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Custom logging in mixed-language environment

I'm tasked of introducing logging to a larger project. I have the following requirements:
Logging to the same file must be enabled from Visual Studio's C++ products, C# products, desktop apps, windows services, and more than one process should be able to write to a log file at once.
Format of logs is custom (semi-colon delimited fields, something like "custom_date;custom_time;the_rest;of_the_fields").
Log files have limit in size.
There's main .log file and older .bak file. .bak file is deleted when new .log file is created and current .log is renamed to .bak.
In one special case name of log files depends on time of creation. There are no multi-process writes in this case.
Now, I can roll my own implementation, but it would be really nice if there are ready made free libraries that satisfy all of the requirements. Does any one know of such libraries?

Many of your requirements (I think all but the language independence) are fulfilled by log4net
As you want to use several software components to use the logger I would suggest to write a windows service by yourself as it can be used by all types of your client software (C++, C#, ...)

Maybe you could simply write to the Event Log.

I would recommend NLog, meets most of your requirements

Use Microsoft's Enterprise Exception and Logging Application blocks. It satisfies all of your requirements. Everything is configurable using the web.config or app.config and allows the use of templates to record specific details. Also note that Microsoft has included a rolling type logger that will automatically start a new file based upon size or date/time. It's a complete package for any type of logging you want to do, MSMQ, SQL, flat file, windows event log, etc.

Log4Net can help you with your 2 and 3 point, for your 1,4 and 5 point i suggest you write a WebService that do all the work for writting in the logs, create, delete, etc.

Logging in someone else's code

I have just begun working on a C# application (using log4net). I'm supposed to add logs to code written by someone else. Hence, it's not possible for me to understand well the context each time (too much code, too less time :) ).
I have been following a convention which seems quite crude. I display log level, datetime, class name, method name with every log. I print the log on entering and exiting every method (most of them, i try to exclude method within big loops), in constructors, some events and every catch statement.
I think I'm overdoing it at places but some degree of uniformity is required. Any suggestions on the right (or better) approach?

Logging the start and end of every method, and the constructors, is certainly overkill.
I'm afraid that if you don't understand the method you won't be able to log the appropriate issues.
At the very least log all exceptions, never catch an exception without doing something with it.

I suggest, you first of all find out what is the reason why the log is needed now. After all, the code you are working on was apperntly developed without much logging, so what is the reason they want you to add logging: do they have problems locating bugs (what kind of bugs?) do they want be able to see in retrospect when the program did something (what is this something?)?

From my pov it is okay to log every function/constructor with input parameters and results in debug.
But keep the log messages as brief as possible as most data can be added/changed via the log4net configuration. Thus log only parameter1: value1 parameter2: value2 and result:x

Maybe you should also consider to log important conditional statements and their branches (entries), such as if/else if/else, and switch/case/default etc. This will help you provide more logic details inside the methods.

logging- changing implementation?

We have been using log4net for logging our asp.net web forms application. Our logging is typically in our business layer and the typical implementation is like this
SomeMethodCall(MethodParams)
{
Log.Start("Starting Some Method");
try
{
//do something
}
catch(Exception ex)
{
Log.Exception("exception in SomeMethodCall" + ex.message);
}
Log.End("End SomeMethod");
}
In my opinon, it is a bit clumsy. Is there a cleaner way of doing it without using AOP ?I am not sure if I would need to have the overhead of adding a framework, just for logging, thought I understand that it would give me a lot of other options (which I do not need)
I am thinking of using some AOP frameworks to do it in a more cleaner way, just by marking the methods with attributes to log and handle exceptions.
There are 2 things I am concerned with AOP (after my initial reading).
Some frameworks inject code into your IL(as per my understanding) and am concerned if it would misguide me. I might be looking at line x, given by my AOP framework, where as it actually might be line y in my application. Is my fear unfounded?
Performance: How much of a performance overhead would be added if using an AOP framework.
Edit: I was also looking into PolicyInjectionApplicationBlock. Unfortunately, I do not have the luxury of changing implementaions inside my business logic

Are you saying that most / all of your methods take this structure? If so, my advice would be to tone down your logging. (I stress that this is advice, and possibly controversial advice as well).
There shouldn't really be a need to use a blanket log of the entry and exit of every single method in this way, instead place your log entries more strategically. I like to ensure that all log messages have a "purpose" - if I can't think of a scenario where that log message would help or aid me in some way while debugging then it has no business being in my code!

You can read this, also read 7 approaches for AOP in .Net, I use AOP in Java and i didn't see performance problem. Anyway check here to be careful....
Additional
Spring.Net AOP

Can log4net count how many error traces have been logged?

For a C# regression test simulation of some hardware we're using log4net to trace the execution of the simulation. Errors are logged every time something goes wrong, and there should be zero errors, of course. We use the error count to determine pass/fail of the test, currently we search the log for ERROR to determine this.
Is it possible to retrieve an error count from log4net that increments every time an error is logged? I see that it is possible to log errors to a separate file but this is not exactly what we want, although with some fiddling we could extract the information indirectly from it, of course.
Thanks in advance.

I would log directly to a database or create a routine to import the log files into a database. Once the logs are in a database table they can be easily queried with SQL.

I can not think of anything directly built into log4net.
Either use some built-in appender that will let you count error occurences as Jaime suggested or alternatively create your own appender that will do exactly you want.
It is not too complicated especially since you plan using log4net to automate your whole testing process.

