Project File Language Service

If a custom language defines a ProjectFileType, it must also associate that file type with a PSI language type, which ReSharper can use to build a PSI tree of the file's contents. The IProjectFileLanguageService interface provides this functionality, and can be considered as a bridge between the Project Model and the PSI.

The project file language service is implemented as a per-file type component. Given a ProjectFileType, it is possible to retrieve that file type's project file language service, which exposes the custom language's PsiLanguageType. This can be used as the key to retrieve all per-language components, and get access to the custom language's lexer and parser, to build a PSI tree.

The IProjectFileLanguageService interface is as follows:

public interface IProjectFileLanguageService
{
  ProjectFileType LanguageType { get; }
  IconId Icon { get; }
  PsiLanguageType GetPsiLanguageType(IProjectFile projectFile);
  IPsiSourceFileProperties GetPsiProperties(IProjectFile projectFile, IPsiSourceFile sourceFile);
  ILexerFactory GetMixedLexerFactory(ISolution solution, IBuffer buffer, IPsiSourceFile sourceFile = null);
  PreProcessingDirective[] GetPreprocessorDefines(IProject project, TargetFrameworkId targetFrameworkId);
}

The members are as follows:

LanguageType - confusingly called LanguageType, this is the singleton instance of the derived ProjectFileType class that represents the Project Model file type of this language.
Icon - the IconId of the icon used to represent this file. This is used whenever ReSharper needs to display the icon for a file of this type, such as in the Find Results tool window. See the section on icons for more details on icons.
GetPreprocessorDefines - retrieves a list of preprocessor symbols that are defined for the current project, as this affects the parsing of the PSI tree. This is typically only required for compiled languages in a project where the project itself has a specific language, e.g. a C# project.

The GetPsiLanguageType method

The GetPsiLanguageType method returns the PsiLanguageType derived instance that represents the custom language of the given project file. Generally speaking, this is very simple to implement, and involves returning the static instance of the PSI language type, after checking the IProjectFile.LanguageType is the required ProjectFileType (or a derived instance thereof). For example:

public PsiLanguageType GetPsiLanguageType(IProjectFile projectFile)
{
  // Return UnknownLanguage if the actual language instance is null
  // This can happen if the language feature is disabled
  if (projectFile.LanguageType.Is<CssProjectFileType>())
    return (PsiLanguageType) CssLanguage.Instance ?? UnknownLanguage.Instance;
  return UnknownLanguage.Instance;
}

Note that we return UnknownLanguage.Instance if the actual language instance isn't available. This can happen if the language feature has been disabled.

This method can be a bit more complex. For example, the JavaScriptProjectFileLanguageService checks to see if the current project is a Windows JavaScript application, in which case, it returns JavaScriptWinRTLanguage.Instance. Otherwise it returns the standard JavaScriptLanguage.Instance.

PSI source file properties

The GetPsiProperties method returns an instance of IPsiSourceFileProperties that defines how the given file should be handled by the PSI, such as if a PSI tree should be built for the file.

Typically, this can be an instance of, or derived from DefaultPsiProjectFileProperties, which provides a default implementation, given an IProjectFile and an IPsiSourceFile.

The IPsiSourceFileProperties interface is as follows:

public interface IPsiSourceFileProperties
{
  bool ShouldBuildPsi { get; }
  bool IsGeneratedFile { get; }
  bool IsICacheParticipant { get; }
  bool ProvidesCodeModel { get; }
  bool IsNonUserFile { get; }
  IEnumerable<string> GetPreImportedNamespaces();
  string GetDefaultNamespace();
  ICollection<PreProcessingDirective> GetDefines();
}

The interface members are:

ShouldBuildPsi - return true if ReSharper should build a PSI tree of this file. The default implementation will return true if the ProjectFileType is a known language.
IsGeneratedFile - return true if the file is generated. ReSharper will build a PSI tree for the file, but won't run inspections. The default implementation defers to GeneratedUtils.IsGeneratedFile, which verifies the file name and path against the masks in the Generated Code options page.
IsICacheParticipant - this is a flag to indicate if the caching subsystem should be notified whenever the file is changed. This is typically true.
ProvidesCodeModel - return true if this file contributes to the semantic code model of the project. If false, functionality such as inspections and rename is not available. Generally, this value matches ShouldBuildPsi, but can be different for features such as "external sources" - decompiled or downloaded source files.
IsNonUserFile - return true if the file is not a user-editable file. This is similar to IsGeneratedFile but does not indicate that the code is generated. Inspections and modifications are disabled when this value is true.
GetPreImportedNamespaces - returns a list of namespaces that are implicitly imported. For example, .aspx pages can have namespaces imported by entries in the web.config file.
GetDefaultNamespace - returns the default namespace of this file. Typically defined in the project settings of a CLR project. This setting is not used for non-CLR project files, e.g. JavaScript. Should return string.Empty if not required.
GetDefines - returns the pre-processor symbols defined for the project. Again, this comes from the project settings, and only makes sense for compiled languages, such as C#.

Lexer factory

The GetMixedLexerFactory method returns a factory that is used to create the lexer for a file of this type. Typically, this is just a call to the PSI language service's GetPrimaryLexerFactory method.

public override ILexerFactory GetMixedLexerFactory(ISolution solution, IBuffer buffer, IPsiSourceFile sourceFile = null)
{
  var languageService = My.Instance.LanguageService();
  if (languageService != null)
    return languageService.GetPrimaryLexerFactory();
  return null;
}

However, if the file type is a "mixed" file, that is, it contains secondary PSI trees, such as an .aspx or HTML file that can contain HTML, CSS, JavaScript and even C#, this method should return a "mixed" lexer. Deriving from the MixedProjectFileLanguageService will provide a suitable implementation here. See the section on Secondary PSI files for more details.

Default base classes

ReSharper provides the ProjectFileLanguageService and MixedProjectFileLanguageService abstract base classes that provide a good base implementation of IProjectFileLanguageService. While many of the type members are virtual, the only abstract members that need implementing are:

PsiLanguageType
Icon
GetMixedLexerFactory - MixedProjectFileLanguageService provides a default implementation of this.

Custom PSI properties provider

Some languages also implement IProjectFileCustomPsiPropertiesProvider, which is a per-file type component that will return a specific subclass of ICustomPsiSourceFileProperties.

This is also a bridge between the Project Model and the PSI, as it provides access to custom project properties, via the GetCustomProperties<T> extension method on IPsiSourceFile. For example, using this extension method on a C# file can retrieve the ICSharpPsiSourceFileProperties interface, which provides a WarningsAsErrors property. This allows PSI based inspections and analyses to set the severity level of a highlight.

Last modified: 04 July 2023