How to unit test SourceGenerator?

Question

I writed a SourceGenerator, but how do I test it?

Main issue is how to imitate GeneratorExecutionContext (or just Compilation inside it) which generator gets into Execute method. I think there is a proper way to make fake SyntaxTrees for unit testing, but I cant find it. There are many articles about source generators itself, but none of them explain how to test generators.

Answer 1

You should look at official Source Generators Cookbook

There is example from it:

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Diagnostics;
using System.Linq;
using System.Reflection;

namespace GeneratorTests.Tests
{
    [TestClass]
    public class GeneratorTests
    {
        [TestMethod]
        public void SimpleGeneratorTest()
        {
            // Create the 'input' compilation that the generator will act on
            Compilation inputCompilation = CreateCompilation(@"
namespace MyCode
{
    public class Program
    {
        public static void Main(string[] args)
        {
        }
    }
}
");

            // directly create an instance of the generator
            // (Note: in the compiler this is loaded from an assembly, and created via reflection at runtime)
            CustomGenerator generator = new CustomGenerator();

            // Create the driver that will control the generation, passing in our generator
            GeneratorDriver driver = CSharpGeneratorDriver.Create(generator);

            // Run the generation pass
            // (Note: the generator driver itself is immutable, and all calls return an updated version of the driver that you should use for subsequent calls)
            driver = driver.RunGeneratorsAndUpdateCompilation(inputCompilation, out var outputCompilation, out var diagnostics);

            // We can now assert things about the resulting compilation:
            Debug.Assert(diagnostics.IsEmpty); // there were no diagnostics created by the generators
            Debug.Assert(outputCompilation.SyntaxTrees.Count() == 2); // we have two syntax trees, the original 'user' provided one, and the one added by the generator
            Debug.Assert(outputCompilation.GetDiagnostics().IsEmpty); // verify the compilation with the added source has no diagnostics

            // Or we can look at the results directly:
            GeneratorDriverRunResult runResult = driver.GetRunResult();

            // The runResult contains the combined results of all generators passed to the driver
            Debug.Assert(runResult.GeneratedTrees.Length == 1);
            Debug.Assert(runResult.Diagnostics.IsEmpty);

            // Or you can access the individual results on a by-generator basis
            GeneratorRunResult generatorResult = runResult.Results[0];
            Debug.Assert(generatorResult.Generator == generator);
            Debug.Assert(generatorResult.Diagnostics.IsEmpty);
            Debug.Assert(generatorResult.GeneratedSources.Length == 1);
            Debug.Assert(generatorResult.Exception is null);
        }

        private static Compilation CreateCompilation(string source)
            => CSharpCompilation.Create("compilation",
                new[] { CSharpSyntaxTree.ParseText(source) },
                new[] { MetadataReference.CreateFromFile(typeof(Binder).GetTypeInfo().Assembly.Location) },
                new CSharpCompilationOptions(OutputKind.ConsoleApplication));
    }
}