codegen/Generator/MySourceGenerator.cs

using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Text;

namespace MySourceGenerator {
	[Generator]
	public class MySourceGenerator : ISourceGenerator {
		public void Execute(GeneratorExecutionContext context) {
			var sb = new StringBuilder();
			sb.AppendLine("using System;");
			sb.AppendLine("namespace Generated;");
			sb.AppendLine("public static class HelloWorld {");
			sb.AppendLine("	public static string SayHello() {");
			sb.AppendLine("		return \"Hello World!!!\";");
			sb.AppendLine("	}");
			sb.AppendLine("}");

			context.AddSource("HelleWorld.generated", SourceText.From(sb.ToString(), Encoding.UTF8));
		}

		public void Initialize(GeneratorInitializationContext context) {
		}
	}
}