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Caching bug in .Net 4.7

Recently, one member in my team reported a weird error: our web application kept throwing NullReferenceException after he install .Net 4.7 on his machine. Frankly, this is not the first time that installing a .Net update broke something, but the weirdo was that the issue didn’t happen on other machines this time. Although reproducing it wasn’t possible, collected stack trace from the “faulty” machine suggested that somehow all items getting from Cache were null. Googling the issue showed me that it is a bug of .Net 4.7. Another question in forum spotted the exact issue in code:

The buggy method

Fixing the issue for my own code without having to wait for a fix from .Net was easy: use UTC DateTime for the absoluteExpiration parameter.

But why did it happen on just one machine? Thinking about it, an object getting from cache is null means it has expired. This in turn means that the absoluteExpiration time had to be before GMT+0 on that machine but be after GMT+0 on other machines. Yeah, time zone was the reason. That specific machine used GMT-8 while all other machine used GMT+7.

Analysis: this post is written as an example about a short post a problems I encounter with my daily job. The post needs to start from what the problem is to how to solve and ends with some additionally useful information.

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