File-Logger | NET Core for .NET 5 Web API

I'm trying to implement a simple file-logger object, with the possibility to truncate the file when its size reaches a threshold.

I am using a StreamWriter, which gets written to at each call of the method Log(). In order to decide when to truncate, I am checking the StreamWriter.BaseStream.Length property before each write and, if it is bigger than the threshold, I close the StreamWriter, create a new file and open the StreamWriter on that file. For example, if I set the threshold to 10Mb files, it will create a new file each 10Mb of written data.

Under normal load (let's say 3-4 seconds between calls to Log()), everything works as it should. However, the product which is going to use this logger will work with lots of data and required logging every 1 second, even less.

The problem is that the logger seems to completely ignore the creation of the new file(and opening the new stream), failing to truncate it and keeps writing to the existing stream.

I also tried to manually compute the stream's length, hoping it would be a problem with the stream, but it does not work.

I have found out that going step by step with the debugger makes it work correctly, but it does not solve my problem. Logging each second seems to make the program skip the UpdateFile() method entirely.