Unit testing for source generator that use <AdditionalFiles> as input, based on https://gist.github.com/tm-nti/bb374b36d815a1166bc0cbc4febb8515

HOWTO.md

How to add XUnit test for source generator that use <AdditionalFiles> (AdditionalText) as input:

1. Add TestAdditionalText class
2. Add TestDataLoader class
3. In Unit test, use method CSharpGeneratorDriver.AddAdditionalTexts to add generator input file

return CSharpGeneratorDriver.Create(generator)
    .AddAdditionalTexts(texts)
    .RunGenerators(compilation);

OpenApiClientGeneratorTest.cs

using Example.Tests.TestData;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Text;
using System.Collections.Immutable;

namespace Example.UnitTests.TestData;

public class OpenApiClientGeneratorTest
{
    private const string ExampleApiYaml = "example_api.yaml";
    private readonly GeneratorDriver _driver = RunOpenApiClientGenerator();

    [Fact]
    public void Can_parse_OpenAPI_schema()
    {
        GeneratorDriverRunResult driverResult = _driver.GetRunResult();
        Assert.True(driverResult.Diagnostics.IsEmpty);
        // TODO: add more asserts here.
    }

    private static GeneratorDriver RunOpenApiClientGenerator()
    {
        OpenApiClientGenerator generator = new();
        ImmutableArray<AdditionalText> texts =
        [
            TestDataLoader.LoadAdditionalText(ExampleApiYaml)
        ];

        CSharpCompilation compilation = CSharpCompilation.Create(GetTestAssemblyName());
        return CSharpGeneratorDriver.Create(generator)
            .AddAdditionalTexts(texts)
            .RunGenerators(compilation);
    }

    private static string? GetTestAssemblyName()
    {
        return typeof(OpenApiClientGeneratorTest).Assembly.FullName;
    }
}

TestAdditionalText.cs

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

namespace Example.UnitTests;

public class TestAdditionalText(string path) : AdditionalText
{
    private readonly SourceText _sourceText = SourceText.From(File.ReadAllText(path), System.Text.Encoding.UTF8);

    public override string Path { get; } = path;

    public override SourceText? GetText(CancellationToken cancellationToken = default)
    {
        return _sourceText;
    }
}

TestDataLoader.cs

using Example.UnitTests;
using Microsoft.CodeAnalysis;
using System.Runtime.CompilerServices;

namespace Example.UnitTests.TestData;

// NOTE: this file must be placed in the test files directory
public static class TestDataLoader
{
    public static AdditionalText LoadAdditionalText(string fileName)
    {
        string filePath = Path.Combine(GetDataDirectoryPath(), fileName);
        return new TestAdditionalText(filePath);
    }

    private static string GetDataDirectoryPath([CallerFilePath] string path = "")
    {
        return Path.GetDirectoryName(path)
               ?? throw new ArgumentException($"Cannot get directory path from {path}");
    }
}