The collection's state is no longer correct Exception

[Coder3333]

I randomly get this exception when calling Evaluate from the middleware of my ASP.Net Core web application. All I do is pass a string into the expression. I cannot think of any way that I can be affecting any collections in a non-concurrent way. Any suggestions?

Sometimes I get this error:

NCalc.EvaluationException: Operations that change non-concurrent collections must have exclusive access.
A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct. at NCalc.Expression.Evaluate()

Sometimes the error looks like this:

NCalc.EvaluationException: Collection was modified; enumeration operation may not execute. at NCalc.Expression.Evaluate()

            var eap = new NCalc.Expression(expression);
            return eap.Evaluate();

[yallie]

Could you please create a unit test to demonstrate the issue?
I'd be glad to accept a PR.

[Coder3333]

The expression I am using is something as simple as the string "400 = 200", which I expect to return false. I have never been able to replicate the issue in a unit tests. I only see it in production, which makes me think there is a static collection somewhere in the code that is being shared by threads, but I have not been able to find it.

[Coder3333]

I am starting to think this is a bug in .Net Core. I am going to update to Microsoft.AspNetCore.All 2.2.3 from 2.0.0 and see if the issue goes away.

I did find a static Dictionary in the NCalc codebase that is likely what is throwing the exception. Without having spent too much time figuring out what this is for, I am wondering if this would be better handled with a ConcurrentDictionary, so the ReaderWriterLock would not need to be used.

private static Dictionary<string, WeakReference> _compiledExpressions = new Dictionary<string, WeakReference>();

[yallie]

I did find a static Dictionary in the NCalc codebase that is likely what is throwing the exception.

Yes, that looks suspicious.
By the way, does the issue reproduce when you set CacheEnabled = false?

[codermrrob]

I am also getting this error in production. It happens rarely and I have been unable to reproduce it in a dev/testing environment.

Expressions are always simple. The most recent fail was evaluating 403 * -1

My suspicions have also been on the static dictionary. I will disable the cache, and it seems like that will stop using the static dictionary.

[gumbarros]

@codermrrob we are now using a ConcurrentDictionary at v3.12, now the cache is truly thread-safe. Feel free to close the issue if everything is working or more details if the error still happens.