Custom Icon Types

It is possible to create a custom icon type), if the existing item types do not meet requirements. For example, the image could be retrieved from the filesystem, rather than compiled into an assembly.

To create a custom file type, you need to create a class that derives from IconId. This class identifies your image, and should provide enough information to be able to create an image. It should be a value type, and must override the various Equals, CompareTo and GetHashCode methods to be able to properly compare two instances as the same.

Each IconId is associated with a class that implements IIconIdOwner, and is marked with the [ShellComponent] attribute. This class is responsible for taking the information in the IconId derived class and retrieving the icon.

public interface IIconIdOwner
{
    Type IconIdType { get; }
    ImageSource TryGetImage(IconId iconid, IconTheme theme, IThemedIconManagerPerThemeCache themedIconManagerPerThemeCache, OnError onerror);
}

The IconIdType member should return the Type of the derived IconId that the owner handles. When trying to get an image from an IconId, this type is used to lookup which class can handle the icon.

The TryGetImage method is called to retrieve the image. The owner class should downcast the passed icon ID to the type of IconId it supports, extract the information and use that to retrieve the image. It should always return something. If it can't find an image, or if an error happens, it should return IconPlaceholder.IconPlaceholderAvalon, which is a WPF ImageSource that represents a broken icon (an icon with a cross).

In the case of an error, the class should pass any exception to the Handle method on the onerror parameter, and return the placeholder image.

Using existing icons

If the icon owner needs to use other icons, perhaps to apply an overlay (although the CompositeIconId should handle this), it can use the IThemedIconManagerPerThemeCache instance that gets passed in. This interface is a cache of existing icons for the current theme, and can be used to get an icon image, as a WPF ImageSource or a System.Drawing.Bitmap for a particular DPI.

public interface IThemedIconManagerPerThemeCache
{
    ImageSource GetIconImageSource([NotNull] IconId iconid);
    Bitmap GetIconGdipBitmap([NotNull] IconId id, RasterizationResolution resolution);
    ImageSource TryGetIconImageSource([NotNull] IconId id);
}

The images are created on demand, or pulled from the in-memory cache. The GetIconImageSource will return the appropriate image, or will return the placeholder. The TryGetIconImageSource method will return null if it can't find the requested icon.

Theming

It is up to the icon owner to handle theming. Some icons don't partake in theming, for example, ColorIconId returns a solid block of colour, which is not appropriate for theming.

When asked for an icon, one of the items to be passed in is an instance of IconTheme. An icon theme is made up of a number of aspects, represented by an array of IconThemeAspect instances. There are currently two types of aspect that are recognised - "Generic" and "PsiSymbol".

The generic aspect represents the colour scheme used by the current theme - colour, gray or dark gray. This can be set explicitly by the user, or set to "automatic", where the aspect is chosen based on the Visual Studio theme. ReSharper analyses the theme to see if it is monochrome, and to see if it is light or dark. For the Blue theme, this gives GenericThemeAspect.Color. For the Light theme, GenericThemeAspect.Gray, and the Dark theme gives GenericThemeAspect.DarkGray.

The "PsiSymbol" aspect is used to choose which icon set to use for PSI elements such as methods, properties, etc. in code completion lists and file structure windows and so on. Again, this value can be set by the user, but by default is set to automatic. ReSharper will look at the current Visual Studio theme and version, and choose the appropriate symbol set:

PsiSymbolIconThemeAspect.SymbolsIdea provides symbols based on IntelliJ IDEA
PsiSymbolIconThemeAspect.SymbolsVs08 provides symbols used in Visual Studio 2008
PsiSymbolIconThemeAspect.SymbolsVsColor provides colour symbols as used in Visual Studio 2012 and later
PsiSymbolIconThemeAspect.SymbolsVs11Gray provides monochrome VS2012 symbols for use on a light background
PsiSymbolIconThemeAspect.SymbolsVs11GrayDark provides monochrome VS2012 symbols for use on a dark background

When the IIconIdOwner class needs to return a themed icon, it should look at IconTheme.Aspects for aspects it recognises. The array is in priority order, and there may be more than one entry for each aspect kind, to allow for fallbacks. The icon owner should compare the aspect against the known static aspects listed above. When it recognises that aspect, it should provide the appropriate icon.

How the icon owner chooses to get a theme aspect specific icon is up to the implementation of the icon owner. CompiledIconId icons embed several different versions of the icons into an assembly's resources, and the owner simply retrieves the version of the icon that matches the aspect. The ShellFileIconIdOwner on the other hand, retrieves a file type icon from the operating system, looks for the GenericIconThemeAspect.Gray theme aspect and makes the icon monochrome using ColorManagement.MakeMonochrome.

BitmapSource bmp = GetFileIcon(iconid.ExtensionWithDot);
if (theme.Aspects.Contains(GenericIconThemeAspect.Gray))
  return ColorManagement.MakeMonochrome(bmp);

Both the Light and Dark themes will contain the Gray aspect, but the priority ordering is different. This allows for fallback icons.

Last modified: 07 June 2022