Thursday, January 25, 2024

Never, ever make your Random static

Years ago it was popular to advocate for sharing the instance of the Random class. There are numerous sources, including blogs, the stack, etc. Ocasionally, someone dared to write that it could not be the best idea. But still, looking for information, people stumle across this decade-old recommendation.
That was the case of one of our apps. In one critical place, a shared instance of random was used

public static Random _rnd = new Random();

This worked. For years. Until one day, it failed miserably.
You see, the random has two internal variables it overwrites each time Next is called so that it can advance to the next random value. If your app is just winning a lottery ticket in its concurrent execution, the two values can end up being overwritten with the same value.
And guess what, random starts to return 0 each time Next is called! For us, it was the case of an internal stress test, where a heavy traffic was directed onto the application but it can happen just anytime.
There are numerous solutions. First, starting from .NET6 there's the Random.Shared static property, marked as thread-safe. In older frameworks, one of alternative approaches should be used, e.g. this

No comments: