This is a quick post to give you some feedback about an experiment I just made with the Scriban Text Templating Library to add support for async/await automatically from the existing synchronous code, all of this done by using Roslyn.

If you have an existing code base that works beautifully in a synchronous manner, but you would like also to provide a path for async/await patterns, you don't want to rewrite your entire code base to the async/await pattern, or to drop synchronous code for async/await only. It would either be a huge burden to maintain two code paths doing almost the same thing, or the use async/await only would actually perform significantly worse than the synchronous version, even if you are using the recently introduced ValueTask<T>

For Scriban, I wanted to have all this async/await version of the library entirely generated, so that whenever I update the synchronous code, I can automatically generate the async/await version. While implementing this, I found an old GitHub issue on Roslyn "Automatically create non-async methods from async methods" and that it was also experimented on the project npgsql via AsyncRewriter done by Shay Rojansky (Hey Shay!). The method used by AsyncRewriter was a bit different from what I was looking for, specifically that I didn't want to modify my existing code with attributes but let the codegen find the relevant methods transitively with just a few bootstrap hints.

It's an interesting challenge and can be a very common point of library design issue (Should I maintain async/await only, or have both? How can I maintain them?), so let's see how that was done.

Identifying async/await methods

The first step was to identify in the existing codebase where async/await can be actually used. There are mainly two end usages:

  • The interface IScriptOutput is a tiny abstraction used by the engine to write a string to an output, that can be implemented by a StreamWriter, or a StringBuilder, or whatever. This interface has mainly a method Write(string text, int offset, int count)
  • The interface IScriptCustomFunction providing user custom functions/delegates that can be used within the template scripts. This interface has mainly a method object Invoke(TemplateContext context, ScriptNode callerContext, ScriptArray arguments, ScriptBlockStatement blockStatement);

Because the code was also using some virtual/abstract methods for the model of the syntax tree, I had to help finding the methods by telling that I also wanted to compile all the method inheriting from ScriptNode.Evaluate

This first selection of methods was very easy to express with Roslyn:

var workspace = MSBuildWorkspace.Create();

var solution = await workspace.OpenSolutionAsync(@"..\..\..\..\scriban.sln");
var project = solution.Projects.First(p => Path.GetFileName(p.FilePath) == "Scriban.csproj");
var compilation = await project.GetCompilationAsync();
var models = compilation.SyntaxTrees.Select(tree => compilation.GetSemanticModel(tree)).ToList();

var methods = new Stack<IMethodSymbol>();
var visited = new HashSet<IMethodSymbol>();

// ----------------------------------------------------------------------------------
// 1) Collect origin methods from IScriptOutput.Write and all ScriptNode.Evaluate methods
// ----------------------------------------------------------------------------------
foreach (var model in models)
    foreach (var methodDeclaration in model.SyntaxTree.GetRoot().DescendantNodes().OfType<MethodDeclarationSyntax>())
        if (methodDeclaration.Parent is InterfaceDeclarationSyntax)
            var interfaceDecl = (InterfaceDeclarationSyntax) methodDeclaration.Parent;

            var interfaceType = model.GetDeclaredSymbol(interfaceDecl);
            if (interfaceType != null && interfaceType.ContainingNamespace.Name == "Runtime" && (interfaceType.Name == "IScriptOutput" || interfaceType.Name == "IScriptCustomFunction"))
                var method = model.GetDeclaredSymbol(methodDeclaration);
                if (visited.Add(method))
            var methodModel = model.GetDeclaredSymbol(methodDeclaration);
            if (!methodModel.IsStatic && methodModel.Name == "Evaluate" && methodModel.Parameters.Length == 1 && methodModel.Parameters[0].Type.Name == "TemplateContext" && InheritFrom(methodModel.ReceiverType, "Syntax", "ScriptNode"))
                while (methodModel != null)
                    if (visited.Add(methodModel))
                    methodModel = methodModel.OverriddenMethod;

Building a graph of method calls

Once we have this first set of mandatory async/await methods, we want to go through all their usages, and tag transitively all methods being "infected" by these async/await methods.

