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Did you notice a MemoryCache problem with decimal precision?

Posted in Did you notice?

A little bit of context first

Hi everyone. A couple months ago as I was building the CoinzProfit API, I ran into a weird issue with MemoryCache. In order to avoid hitting too often the various APIs CoinzProfit depends on like Coinbase’s, I decided to implement caching. Indeed, a cache allows keeping user calculated profits and various currency rates without having to fetch data too often. In order to save on costs since the app is free and has no ads, I used .NET Core in-memory caching.

Basically when I compute profits and investments and so on, I would cache my computed result in GBP if my app is set to display amounts in GBP. When querying the data again, then the if the app query currency does not match the cache I convert it. Similarly, if I change my app settings to display USD then the data displays in USD. This allows for successive request that do not require to call Coinbase and Binance APIs earlier than needed.  All was good and well until I noticed an issue with my computed profits that would change dramatically in a seemingly random fashion.


Originally I thought that maybe, the conversion rates I retrieved were not accurate enough. This could perfectly explain why the conversion works in one direction but goes random in the other. After a couple of hours playing hide-and-seek (or “cache-cache” in French), MemoryCache revealed itself to be the source of my problem. Please do not act surprised, it was the only suspect and kinda is the focus of this post.

The reason why my conversion was all over the place is that my conversion rates were truncated when cached. To validate that assertion I injected a caching implementation that would always fail to return data so that I always get fresh data and conversion rates. Once I confirmed the issue I resorted to serialisation to preserve decimal precision on the data I needed to cache.

Two months later

It has now been two months since I ran into that issue. I originally planned on writing this post way earlier but I had a lot on my plate. That precision issue may be completely gone by now. This is what I will try and test today. I will just setup a simple .NET Core console application and write some decimal data through MemoryCache and read it to see if it now preserves precision. Note that I built the API using one of the .NET Core 2.0.* versions and that the 2.1.1. version is currently available. Therefore today I will use the later for this little experiment .

If you want to try that experiment at home, you can install VSCode and .NET Core if not done yet then run the following commands.

In order these commands:

  1. Create a .NET Core console app project
  2. Move you into the project folder
  3. Install the `Microsoft.Extensions.Caching.Memory` package from nuget.
  4. Restore the package and builds the project

Now you can copy the code from the file below in your Program.cs

Are you ready to run this? Run the command dotnet run

classic memorycache

What do you see right now? Exactly, it does work now. I’m not even mad. It’s kinda amazing that Microsoft fixed that thing in such a short period of time. Will try later on to reproduce the bug whenever I find the exact version of .NET Core I used when I ran into that issue. Expect an update or a part 2 to this post at some point. That is unless I forget about it. Do let me know if you run into the same issue which version of .NET Core you have installed.

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