Message sinks!

A very small logging library   

Several years ago (and still today), log4net was a very popular logging library for C#/.NET. And, several years ago, I had the idea to distill the most important, popular libraries into a single library. So I took a look at log4net - admittedly, without ever actually using it before - with the goal of producing a much smaller logging library within Loyc.Essentials that provided the most commonly-used features of log4net.

That didn’t work out. It turned out that log4net was not merely a large library (larger than all of Loyc.Essentials), but it was also very complicated, a spaghetti of interwoven interfaces and dependencies. It was difficult to follow how it worked internally, and I wasn’t sure where the useful “core” was that would retain compatibility with the most commonly-used features.

Instead I just designed a small logging library without regard for compatibility with log4net - something that would be useful not only for logging but also for other situations where messages are produced, such as error messages in my compilers.

I call it the “message sink” - a drain you dump messages into. The word “sink” is consistent with the naming convention of other “sources” and “sinks” in Loyc.Essentials, e.g. ICollectionSink<in T> is a subset of ICollection<T> that lets you adding or removing items, but not get items out, while IListSource<out T> is like IList<T> but you can only get items, not add or remove items.

The interface

IMessageSink is a simple interface designed to make it easy to define your own implementations, so that you can easily extend it with more functionality as you need it - while balancing the need for good performance.

// Alias for IMessageSink<object>
public interface IMessageSink : IMessageSink<object>
public interface IMessageSink<in TContext>
	/// <summary>Returns true if messages of the specified type will actually be 
	/// printed, or false if Write(type, ...) has no effect.</summary>
	bool IsEnabled(Severity type);
	/// <summary>Writes a message to a log or other target.</summary>
	void Write(Severity level, TContext context, string format);
	void Write(Severity level, TContext context, string format, object arg0, object arg1 = null);
	void Write(Severity level, TContext context, string format, params object[] args);

Most people will just use IMessageSink, but you can also customize the meaning of the context parameter. Notice the in in IMessageSink<in TContext>: this means that any IMessageSink<object> is implicitly convertible to IMessageSink<C> for any class C. Because of this, you can, for example, use ConsoleMessageSink as if it were IMessageSink<C> even though it only implements IMessageSink.

The first method, IsEnabled(c), lets you find out before printing a message whether that message could actually be printed (if it returns false, messages in category c are being filtered out.) This lets you avoid doing work to construct a message that will be discarded.

A message has four parts:

  1. Severity type: an enum that indicates what kind of message this is (how “serious” or how “common”) on a numeric scale. Commonly used values include Severity.Error, Severity.Warning, and Severity.Debug.
  2. TContext context: an object that represents the “context” or “location” that the message relates to. Typically TContext = object, so this parameter could be anything, and the exact meaning of the context can vary from application to application.
  3. string format: a message to be logged, with optional argument placeholders like {0} and {1}.
  4. format arguments: objects or strings to insert into the format string.

Now, log4net lets you write an object or a string, but to me it made more sense to write an object and a string, where the object provides some sort of “context” for the message. In addition to this interface, many log4net-like extension methods are provided that you can use as shortcuts (e.g. Warn("It's cold outside!"), which uses level: Severity.Warning and context: null.)

Only a single Write() method is truly needed, but it is expected to be fairly common that a message sink will drop some or all messages without printing them, e.g. if a message sink is used for logging, verbose messages might be “off” by default. It would be wasteful to actually format a message if the message will not actually be printed, and it would even be wasteful to create an array of objects to hold the arguments if they are just going to be discarded. With that in mind, since most formatting requests only need a couple of arguments, there is an overload of Write() that accepts up to two arguments without the need to package them into an params array.

So there’s three Writes:

Message sinks may perform localization using Localize.Localized().

