C# CDK Aspect

I recently stumbled across CDK Aspects on the AWS open source news and updates new letter. There was a link to an example Tag checker that I thought I would try out in C#.

I originally had the class defined as 

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public class TagAspect :  IAspect
{
    private readonly IList<string> _requiredTags;

    public TagAspect(IList<string> requiredTags)
    {
        _requiredTags = requiredTags;
    }

    public void Visit(IConstruct node)
    {
        if (node is Stack stack)
        {
            foreach (var requiredTag in _requiredTags)
            {
                if (!stack.Tags.TagValues().ContainsKey(requiredTag))
                {
                    Annotations.Of(node).AddError($"Missing required tag {requiredTag} on stack with id {stack.StackName}");
                }
            }
        }
    }
}

But when I ran CDK synth I got this error

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Unhandled exception. System.ArgumentException: Could not convert argument 'DeployLambda.TagAspect' to Jsii (Parameter 'arguments')
   at Amazon.JSII.Runtime.Deputy.DeputyBase.<>c__DisplayClass20_0.<ConvertArguments>b__0(Parameter parameter, Object frameworkArgument)

After a bit of Googling I found there is a known issue with an easy fix. You just need to make sure the aspect class inherites from Amazon.Jsii.Runtime.Deputy.DeputyBase

The working code is

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public class TagAspect : Amazon.JSII.Runtime.Deputy.DeputyBase, IAspect
{
    private readonly IList<string> _requiredTags;

    public TagAspect(IList<string> requiredTags)
    {
        _requiredTags = requiredTags;
    }

    public void Visit(IConstruct node)
    {
        if (node is Stack stack)
        {
            foreach (var requiredTag in _requiredTags)
            {
                if (!stack.Tags.TagValues().ContainsKey(requiredTag))
                {
                    Annotations.Of(node).AddError($"Missing required tag {requiredTag} on stack with id {stack.StackName}");
                }
            }
        }
    }
}

You wire this in to the program.cs 

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Aspects.Of(app).Add(new TagAspect(new List<string>{"owner"}));

Which gives this error is we don’t have a owner tag set

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[Error at /lambda-pipeline] Missing required tag owner on stack with id lambda-pipeline
Found errors

Excample code