Filename To Icon Converter

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Introduction

This article describes FileToIconConverter, which is a MultiBinding Converter that can retrieve an Icon from system based on a filename(exist or not) and size.

Background

I am working on a file explorer, which shows a file list, inside the filelist, which require to place a file icon next to each file in a folder. In my first implementation, I added a Icon property in the data model of the file, it works fine. When I implements the Icon View, I added a Large Icon property, then when I add Thumbnail property for the Thumbnail view support.

This implementation has a number of problems,

Three Bitmaps (Icon, LargeIcon, Thumbnail), Five if include ExtraLarge and Jumbo is holds in the Datamodel, they are usually duplicated (e.g. folder with jpgs), and unused (who will change views regularly anyway)
Those code are not related to the DataModel, it shoudnt be placed there.
Icon resize (like the slidebar in vista explorer) become very difficult, as there are 3-5 properties to bind with. So I changed by design, I believe a converter with cache is best suite for this purpose
Icons are cached in a dictionary, files with same extension occupied only one copy of memory.
Load on demand, (e.g. if the view need Icon, then load Icon only).
Reusable, coder just need to bind filename and size Thumbnails are loaded in separate thread (no longer jam the UI), Jumbo icon are shown before the thumbnail is loaded.

How to use?

The converter is a MultiBinding Converter, which takes 2 parameters, filename and size,

Filename is not necessary to be exists (e.g. abc.txt works),
Size determine which icon to obtain (Optional)

<= 16 - Small
<= 32 - Large
<= 48 - Extra Large
<= 100 - Jumbo
Otherwise, Thumbnail if is an image, Jumbo if not an image.

How it works?

MultiBinding Converter is similar to the normal Binding IValueConverter except it takes multiple value to convert, the Convert is like below :

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)

{

int size = defaultSize;

if (values.Length > 1 && values[1] is double)

size = (int)(float)(double)values[1];

if (values[0] is string)

return imageDic[getIconKey(values[0] as string, size)];

else return imageDic[getIconKey("", size)];

}

parameter 2 is convert to a local variable named size.
parameter 1 is convert to a Icon in imageDic with a key (getIconKey() method, see below) Icon are retrieved based on Size and Extensions
Thumbnail
- Image - Return WritableBitmap (new in dotNet 3.5), which acts like BitmapImage but allow change after initalization.
- otherwise - treat as Icon
Jumbo / ExtraLarge
- all (include exe) - load from SystemImageList
Small / Large
- Load using SHGetFileInfo (Win32 api) directly.

I try to avoid SystemImageList because it have it's own cache system which will cause some overhead. There are two cache, iconCache and thumbnailCache, all Icons and thumbnail are stored in cache.

iconCache is static, for small - Jumbo Icons, common for all FileToIconConverter.
thumbnailCache is instanced, for thumbnail, you can call ClearInstanceCache() method to clear this cache.
addToDic() method will add the Icon or Thumbnail (from getImage()) to it's cache
returnKey() method will return a key for dictionary based on fileName and size,
- e.g. (".txt+L" for Large Text Icon, ".jpg+S" for Small JPEG icon)
loadBitmap() method takes a Bitmap and return a BitmapSource, it's actually calling Imaging.CreateBitmapSourceFromHBitmap() method, and is required because Image UIelement dont take a bitmap directly. getImage() method retrieve Icon and Thumbnail, it's thumbnail loading code looks like below :

//Load as jumbo icon first.
WriteableBitmap bitmap = new WriteableBitmap(addToDic(fileName, IconSize.jumbo) as BitmapSource);
ThreadPool.QueueUserWorkItem(new WaitCallback(PollThumbnailCallback), new thumbnailInfo(bitmap, fileName));
return bitmap;
The code will try to load the jumbo icon first, which usually cached already, and much faster to load even not cache, then when the processor is free, it will call PollThumbnailCallback() in background thread. This implementation will prevent UI thread hogging problem. PollThumbnailCallback() method is not too complicate, it's actually get and resize the thumbnail bitmap (line 4-8), turn it to BitmapSource (Line 9-C), and Write it to the WriteableBitmap obtained (Line 6, D-Q), Line M to Line Q is executed in UI thread, so only work that required to process the writeBitmap is placed there.

1) private void PollThumbnailCallback(object state)

2) {

3) //Non UIThread

4) thumbnailInfo input = state as thumbnailInfo;

5) string fileName = input.fullPath;

6) WriteableBitmap writeBitmap = input.bitmap;

7) Bitmap origBitmap = new Bitmap(fileName);

8) Bitmap inputBitmap = resizeImage(origBitmap, new System.Drawing.Size(256, 256));

9) BitmapSource inputBitmapSource = Imaging.CreateBitmapSourceFromHBitmap(inputBitmap.GetHbitmap(),

A) IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());

B) origBitmap.Dispose();

C) inputBitmap.Dispose();

D) int width = inputBitmapSource.PixelWidth;

E) int height = inputBitmapSource.PixelHeight;

F) int stride = width * ((inputBitmapSource.Format.BitsPerPixel + 7) / 8);

G) byte[] bits = new byte[height * stride];

H) inputBitmapSource.CopyPixels(bits, stride, 0);

I) inputBitmapSource = null;

J) writeBitmap.Dispatcher.Invoke(DispatcherPriority.Background,

K) new ThreadStart(delegate

L) {

M) //UI Thread

N) Int32Rect outRect = new Int32Rect(0, (int)(writeBitmap.Height - height) / 2, width, height);

O) writeBitmap.WritePixels(outRect, bits, stride, 0);

P) }));

Q) }

Issues

The component is still very resource hogging. , fixed, I just noticed I have to clear the created object myself using DeleteObject if I called Bitmap.GHbitmap, like below

IntPtr hBitmap = source.GetHbitmap();

return Imaging.CreateBitmapSourceFromHBitmap(hBitmap, IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());

DeleteObject(hBitmap);

References

System Image List (Steve McMahon, vbaccelerator)
ResizeImage routine
How to dispose BitmapSource

History

26-12-08 Initial version 28-12-08 version 1, load thumbnail in thread,.
28-12-08 version 2, component updated, demo updated.
28-12-08 article updated.
28-12-08 Version 3, border added for thumbnail, and extra large icon.
29-12-08 Version 4, folder support, memory usage reduced.
30-12-08 Version 5, fixed memory leak, thread exe icon loading.

License

This article, along with any associated source code and files, is licensed under The GNU Lesser General Public License.