In our case, we are navigating through these methods using the SymbolFinder.FindCallersAsync. I originally tried to use the Syntax Tree but I as actually having a problem storing these in a hashmap, asking this on stackoverflow "How to collect all MethodDeclarationSyntax transitively with Roslyn?" and it turns out that I could work entirely with IMethodSymbol (the semantic model) instead of bouncing between the syntax tree and semantic model (Thanks Marius for the tip!)

// ----------------------------------------------------------------------------------
// 2) Collect method graph calls
// ----------------------------------------------------------------------------------
var methodGraph = new Dictionary<IMethodSymbol, HashSet<ITypeSymbol>>();
var classGraph = new Dictionary<ITypeSymbol, ClassToTransform>();

while (methods.Count > 0)
    var method = methods.Pop();
    if (!visited.Add(method))

    HashSet<ITypeSymbol> callerTypes;
    if (!methodGraph.TryGetValue(method, out callerTypes))
        callerTypes = new HashSet<ITypeSymbol>();
        methodGraph.Add(method, callerTypes);

    var finds = await SymbolFinder.FindCallersAsync(method, solution);
    foreach (var referencer in finds.Where(f => f.IsDirect))
        var callingMethodSymbol = (IMethodSymbol)referencer.CallingSymbol;

        // Push the method overriden
        var methodOverride = callingMethodSymbol;
        while (methodOverride != null && methodOverride.IsOverride && methodOverride.OverriddenMethod != null)
            methodOverride = methodOverride.OverriddenMethod;

        if (callingMethodSymbol.MethodKind == MethodKind.StaticConstructor)

        var callingSyntax = referencer.CallingSymbol.DeclaringSyntaxReferences[0].GetSyntax();
        var callingMethod = (MethodDeclarationSyntax)callingSyntax;

        foreach (var invokeLocation in referencer.Locations)
            var invoke = callingMethod.FindNode(invokeLocation.SourceSpan);
            while (invoke != null && !(invoke is InvocationExpressionSyntax))
                invoke = invoke.Parent;
            Debug.Assert(invoke is InvocationExpressionSyntax);

            var declaredSymbol = callingMethodSymbol.ReceiverType;

            if (declaredSymbol.Name != "TemplateRewriterContext" && callingMethodSymbol.Parameters.All(x => x.Type.Name != "TemplateRewriterContext" && x.Type.Name != "TemplateRewriterOptions")
                && (declaredSymbol.BaseType.Name != "DynamicCustomFunction" || declaredSymbol.Name == "GenericFunctionWrapper"))
                ClassToTransform classToTransform;
                if (!classGraph.TryGetValue(callingMethodSymbol.ReceiverType, out classToTransform))
                    classToTransform = new ClassToTransform(callingMethodSymbol.ReceiverType);
                    classGraph.Add(callingMethodSymbol.ReceiverType, classToTransform);

                // Find an existing method to transform
                var methodToTransform = classToTransform.MethodCalls.FirstOrDefault(x => x.MethodSymbol.Equals(callingMethodSymbol));
                if (methodToTransform == null)
                    methodToTransform = new MethodCallToTransform(callingMethodSymbol, callingMethod);

                // Add a call site

Here, we are just building a list of ClassToTransform with each class to transform having a list of MethodCallToTransform and then each having a list of call sites InvocationExpressionSyntax to change (that we would require to transform from sync to await calls)

Transforming the code

Once we have our graph, we just need to iterate through the syntax tree, duplicate it and modify the methods:

  • Change the signature of the method from XXX(...) to XXXAsync(...)

    // Rename method with `Async` postfix
    method = method.WithIdentifier(Identifier(method.Identifier.Text + "Async"));
  • Add the keyword async to these methods:

    // Add async keyword to the method
    method = method.WithModifiers(method.Modifiers.Add(Token(SyntaxKind.AsyncKeyword).WithTrailingTrivia(Space)));
  • Change the return type from void to Task or from MyTypeXXX to Task<MyTypeXXX>

