logging: ASP .NET Core 5 usage: cannot read log file content - The process cannot access the file '...' because it is being used by another process

Hello. I use your library in ASP .NET Core 5 Web app. In Startup.cs inside ConfigureServices(IServiceCollection services) I have:

services.AddLogging(loggingBuilder => {
                loggingBuilder.AddFile("Logs/app.log", options => {
                   options.Append = true;
                    options.MinLevel = LogLevel.Warning;
                    options.FileSizeLimitBytes = 5 * 1024 * 1024;
                    options.MaxRollingFiles = 1000;
                    });
            });

In Program.cs I resolve Ilogger via host.Services.GetService<ILogger<TelegramBot>>() and pass it to the constructor of a TelegramBot class. Inside the constructor logger is saved in local variable that is later used to log some events with logger.LogWarning() (specifically - bot start\stop events). On some page in my application I’m trying to read log file content with the following code (start\stop events are definitely doesn’t happend and logger.LogWarning() is definitely not called at that moment):

            try
            {
                using (FileStream stream = File.Open(filePath, FileMode.Open, FileAccess.Read, FileShare.Read))
                ....
            }
            catch (IOException ex)
            {
                //Here I get 'The process cannot access the file '...' because it is being used by another process'
            }

but I get IOException

The process cannot access the file ‘…’ because it is being used by another process

So what’s wrong with my library usage? How can I properly read log file content? Can it be that logger unblocks log file only on Dispose() event? Should it live as short as possible (resolve->write log->dispose)?

About this issue

Original URL
State: closed
Created 3 years ago
Reactions: 1
Comments: 24 (9 by maintainers)

Commits related to this issue

Ability to handle file errors and use an alternative log file name if current one cannot be opened (to handle exceptions like 'The process cannot access the file' #28) — committed to nreco/logging by VitaliyMF 3 years ago

Most upvoted comments

@bairog The issue is that what is opening the file needs to have FileShare.ReadWrite (as suggested by @zoran-petrovic-87). This will allow the file to be written to while you are reading it. The docs description is a tad misleading on this but essentially if the file is already open for writing, and you’re only willing to share with others reading it when you try to open it, you come off second best. Note that FileShare.Read seems to be the default in .NET 5 if you’re not explicit.

Why I end up with the file not even readable by the likes of Notepad still is a mystery to me.

There’s a unit test in the above pull request that confirms that with FileShare.ReadWrite you can read the file while it is open.

LeeReid1 on Dec 15, 2021

When using var log = await File.ReadAllTextAsync("app.log"); I get “The process cannot access the file ‘…’ because it is being used by another process”

But this does work for me:

var log = "";
await using (var fileStream = new FileStream("app.log", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
	using (var reader = new StreamReader(fileStream))
	{
		log = await reader.ReadToEndAsync();
	}
}

The problem here is that some logs are still not written to the file, so sometimes I end up with log that is not complete… I need to read log file when my console app is done with execution, so I did this:

// Force file logger to finish writing logs to file and release lock.
var fileProvider = serviceProvider.GetService(typeof(ILoggerProvider)) as FileLoggerProvider;
fileProvider?.Dispose();
var log = await File.ReadAllTextAsync("app.log");

Ugly but it works in my specific case. I think that library should provide a better way to handle this.

@VitaliyMF thanks for this library, I hope that you will continue to maintain it for a long time 😃

zoran-petrovic-87 on Oct 21, 2021