using (var g = Graphics.FromImage(tempBitmap))
            {
                g.Clear(System.Drawing.Color.White);
                g.DrawImageUnscaled(originBitmap, , );
            }
            return new Bitmap(tempBitmap);
        }
    }

    #endregion
}