    TypeSyntax asyncReturnType;
    if (methodModel.ReturnsVoid)
        asyncReturnType = IdentifierName("ValueTask").WithTrailingTrivia(Space);
        var trailingTrivia = method.ReturnType.GetTrailingTrivia();
        asyncReturnType = GenericName(
    method = method.WithReturnType(asyncReturnType);
  • Update classes being modified to add the keyword partial and save the changes at the end (note that I didn't handle nested classes):

    if (typeDecl.Modifiers.All(x => x.Text != "partial"))
        var rootSyntax = typeDecl.SyntaxTree.GetRoot();
        var originalDoc = solution.GetDocument(rootSyntax.SyntaxTree);
        var previousDecl = typeDecl;
        typeDecl = typeDecl.WithModifiers(typeDecl.Modifiers.Add(Token(SyntaxKind.PartialKeyword).WithTrailingTrivia(Space)));
        rootSyntax = rootSyntax.ReplaceNode(previousDecl, typeDecl);
        originalDoc = originalDoc.WithSyntaxRoot(rootSyntax);
        solution = originalDoc.Project.Solution;
  • Update all call sites replacing synchronous code with await expressions with the proper usage of ConfigureAwait (I'm not including all the cases but it mostly boils down to the following):

    method = method.ReplaceNodes(callingMethod.CallSites, (callSite, r) =>
        // We have other cases in the transform (`MemberBindingExpressionSyntax`, `IdentifierNameSyntax`...)
        var m = (MemberAccessExpressionSyntax)newCallSite.Expression;
        var newExpression = m.WithName(IdentifierName(m.Name.ToString() + "Async"));
        newCallSite = newCallSite.WithExpression(newExpression);
        var awaitCall = AwaitExpression(InvocationExpression(
            .WithAwaitKeyword(Token(leadingTrivia, SyntaxKind.AwaitKeyword, TriviaList(Space)));
        return awaitCall;
  • A specific case for IScriptOutput.WriteAsync to accept a last parameter CancellationToken and to flow this argument from (TemplateContext.CancellationToken) to this method. I was surprised when updating the code that TextWriter doesn't expose actually async methods with CancellationToken to later find that there is an issue on corefx "Add CancellationToken to StreamReader.Read* methods"

An example of the code generated is like this:

    public partial class ScriptReturnStatement
        public override async ValueTask<object> EvaluateAsync(TemplateContext context)
            context.FlowState = ScriptFlowState.Return;
            return await context.EvaluateAsync(Expression).ConfigureAwait(false);

while the original method was like this:

        public override object Evaluate(TemplateContext context)
            context.FlowState = ScriptFlowState.Return;
            return context.Evaluate(Expression);

All the generated code is saved to a file ScribanAsync.generated.cs and as you can see, it is in the end significant amount of code!

Performance with ValueTask<T>

I started the codegen using Task<T> but running a benchmark with the synchronized version, to was unsurprisingly generating a huge amount of allocations. I switched easily the codegen to use ValueTask<T> and I was able to divide by 5 the amount of allocations.

For a library like Scriban that is mostly synchronous, it makes a lot more sense to use ValueTask<T>

Compared to the synchronous code, the async/await version is still 3x slower, but considering that it is not necessarily the common usage of Scriban, and that I can keep around the same good old synchronous code, I'm fine with the results (benchmark done with love with BenchmarkDotNet, as always!)

       Method |     Mean |     Error |    StdDev | Gen 0/1k Op | Gen 1/1k Op | Gen 2/1k Op | Allocated Memory/Op |
------------- |---------:|----------:|----------:|------------:|------------:|------------:|--------------------:|
      Scriban | 1.458 ms | 0.0037 ms | 0.0033 ms |     83.9844 |     17.5781 |     17.5781 |           370.53 KB |
 ScribanAsync | 5.067 ms | 0.0618 ms | 0.0516 ms |    132.8125 |           - |           - |            645.5 KB |


I can say that I was really surprised and happy with the results of this work. Bringing async/await to an existing library without having to manually duplicate the code is a huge time/bug/maintenance saver.

Having Roslyn is also what made this experiment possible, and this is just amazing. Though, I must say that the immutable nature of the Roslyn API is quite laborious to work with. I made several mistakes when forgetting to reuse a tree I just modified or incorrectly mixing an old and new tree. But that was really fine, the final code of the codegen being just a few hundred lines of code for generating roughly 1600 lines of code, it was worth to do it and of course, a lot more future proof in case I have to update the async/await code.

I was also surprised to not find many resources related to this particular subject, while I found it quite common, so I'm glad to share it here!

Happy coding!