The Compilation API

The Compilation API

CSharpCompilation is Roslyn's build in a function call: trees + references + options → diagnostics + a pe-stream you can emit. Everything in this module runs inside the sandbox — no dotnet CLI needed:

var comp = CSharpCompilation.Create(
    "policy",
    new[] { CSharpSyntaxTree.ParseText(policySource) },
    references,   // MetadataReference.CreateFromFile(assembly.Location)
    new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

var diags = comp.GetDiagnostics()
    .Where(d => d.Severity == DiagnosticSeverity.Error);

using var pe = new MemoryStream();
var result = comp.Emit(pe);
// result.Success, result.Diagnostics — the real compiler verdict

What the platform's own grader does for every challenge you submit: stage your code, reference the framework assemblies, compile, run the test harness — exactly this API shape, executed in a hardened container.

Three practical notes from real use:

• References come from loaded assemblies: filter AppDomain.CurrentDomain.GetAssemblies() to non-dynamic assemblies with a Location on disk. The Roslyn dlls themselves need explicit refs when you compile code that uses Roslyn (as the harness does for this course).
• GetDiagnostics() vs Emit(): emit also validates — some errors (missing types at link time) appear only when emitting.
• Compilation is expensive (~tens of ms per small program); batch work or keep compilations long-lived instead of rebuilding per request.