Incremental Roslyn Source Generators: Using Additional Files – Part 7

In the previous article the Source Generator itself needed a 3rd-party library Newtonsoft.Json in order to generate new source code. The JSON-strings were hard-coded inside the Source Generator for simplicity reasons. In this article we will see how to process not just .NET code, but also other files, like JSON or XML.

In this article:

pg
Pawel Gerr is architect consultant at Thinktecture. He focuses on backends with .NET Core and knows Entity Framework inside out.

More information about the Smart Enums and the source code can be found on GitHub:

I recommend reading part 6 of the series because this article is based on the results of the previous one.

In the previous article the translations were rendered out of hard-coded JSON. This time, the JSON content should come from a file residing along with the production code.

Creation of an "Additional File"

The (content) files of a project are not automatically available in Source Generators. The files must be tagged as an AdditionalFile to be provided to a Source Generator. An additional file can be anywhere on the file system accessible by the compiler.

The content of previously hard-coded JSON is moved to a new file Translations.json into the project DemoConsoleApplication.

				
					{ 
  "ProductCategory": { 
    "en": "Product category", 
    "de": "Produktkategorie" 
  } 
}
				
			

The JSON file is included into AdditionalFiles to contribute to code generation.

				
					<Project Sdk="Microsoft.NET.Sdk"> 
   ...
 
   <ItemGroup> 
      <AdditionalFiles Include="Translations.json" /> 
   </ItemGroup> 
 
</Project> 

				
			

Accessing Additional Files

The additional files are accessible by AdditionalTextsProvider and provide the same capabilities as the SyntaxProvider. All performance considerations described in part 4 are applicable here as well. After filtering the files by name, the file contents are combined with enumTypes and generators which leads to a change of the signature of the method GenerateCode.

				
					[Generator] 
public class DemoSourceGenerator : IIncrementalGenerator 
{ 
   ...
   
   public void Initialize(IncrementalGeneratorInitializationContext context) 
   { 
      var enumTypes = context.SyntaxProvider 
                             .CreateSyntaxProvider(CouldBeEnumerationAsync, GetEnumInfoOrNull) 
                             .Where(type => type is not null)! 
                             .Collect<DemoEnumInfo>() 
                             .SelectMany((enumInfos, _) => enumInfos.Distinct()); 
 
      var translations = context.AdditionalTextsProvider 
                                .Where(text => text.Path.EndsWith("translations.json",
                                               StringComparison.OrdinalIgnoreCase)) 
                                .Select((text, token) => text.GetText(token)?.ToString()) 
                                .Where(text => text is not null)! 
                                .Collect<string>(); 
 
      var generators = context.GetMetadataReferencesProvider() 
            .SelectMany(static (reference, _) => TryGetCodeGenerator(reference, out var factory) 
                                                  ? ImmutableArray.Create(factory) 
                                                  : ImmutableArray<ICodeGenerator>.Empty) 
                              .Collect(); 
 
      context.RegisterSourceOutput(enumTypes.Combine(translations)
                                            .Combine(generators),
                                   GenerateCode); 
   } 
 
   ...
    
   private static void GenerateCode( 
      SourceProductionContext context, 
      ((DemoEnumInfo, ImmutableArray<string>), ImmutableArray<ICodeGenerator>) args) 
   { 
      var ((enumInfo, translationsAsJson), generators) = args; 
 
      if (generators.IsDefaultOrEmpty) 
         return; 
 
      var translationsByClassName = GetTranslationsByClassName(context, translationsAsJson); 
 
      foreach (var generator in generators.Distinct()) 
      { 
         if (translationsByClassName is null
            || !translationsByClassName.TryGetValue(enumInfo.Name, out var translations))
        {
            translations = _noTranslations; 
        }
 
         var ns = enumInfo.Namespace is null ? null : $"{enumInfo.Namespace}."; 
         var code = generator.Generate(enumInfo, translations); 
 
         if (!String.IsNullOrWhiteSpace(code)) 
            context.AddSource($"{ns}{enumInfo.Name}{generator.FileHintSuffix}.g.cs", code); 
      } 
   }
   
   ...
				
			

Parsing of the JSON contents is delegated to a new method GetTranslationsByClassName. This method is not just parsing the JSON, but makes a few sanity checks and catches potential parsing errors.

				
					   private static Dictionary<string, IReadOnlyDictionary<string, string>>? 
        GetTranslationsByClassName(SourceProductionContext context, 
                                   ImmutableArray<string> translationsAsJson) 
   { 
      if (translationsAsJson.Length <= 0) 
         return null; 
 
      if (translationsAsJson.Length > 1) 
      { 
         var error = Diagnostic.Create(DemoDiagnosticsDescriptors.MultipleTranslationsFound,
                                       null); 
         context.ReportDiagnostic(error); 
      } 
 
      try 
      { 
         return JsonConvert.DeserializeObject<
            Dictionary<string, IReadOnlyDictionary<string, string>>>(translationsAsJson[0]); 
      } 
      catch (Exception ex) 
      { 
         var error = Diagnostic.Create(DemoDiagnosticsDescriptors.TranslationDeserializationError, 
                                       null, 
                                       ex.ToString()); 
         context.ReportDiagnostic(error); 
 
         return null; 
      } 
   }
				
			
The diagnostic descriptors MultipleTranslationsFound and TranslationDeserializationError are defined in DemoDiagnosticsDescriptors, which is in the same project as the Source Generator.
				
					public static class DemoDiagnosticsDescriptors 
{ 
   ...
   
   public static readonly DiagnosticDescriptor MultipleTranslationsFound 
      = new("DEMO002", 
            "Multiple translations found", 
            "Multiple translations found", 
            "DemoSourceGenerator", 
            DiagnosticSeverity.Error, 
            true); 
 
   public static readonly DiagnosticDescriptor TranslationDeserializationError 
      = new("DEMO003", 
            "Translations could not be deserialized", 
            "Translations could not be deserialized: {0}", 
            "DemoSourceGenerator", 
            DiagnosticSeverity.Error, 
            true); 
} 

				
			

Summary

Additional files are an excellent way to generate code out of (rather static) files, like JSON or XML.

One of the use cases is translation management. Whether the translations come from an external company or are maintained by the developers themselves, a Source Generator can generate constants instead of using magic strings. That way, the translations become virtually type-safe, and there will be no issue with missing translations anymore.

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