Understanding arithmetic overflow checking in 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!

Default context = unchecked

By default, arithmetic operations and conversions in C# are executed in an unchecked context. This means that for a signed integer it overflows from int.MaxValue to int.MinValue and underflows from int.MinValue to int.MaxValue, hence both statements below evaluates to true:

(int.MinValue – 1) == int.MaxValue;
(int.MaxValue + 1) == int.MinValue;

Similarly, for an unsigned integer it will underflow from 0 to uint.MaxValue and overflow from uint.MaxValue to 0:

(0 – 1) == uint.MaxValue; // uint.MinValue = 0
(uint.MaxValue + 1) == 0;

This also applies to conversions too:

(int)((long) int.MaxValue + 1) == int.MinValue; // true


This default behaviour of swallowing the exceptions seems rather strange and unexpected to me, given that when an overflow happens it’s usually of interest to me as a developer to know about it and deal with it as appropriate because:

  • if an overflow is to happen during normal usage of the application it probably means there’s something wrong with the design/assumptions of the applicatoin
  • if an overflow is to happen and the application is allowed to continue and persist the overflowed/underflowed value it could mean important pieces of data are left in an invalid state which is difficult/impossible to revert
  • the jump is large! Imagine seeing your account balance go from £2147483647 to £–2147483648 after crediting £1 into it..

With that said, unchecked context performs significantly better than checked implementation, which is probably why it was chosen as the default.

Also, you will probably want to use unchecked blocks for calculating hash codes where it’s the bit patterns that matters not the magnitude of the integer value, i.e.:

public class MyClass
    public override int GetHashCode()
            return ….


There are two things to keep in mind when using checked/unchecked blocks:

They’re always LOCAL to the method

Which means if you call another method from within a checked block the method will still execute in the default context (unchecked):

    // this method will still execute in unchecked context
public void DoSomethingThatOverflows()
    // no overflow exception is thrown...
    var overflowed = int.MaxValue + 1;

The context a line of code executes in is determined by the MOST inner checked/unchecked statement block

Hence the following code will execute in unchecked context:

        var over = int.MaxValue + 1;

Project-wide arithmetic overflow/underflow checks

If you require arithmetic overflow/underflow checking on a project-wide scale, there is a property you can set from within Visual Studio. Go to project properties and find the Build tab, click “Advanced…” and tick the “Check for arithmetic overflow/underflow” box. But remember, you would probably still want to make sure GetHashCode is executed in an unchecked context.

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!