Code generation with Roslyn

I recently wrote an answer on regarding what I've learned doing code generation with Roslyn.

My conclusion after a few weeks playing around is that usingĀ a combination of inline code snippets (parsed using CSharpSyntaxTree.ParseText) and manually generated SyntaxNodes, works well. But I felt a strong preference for the former. I have also used T4 in the past but am moving away from them due to general lack of integration & capability. I compare the three approaches below and conclude with some tips about doing code gen with Roslyn:

Advantages/disadvantages of each:

Roslyn ParseText

  • Generates arguably more readable code-generator code.
  • Allows 'text templating' approach e.g. using C# 6 string interpolation.
  • Less verbose.
  • Guarantees valid syntax trees.
  • Can be more performant.
  • Easier to get started.
  • Text can become harder to read than SyntaxNodes if majority is procedural.

Roslyn SyntaxNode building

  • Better for transforming existing syntax trees - no need to start from scratch.
    • But existing trivia can make this confusing/complex.
  • More verbose. Arguably harder to read and build.
    • Syntax trees are often more complex than you imagine
  • SyntaxFactory API provides guidance on valid syntax.
  • Roslyn Quoter helps you transform textual code to factory code.
  • Syntax trees are not necessarily valid.
  • Code is perhaps more robust once written.

T4 templates

  • Good if majority of code to be generated is boiler plate.
  • No proper CI support.
  • No syntax highlighting or intellisense without 3rd party extensions.
  • One to one mapping between input and output files.
    • Not ideal if you are doing more complex generation e.g. entire class hierarchy based on single input.
  • Still probably want to use Roslyn to "reflect" on input types, otherwise you will get into trouble with System.Reflection and file locks etc.
  • Less discoverable API. T4 includes, parameters etc. can be confusing to learn.

Roslyn code-gen tips

  • If you are only parsing snippets of code e.g. method statements, then you will need to use CSharpParseOptions.Default.WithKind(SourceCodeKind.Script) to get the right syntax nodes back.
  • If you are parsing a whole block of code for a method body then you will want to parse it as a GlobalStatementSyntax and then access the Statement property as a BlockSyntax.
  • Use a helper method to parse single SyntaxNodes:
        private static TSyntax ParseText<TSyntax>(string code, bool asScript = false)
            var options = asScript
                ? CSharpParseOptions.Default.WithKind(SourceCodeKind.Script)
                : CSharpParseOptions.Default;
            var syntaxNodes =
                CSharpSyntaxTree.ParseText(code, options)
            return syntaxNodes.OfType<TSyntax>().First();
  • When building SyntaxNodes by hand you will typically want to make a final call to SyntaxTree.NormalizeWhitespace(elasticTrivia: true) to make the code "round-trippable".
  • Typically you will want to use SyntaxNode.ToFullString() to get the actual code text including trivia.
  • Use SyntaxTree.WithFilePath() as a convenient place to store the eventual file name for when you come to write out the code.
  • If your goal is to output source files, the end game is to end up with valid CompilationUnitSyntaxs.
  • Don't forget to pretty-print using Formatter.Format as one of the final steps.
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