Basic sinks

The following “basic” sinks are built into Loyc.Essentials (the ones with .Value are singletons - you usually don’t need to create new instances):

ConsoleMessageSink and TraceMessageSink produce similar output; for example

ConsoleMessageSink.Value.Write(Severity.Error, "Foo.csv", "Syntax error")

comes out as

Error: Foo.csv: Syntax error

By default, ConsoleMessageSink (but not TraceMessageSink) leaves out the severity for lower-level messages (anything below Warning), so the text color alone indicates the Severity.

Note: Message sinks convert the context object to a string by calling MessageSink.ContextToString, see below.

Wrapper sinks

Some sink types are wrapper objects that modify an “inner” or “target” sink:

Extension methods

IMessageSink has a series of extension methods like these, which lets you use it like log4net:

public static bool IsErrorEnabled<C>(this IMessageSink<C> sink)
	return sink.IsEnabled(Severity.Error);
public static void Error(this IMessageSink<object> sink, string format)
	sink.Write(Severity.Error, null, format);
public static void ErrorFormat(this IMessageSink<object> sink, string format, params object[] args)
	sink.Write(Severity.Error, null, format, args);
public static void Error<C>(this IMessageSink<C> sink, C context, string format)
	sink.Write(Severity.Error, context, format);
/* ...more Error methods... */

public static bool IsWarnEnabled<C>(this IMessageSink<C> sink)
	return sink.IsEnabled(Severity.Warning);
public static void Warn(this IMessageSink<object> sink, string format)
	sink.Write(Severity.Warning, null, format);
public static void WarnFormat(this IMessageSink<object> sink, string format, params object[] args)
	sink.Write(Severity.Warning, null, format, args);
public static void Warning<C>(this IMessageSink<C> sink, C context, string format)
	sink.Write(Severity.Warning, context, format);

/* ...more Warn methods... */

The names Warn and WarnFormat come directly from log4net.

Note: the methods called ErrorFormat (and WarnFormat, etc.), which do not take a context parameter, actually cannot be called Error instead. If the method had been called Error rather than ErrorFormat then if you call

messageSink.Error("", "");

the call would be ambiguous between IMessageSink<C>.Error(C context, string format) and IMessageSink.Error(string format, object arg0). So be careful: the word Format is needed to tell the compiler that there is no context parameter! There is no way, unfortunately, to tell the compiler that the context will never be a string.

For warnings, specifically, log4net calls them Warn but I call them Warning. So I decided that when providing a context parameter, the method would be called Warning. This ensures that when you call Warn but you actually intended to call WarnFormat, you’ll get a compiler error instead of calling the wrong method.

Comparison with log4net

log4net is typically configured via XML files. It would be nice if a volunteer would step up to add a similar feature to Loyc Core, but I don’t personally need XML-based configuration, and in the interest of keeping Loyc.Essentials small, that feature would probably end up in a separate assembly unless the feature can be implemented in quite a compact way.

In log4net there is a convention of defining a static field in each of your classes to provide logging:

private static readonly log4net.ILog log = log4net.LogManager.GetLogger

You can do something similar with message sinks:

private static readonly IMessageSink log = MessageSink.WithContext

This will send messages to the default message sink (MessageSink.Default) using the Type of the current class as the default context parameter (when the context given to Write is null).

If you want to play with message sinks in code that uses log4net already, you could even add a ”fake” log4net so that the original code keeps working:

using Loyc;

namespace log4net
    interface ILog : IMessageSink
        // If necessary, Add things from the real ILog

    class MessageSinkAsILog : WrapperBase<IMessageSink<object>>, ILog
        public MessageSinkAsILog(IMessageSink<object> wrappedObj) : base(wrappedObj) { }

        public bool IsEnabled(Severity level)
            return _obj.IsEnabled(level);
        public void Write(Severity level, object context, [Localizable] string format)
            _obj.Write(level, context, format);
        public void Write(Severity level, object context, [Localizable] string format, 
                          params object[] args)
            _obj.Write(level, context, format, args);
        public void Write(Severity level, object context, [Localizable] string format, 
                          object arg0, object arg1 = null)
            _obj.Write(level, context, format, arg0, arg1);

    class LogManager
        public static ILog GetLogger(object type) {
            return new MessageSinkAsILog(MessageSink.WithContext(type));

If you want to do Type-specific filtering (e.g. filtering out Debug messages in certain types but not others), Loyc.Essentials doesn’t currently support that directly; you’d need to write some custom code.

