CliFx/CliFx.Tests/Utils/DynamicCommandBuilder.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using Basic.Reference.Assemblies;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Text;

namespace CliFx.Tests.Utils;

// This class uses Roslyn to compile commands dynamically.
// It allows us to collocate commands with tests more easily, which helps a lot when reasoning about them.
// Unfortunately, this comes at a cost of static typing, but this is still a worthwhile trade off.
// Maybe one day C# will allow declaring classes inside methods and doing this will no longer be necessary.
// Language proposal: https://github.com/dotnet/csharplang/discussions/130
internal static class DynamicCommandBuilder
{
    public static IReadOnlyList<Type> CompileMany(string sourceCode)
    {
        // Get default system namespaces
        var defaultSystemNamespaces = new[]
        {
            "System",
            "System.Collections",
            "System.Collections.Generic",
            "System.Linq",
            "System.Threading.Tasks",
            "System.Globalization"
        };

        // Get default CliFx namespaces
        var defaultCliFxNamespaces = typeof(ICommand).Assembly
            .GetTypes()
            .Where(t => t.IsPublic)
            .Select(t => t.Namespace)
            .Distinct()
            .ToArray();

        // Append default imports to the source code
        var sourceCodeWithUsings =
            string.Join(Environment.NewLine, defaultSystemNamespaces.Select(n => $"using {n};"))
            + string.Join(Environment.NewLine, defaultCliFxNamespaces.Select(n => $"using {n};"))
            + Environment.NewLine
            + sourceCode;

        // Parse the source code
        var ast = SyntaxFactory.ParseSyntaxTree(
            SourceText.From(sourceCodeWithUsings),
            CSharpParseOptions.Default
        );

        // Compile the code to IL
        var compilation = CSharpCompilation.Create(
            "CliFxTests_DynamicAssembly_" + Guid.NewGuid(),
            new[] { ast },
            Net70.References.All
                .Append(MetadataReference.CreateFromFile(typeof(ICommand).Assembly.Location))
                .Append(
                    MetadataReference.CreateFromFile(
                        typeof(DynamicCommandBuilder).Assembly.Location
                    )
                ),
            // DLL to avoid having to define the Main() method
            new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary)
        );

        var compilationErrors = compilation
            .GetDiagnostics()
            .Where(d => d.Severity >= DiagnosticSeverity.Error)
            .ToArray();

        if (compilationErrors.Any())
        {
            throw new InvalidOperationException(
                $"""
                Failed to compile code.
                {string.Join(Environment.NewLine, compilationErrors.Select(e => e.ToString()))}
                """
            );
        }

        // Emit the code to an in-memory buffer
        using var buffer = new MemoryStream();
        var emit = compilation.Emit(buffer);

        var emitErrors = emit.Diagnostics
            .Where(d => d.Severity >= DiagnosticSeverity.Error)
            .ToArray();

        if (emitErrors.Any())
        {
            throw new InvalidOperationException(
                $"""
                Failed to emit code.
                {string.Join(Environment.NewLine, emitErrors.Select(e => e.ToString()))}
                """
            );
        }

        // Load the generated assembly
        var generatedAssembly = Assembly.Load(buffer.ToArray());

        // Return all defined commands
        var commandTypes = generatedAssembly
            .GetTypes()
            .Where(t => t.IsAssignableTo(typeof(ICommand)) && !t.IsAbstract)
            .ToArray();

        if (commandTypes.Length <= 0)
        {
            throw new InvalidOperationException(
                "There are no command definitions in the provided source code."
            );
        }

        return commandTypes;
    }

    public static Type Compile(string sourceCode)
    {
        var commandTypes = CompileMany(sourceCode);
        if (commandTypes.Count > 1)
        {
            throw new InvalidOperationException(
                "There are more than one command definitions in the provided source code."
            );
        }

        return commandTypes.Single();
    }
}