As Konrad suggests, roll your own. You should subclassForwardingAppender, making it count messages on their way to the "real" appenders. The appender could log the actual counts to a separate appender.

Error logging in C#

I am making my switch from coding in C++ to C#. I need to replace my C++ error logging/reporting macro system with something similar in C#.
In my C++ source I can write
LOGERR("Some error");
or
LOGERR("Error with inputs %s and %d", stringvar, intvar);
The macro & supporting library code then passes the (possibly varargs) formatted message into a database along with the source file, source line, user name, and time. The same data is also stuffed into a data structure for later reporting to the user.
Does anybody have C# code snippets or pointers to examples that do this basic error reporting/logging?
Edit: At the time I asked this question I was really new to .NET and was unaware of System.Diagnostics.Trace. System.Diagnostics.Trace was what I needed at that time. Since then I have used log4net on projects where the logging requirements were larger and more complex. Just edit that 500 line XML configuration file and log4net will do everything you will ever need :)

Lots of log4net advocates here so I'm sure this will be ignored, but I'll add my own preference:
System.Diagnostics.Trace
This includes listeners that listen for your Trace() methods, and then write to a log file/output window/event log, ones in the framework that are included are DefaultTraceListener, TextWriterTraceListener and the EventLogTraceListener. It allows you to specify levels (Warning,Error,Info) and categories.
Trace class on MSDN
Writing to the Event Log in a Web Application
UdpTraceListener - write log4net compatible XML messages to a log viewer such as log2console

I would highly recommend looking at log4Net. This post covers the majority of what you need to get started.

Another good logging library is NLog, which can log to a lot of different places, such as files, databases, event logger etc.

I use The Object Guy's Logging Framework--as do most people who try it. This guy has some interesting comments about it.

Enterprise Library is a solid alternative to log4net and it offers a bunch of other capabilities as well (caching, exception handling, validation, etc...). I use it on just about every project I build.
Highly recommended.

Even though I personally hate it, log4net seems to be the de facto standard for C# logging. Sample usage:
log4net.ILog log = log4net.LogManager.GetLogger(typeof(Program));
log.Error(“Some error”);
log.ErrorFormat("Error with inputs {0} and {1}", stringvar, intvar);

As I said in another thread, we've been using The Object Guy's Logging Framework in multiple production apps for several years. It's super easy to use and extend.

Log4Net is a rather comprehensive logging framework that will allow you to log to different levels (Debug, Error, Fatal) and output these log statements to may different places (rolling file, web service, windows errors)
I am able to easily log anywhere by creating an instance of the logger
private static readonly ILog _log = LogManager.GetLogger(typeof([Class Name]));
and then logging the error.
_log.Error("Error messsage", ex);

Serilog is late to the party here, but brings some interesting options to the table. It looks much like classical text-based loggers to use:
Log.Information("Hello, {0}", username);
But, unlike earlier frameworks, it only renders the message and arguments into a string when writing text, e.g. to a file or the console.
The idea is that if you're using a 'NoSQL'-style data store for logs, you can record events like:
{
Timestamp: "2014-02-....",
Message: "Hello, nblumhardt",
Properties:
{
"0": "nblumhardt"
}
}
The .NET format string syntax is extended so you can write the above example as:
Log.Information("Hello, {Name}", username);
In this case the property will be called Name (rather than 0), making querying and correlation easier.
There are already a few good options for storage. MongoDB and Azure Table Storage seem to be quite popular for DIY. I originally built Serilog (though it is a community project) and I'm now working on a product called Seq, which provides storage and querying of these kinds of structured log events.

You can use built in .NET logging. Look into TraceSource and TraceListeners, they can be configured in the .config file.

Ditto for log4net. I'm adding my two bits because for actual use, it makes sense to look at some open source implementations to see real world code samples with some handy additions. For log4net, I'd suggest off the top of my head looking at subtext. Particularly take a look at the application start and assemblyinfo bits.

Further to the couple of comments realting to the use of the System.Diagnostics methods for logging, I would also like to point out that the DebugView tool is very neat for checking debug output when needed - unless you require it, there is no need for the apps to produce a log file, you just launch DebugView as and when needed.

The built in tracing in System.Diagnostics is fine in the .NET Framework and I use it on many applications. However, one of the primary reasons I still use log4net is that the built in .NET Framework tracing lacks many of the useful full featured appenders that log4net already supplies built in.
For instance there really isn't a good rolling file trace listener defined in the .NET Framework other than the one in a VB.NET dll which really is not all that full featured.
Depending on your development environment I would recommend using log4net unless 3rd party tools are not available, then I'd say use the System.Diagnostics tracing classes. If you really need a better appender/tracelistener you can always implement it yourself.
For instance many of our customers require that we do not use open source libraries when installed on their corporate machines, so in that case the .NET Framework tracing classes are a perfect fit.
Additionally - http://www.postsharp.org/ is an AOP library I'm looking into that may also assist in logging as demonstrated here on code project:http://www.codeproject.com/KB/dotnet/log4postsharp-intro.aspx.

ExceptionLess is one of the easiest nuget package available to use for logging. Its an open source project. It automatically takes care of unhandled exception, and options for manually logs are available. You can log to online or self host on local server.

Log4Net, as others have said, is fairly common and similar to Log4j which will help you if you ever do any Java.
You also have the option of using the Logging Application Block http://www.codeproject.com/KB/architecture/GetStartedLoggingBlock.aspx