Roslyn slow startup time

c#, roslyn

Solution

I think one issue is using a memory stream, instead you should try using a dynamic module and ModuleBuilder instead. Overall the code is executing faster but still has a heavier first load scenario. I'm pretty new to Roslyn myself so I'm not sure why this is faster but here is the changed code.

        var iters = 300;
        foreach (var i in Enumerable.Range(0, iters))
        {
            // Parse the source file using Roslyn
            SyntaxTree syntaxTree = SyntaxTree.ParseText(@"public class Foo" + i + @" { public void Exec() { } }");

            // Add all the references we need for the compilation
            var references = new List<MetadataReference>();
            references.Add(new MetadataFileReference(typeof(int).Assembly.Location));

            var compilationOptions = new CompilationOptions(outputKind: OutputKind.DynamicallyLinkedLibrary);

            // Note: using a fixed assembly name, which doesn't matter as long as we don't expect cross references of generated assemblies
            var compilation = Compilation.Create("SomeAssemblyName", compilationOptions, new[] { syntaxTree }, references);

            var assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new System.Reflection.AssemblyName("CustomerA"),
            System.Reflection.Emit.AssemblyBuilderAccess.RunAndCollect);

            var moduleBuilder = assemblyBuilder.DefineDynamicModule("MyModule");

            System.Diagnostics.Stopwatch watch = new System.Diagnostics.Stopwatch();
            watch.Start();

            // if we comment out from this line and down, the runtime drops to ~.5 seconds
            var emitResult = compilation.Emit(moduleBuilder);

            watch.Stop();

            System.Diagnostics.Debug.WriteLine(watch.ElapsedMilliseconds);

            if (emitResult.Diagnostics.LongCount() == 0)
            {
                var type = moduleBuilder.GetTypes().Single(t => t.Name == "Foo" + i);

                System.Diagnostics.Debug.Write(type != null);
            }
        }

By using this technique the compilation took just 96 milliseconds, on subsequent iterations it takes around 3 - 15ms. So I think you could be right in terms of the first load scenario adding some overhead.

Sorry I can't explain why it's faster! I'm just researching Roslyn myself and will do more digging later tonight to see if I can find any more evidence of what the ModuleBuilder provides over the memorystream.

Problem

Original source

Related problems

• Using Roslyn Emit method with a ModuleBuilder instead of a MemoryStream