You’ll also need using Loyc so that the extension methods are available.

Note: Calls to IMessageSink aren’t quite source-level compatible with log4net’s ILog. The first reason is that extension methods like IsErrorEnabled() are methods, whereas in log4net they are properties. If Microsoft adds “extension everything” to C#, the extension method could eventually be changed to a property. The second reason is that log4net has methods like Error(object) that take an object without a string, but Loyc.Essentials has a different “ideology” of passing both an object and a string.

Customizing behavior

Of course, you can always implement your own IMessageSink to get custom behavior. You can also quickly create a message sink without implementing the entire IMessageSink interface, by calling MessageSink.FromDelegate:

    var sink = MessageSink.FromDelgate(
        (level, context, fmt, args) => {}, 
        level => /* return true if level is enabled */);

You can set the default message sink by calling MessageSink.SetDefault(). This method returns a using-compatible structure so that if you don’t want to change it permanently, you can change it temporarily. For example:

    // block all messages temporarily
    using (MessageSink.SetDefault(MessageSink.Null)) {
    // old message sink is restored here

This is a case of the Ambient Service Pattern.

Message sinks that need to convert the context to a string should do so by calling MessageSink.ContextToString(context). This method’s default behavior is to check if the object implements the IHasLocation interface:

public interface IHasLocation // in namespace Loyc
    object Location { get; }

If it does, the Location property is called and the returned location is converted to a string; otherwise, ToString() is called on the context itself.

This is useful, for example, in my compilers like Enhanced C#. The context is the syntax tree that has the error, but the location object represents a location in a source file where that syntax tree is located, so that error messages end up having a location like “Foo.ecs(123,21)”.

If this behavior is not what you need, you can override it by calling MessageSink.SetContextToString(context => ...).


Sometimes you’d like to add more details about a message that was just written. You can do that with a message sink with code like this:

MessageSink.Write(Severity.Error, location, "Expected closing brace here");
MessageSink.Write(Severity.ErrorDetail, openlocation, "(Opening brace was here)");

Each normal value of Severity is an even number, with an associated Detail severity that is one less. For example, Severity.Warning is 60 and Severity.WarningDetail is 59.

When using SeverityMessageFilter, you should prefer to use a Detail level as the MinSeverity:

s = new SeverityMessageFilter(c.Sink, Severity.NoteDetail);

In anticipation that users would accidentally write Severity.Note when they mean Severity.NoteDetail, or Severity.Warning when they mean Severity.WarningDetail, SeverityMessageFilter has a third parameter that defaults to true:

new SeverityMessageFilter(c.Sink, Severity.Warning, includeDetails: true);

This third parameter simply decrements the second parameter if the second parameter is an even number, so that details are included unless you specifically set that parameter to false. However, no such trickery happens when you set the SeverityMessageFilter.MinSeverity property.

Other stuff

Messages that you store in a MessageHolder have type LogMessage. Its basic properties are Severity, Context, Format and Args, and there is also a Formatted property that localizes and formats the format string (not including the Severity and Context). The ToString() method, however, combines all four elements in one message by calling MessageSink.FormatMessage().

The LogException exception takes four arguments just like a message sink:

new LogException(severity, context, format, args)

The severity argument is not required; the default is Severity.Error.

LogException has a Msg property of type LogMessage where it stores this information. (It is not stored in the Exception’s Data dictionary because items in Data must be serializable, but the context object is not necessarily serializable.)

Get it

In Visual Studio, you can use NuGet to install Loyc.Essentials which includes everything described here. The source code is here. Thanks for reading!