AOP – FIFO Memoizer with PostSharp

You can become a serverless blackbelt. Enrol to my 4-week online workshop Production-Ready Serverless and gain hands-on experience building something from scratch using serverless technologies. At the end of the workshop, you should have a broader view of the challenges you will face as your serverless architecture matures and expands. You should also have a firm grasp on when serverless is a good fit for your system as well as common pitfalls you need to avoid. Sign up now and get 15% discount with the code yanprs15!

Back in my first post on AOP, I mentioned the Memoizer on D. Patrick Caldwell’s blog, well, today I came across a situation where I was able to use it but first I needed to make a few modifications because the original implementation didn’t satisfy some of my requirements:

  • There is no cap on the size of the dictionary, I want to avoid a situations where my application uses too much memory, and in the extreme case throws a OutOfMemoryException;
  • There is one static cache shared across all methods, so even with a cap on the size of the dictionary it won’t stop one method from taking up all the available spaces in the dictionary
  • The order of the memos is not kept, so you won’t be able to implement have FIFO strategy for removing old memo entries

In case you’re wondering why I would require these functionalities, I’m building an image viewer with zip support and it doesn’t make sense to load all the images in the zip file into memory at the start (the zip files can be typically hundreds of megs big).

The images are sorted, and as you’re navigating through the images it behaves like a LinkedList anyway, so I’m implementing a sliding window based on where you are in the list and load the previous and next 10 images into cache. Therefore a FIFO memoizer can help reduce the amount of reads from the zip file I need to perform (decompression is an expensive operation).

And here’s the modified version of the Memoizer attribute:

[Serializable]
[AttributeUsage(AttributeTargets.Method)] // only allowed on methods
[DebuggerStepThrough]
public sealed class MemoizeAttribute : OnMethodInvocationAspect
{
    private const int DefaultMemoSize = 100; // default memo size is 100
    // private field to store memos
    private readonly Dictionary<string, object> _memos = new Dictionary<string, object>();
    // private queue to keep track of the order the memos are put in
    private readonly Queue<string> _queue = new Queue<string>();

    #region Constructors
    public MemoizeAttribute() : this(DefaultMemoSize)
    {
    }
    public MemoizeAttribute(int memoSize)
    {
        MemoSize = memoSize;
    }
    #endregion

    public int MemoSize { get; set; } // how many items to keep in the memo

    // intercept the method invocation
    public override void OnInvocation(MethodInvocationEventArgs eventArgs)
    {
        // get the arguments that were passed to the method
        var args = eventArgs.GetArgumentArray();
        var keyBuilder = new StringBuilder();
        // append the hashcode of each arg to the key
        // this limits us to value types (and strings)
        // i need a better way to do this (and preferably
        // a faster one)
        for (var i = 0; i < args.Length; i++)
            keyBuilder.Append(args[i].GetHashCode());
        var key = keyBuilder.ToString();
        // if the key doesn't exist, invoke the original method
        // passing the original arguments and store the result
        if (!_memos.ContainsKey(key))
        {
            _memos[key] = eventArgs.Method.Invoke(eventArgs.Instance, args);
            _queue.Enqueue(key);
            // if we've exceeded the set memo size, then remove the earliest entry
            if (_queue.Count > MemoSize)
            {
                var deQueueKey = _queue.Dequeue();
                _memos.Remove(deQueueKey);
            }
        }
        // return the memo
        eventArgs.ReturnValue = _memos[key];
    }
}

And here’s how you use it, remember, the size cap applies to all the calls to this method (not limited to a particular instance):

[Memoize(5)]
private MemoryStream GetImageStream(ZipFile zipFile, string fileName)
{
    using (var memoryStream = new MemoryStream())
    {
        zipFile[fileName].Extract(memoryStream);
        return memoryStream;
    }
}
Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter


Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.


Skill up your serverless game with this hands-on workshop.

My 4-week Production-Ready Serverless online workshop is back!

This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.

We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

If you enrol now you can also get 15% OFF with the promo code “yanprs15”.

Enrol now and SAVE 15%.


Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.


Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!


Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!