Closure in Javascript vs C#

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!

Java vs C#

As Jon Skeet pointed out in this excellent article on closures, the strategies of capturing the execution context which the behaviour is bound to different between Java and C#. C# captures the variable itself, whereas Java captures the value of the variable. To illustrate the difference, here’s Jon’s original example in C#:

// First build a list of actions
List<Action> actions = new List<Action>();

for (int counter = 0; counter < 10; counter++)
{
    actions.Add(() => Console.WriteLine(counter));
}

// Then execute them
foreach (Action action in actions)
{
    action();
}

This code actually writes 10 ten times instead of 0 to 9 because the context the actions are bound to is the variable counter, and at the time of execution the value of counter is 10 hence why each Action delegate writes 10 instead of the value of counter at the time the Action delegate was created.

The equivalent code in Java would print 0 to 9 because Java’s closure implementation captures the value of the variable instead.

However, as Jon pointed out, whilst Java’s implementation is more intuitive and easier to understand (less WTF bugs), C#’s implementation is more flexible as you can just easily mimic the effect of capturing the variable’s value by using a local variable:

// First build a list of actions
List<Action> actions = new List<Action>();

for (int counter = 0; counter < 10; counter++)
{
    int copy = counter;
    actions.Add(() => Console.WriteLine(copy));
}

// Then execute them
foreach (Action action in actions)
{
    action();
}

Javascipt vs C# (Updated 16/01/2011)

Thanks for Jens for pointing out my initial incorrect assessment (see comment), so I revised my test and used a more straight forward test using just one private variable:

function test () {
    var i = 10;
    
    // create the closure
    var func = function () { return i; };

    // 10
    alert(func());

    i = 11;
    // 11? or 10?
    alert(func());
}

The test here is simple, we create a function func which returns the value of the private variable i, whose value is then changed later on. If Javascript captures the value of the variable then the second time func is invoke the alert message would still be 10, otherwise it’ll be 11 which is the current value of i.

And the answer is?

.

.

.

Javascript captures the reference of the variable, and the second alert message is 11.

References:

Jon Skeet’s article on closures

Article on Javascript closures

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!