The recording of the first ever ReactiveUI virtual conference is available on our YouTube Channel.


One thing that motivates me to write my own instead of using the legion of others, is that most loggers give zero thought to perf concerns on mobile devices - they're all written for servers, so none of them think about CPU perf or allocations. The best imho is Serilog, but it allocates way too much stuff imho to be usable on mobile — Anaïs Betts (2014) issue 46#issuecomment-56550457


ReactiveUI has an underlying framework called Splat which provides a logging interface which can be used to debug your applications as well as ReactiveUI itself. You may ask yourself, "Seriously, another logging interface \ framework?". The reason RxUI does this itself is for portability - none of the common popular logging frameworks support all of the platforms that ReactiveUI supports, and many are server-oriented frameworks ill-suited for simple mobile app logging.

Splat has taken the approach of allowing the developer to decide the target logging framework, whilst the interface aims to keep ReactiveUI and any other users of Splat as agnostic as possible. Logging frameworks have evolved since 2014 but there is still the challenge of performance and flexibility. There has also been a move within Splat to offer better integration into other frameworks so you don't have to make accomodations for Splat or vice versa.

Getting started

By default Splat uses a NullLogger (i.e. it doesn't log). To set up logging:

  1. Register an implementation of ILogger using Service Location.
  2. In the class in which you want to log stuff, "implement" the IEnableLogger interface (this is a tag interface, no implementation actually needed).
  3. Call the Log method to write log entries:

Available logging adapters

Splat has support for the following logging frameworks:

Target Package NuGet
Console Splat SplatBadge
Debug Splat SplatBadge
Log4Net Splat.Log4Net SplatLog4NetBadge
Microsoft Extensions Logging Splat.Microsoft.Extensions.Logging SplatMicrosoftExtensionsLoggingBadge
NLog Splat.NLog SplatNLogBadge
Serilog Splat.Serilog SplatSerilogBadge


This is a simple built in logger, Console logging is not available on every platform and typically carries a performance overhead. It is recommended NOT to use this logger unless you have a specific need for Console output and don't want to use a larger logging framework.

// I only want to hear about errors
var logger = new ConsoleLogger() { Level = LogLevel.Error };
Locator.CurrentMutable.RegisterConstant(logger, typeof(ILogger));


This is a simple built in logger, useful during early development, but you may want to consider using a fully fledged logging package below and using the debug target of that.

// I only want to hear about errors
var logger = new DebugLogger() { Level = LogLevel.Error };
Locator.CurrentMutable.RegisterConstant(logger, typeof(ILogger));


First configure Log4Net. For guidance see configuration

using Splat.Log4Net;

// then in your service locator initialisation


First configure Microsoft.Extensions.Logging. For guidance see logging

using Splat.Microsoft.Extensions.Logging;

// note: this is different from the other adapter extension methods
//       as it needs knowledge of the logger factory
//       also the "container" is how you configured the Microsoft.Logging.Extensions
var loggerFactory = container.Resolve<ILoggerFactory>();
// in theory it could also be
// var loggerFactory = new LoggerFactory();

/// then in your service locator initialisation


First configure NLog. For guidance see tutorial and configuration file

using Splat.NLog;

//  then in your service locator initialisation


First configure Serilog. For guidance see configuration basics

using Splat.Serilog;

// Then in your service locator initialisation

Using a different logging framework

Splat has tried to cater for offering sufficient scope for logging frameworks. If you need another framework or platform consider the whether your desired target platform is available as an output to one of the other logging frameworks already covered? Using an existing output for an existing framework will make your life easier. For example OpenTelemetry is supported across all the supported frameworks.

If we are missing a target (as times change) then feel free to engage the team at the Splat Issues Page on Github.

If you want to write your own logging integration the source code of an existing projects such as the NLog adapter is a good reference. There are a few interfaces you can use depending on the level of control you need for your implementation.

There are 2 logging interfaces to consider: ILogger and IFullLogger. The full logger is the full detailed interface for logging, ILogger is the minimal interface where you can use the default implementation for sending in the logging requests, and need a minimal implementation to get the detail out into the target platform. In most scenarios you should be able to use WrappingFullLogger and implement a smaller subset of ILogger which you can pass in. Your decision will be based on the flexibility and performance of the logging framework you're trying to use. For example if your logging framework supports a simple Write method that takes a log level you may want to use ILogger with the WrappingFullLogger. However if the target framework has specific methods per log level, or has both offerings but is more performant with the direct calls you may wish to implement IFullLogger.

Logging via this.Log() and IEnableLogger

ReactiveUI's logger works a bit differently than other frameworks - its design is inspired by Rails 'logger'. To use it, make your class implement the IEnableLogger interface:

public class MyClass : IEnableLogger
    // IEnableLogger doesn't actually require anything of us

Now, you can call the Log method on your class. Because of how extension methods work, you must prepend this to it:

this.Log().Info("Downloaded {0} tweets", tweets.Count);

There are five levels of logging, Debug, Info, Warn, Error, and Fatal. Additionally, there are special methods to log exceptions - for example, this.Log().InfoException(ex, "Failed to post the message"). This trick doesn't work for static methods though, you have to settle for an alternate method, LogHost.Default.Info(...).

this is used to set the class name for a logger. this.Log() call doesn't always create a new instance of a logger, instead, it uses a cached version if possible.

Debugging Observables

ReactiveUI has several helpers for debugging IObservables. The most straightforward one is Log, which logs events that happen to an Observable:

// Note: Since Log acts like another Rx operator like Select or Where,
// it won't do anything by itself unless someone Subscribes to it.
this.WhenAnyValue(x => x.Name)
    .SelectMany(async x => GoogleForTheName(x))
    .Log(this, "Result of Search")

Another useful method to debug Observables is LoggedCatch. This method works identically to Rx's Catch operator, except that it also logs the exception to the Logger. For example:

var userAvatar = await FetchUserAvatar()
    .LoggedCatch(this, Observable.Return(default(Avatar)));

Static Logging

For static methods, LogHost.Default can be used as the object to write a log entry for. The Static logger uses a different interface from the main logger to allow capture of additional caller context as it doesn't have the details of the class instance etc. when compared to the normal logger. To get the benefit of these you don't need to do much as they are optional parameters at the end of the methods that are utilised by the compiler\framework. Currently we only capture CallerMemberName.