GeneratorConsole/GeneratorLib/HelloWorldGenerator.cs

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using System.Collections.Immutable;

namespace GeneratorLib;

[Generator(LanguageNames.CSharp)]
public class HelloWorldGenerator : IIncrementalGenerator
{
    public void Initialize(IncrementalGeneratorInitializationContext context)
    {
        var provider = context.SyntaxProvider.CreateSyntaxProvider(
            static (syntaxNode, _) => syntaxNode is ClassDeclarationSyntax,
            static (generatorSyntaxContext, _) => (ClassDeclarationSyntax)generatorSyntaxContext.Node)
            .Where(static node => node != null);

        var compilation = context.CompilationProvider.Combine(provider.Collect());

        context.RegisterSourceOutput(compilation, ExecuteHelloWorldGeneration);
    }

    private static void ExecuteHelloWorldGeneration(SourceProductionContext context, (Compilation compilation, ImmutableArray<ClassDeclarationSyntax> classDeclarationSyntax) compilationProvider)
    {
        const string hintName = "HelloWorld.g.cs";
        const string sourceText = """
namespace HelloNamespace
{
abcd
    public static class HelloWorld
    {
        public static void SayHello()
        {
            Console.WriteLine(\"Hello, World!\");
        }
    }
    invalidtexthere
}
""";

        //(Compilation compilation, ImmutableArray<ClassDeclarationSyntax> classDeclarationSyntax) = compilationProvider;
        context.AddSource(hintName, sourceText);
    }
}