Registering a PSI Language

Once a custom language's file type has been registered with the Project Model, the language definition needs to be registered with the PSI. If it's not registered with the PSI, ReSharper will not be able to build a PSI tree, and in fact will not consider it registered with the Project Model, either.

The PSI maintains a hierarchy of supported languages, in a similar manner to how the Project Model maintains a hierarchy of supported file types. The base class is PsiLanguageType, and there are again, two directly derived classes - KnownLanguage and UnknownLanguage. The first is the base class for all known languages, and can be used to mean any and all known PSI languages. The second, UnknownLanguage, is a sealed class, and represents a language that the PSI does not support.

A custom language should create a class that derives from KnownLanguage, or a suitable base class (for example, if your language is a subset of HTML, such as a .aspx file, you can derive from HtmlLanguage). The custom class should provide a programmatic name and a presentable name, and it should be marked with the [LanguageDefinition(...)] attribute. For example, C#:

[LanguageDefinition(Name, Edition = ReSharperEditions.Ids.Csharp)]
public class CSharpLanguage : KnownLanguage
{
  // Must be "new" as it overrides a public field in the base
  public new const string Name = "CSHARP";

  // May need "new" if inheriting from another language, such as HTML
  // This value might be null, if the language has been disabled in Options
  [CanBeNull] public static readonly CSharpLanguage Instance;

  private CSharpLanguage() : base(Name, "C#") { }

  protected CSharpLanguage([NotNull] string name) : base(name) { }

  protected CSharpLanguage([NotNull] string name, [NotNull] string presentableName) : base(name, presentableName) { }
}

The [LanguageDefinition] attribute passes in the programmatic name. It also supports an Edition property, which is no longer used. This is a legacy feature to allow languages to be disabled based on which edition of the product has been purchased. This functionality has been replaced by Zones.

Each node in the PSI tree exposes its owning language via the ITreeNode.Language property, which returns an instance of PsiLanguageType.

Languages are instantiated by the ILanguages shell component, which will create the class using the default constructor, and set the static Instance field, meaning all language definitions are singletons.

The ILanguages interface also provides the means to retrieve and enumerate all language definitions.

public interface ILanguages : IViewable<PsiLanguageType>
{
  IEnumerable<PsiLanguageType> All { get; }
  PsiLanguageType GetLanguageByName(string languageName);
}

Last modified: 04 July 2023