Using extension methods on null objects

January 04, 2009 Edit patterns csharp

One feature that is often disregarded in extension methods is that they get called for null objects. While instrumenting parts of OpenRasta, I faced the problem of having some classes not always created through the container. As such, sometimes their Log property would be null, and each call to a logging method would have to check for that condition. After doing a few back-and-forth, I settled on using an extension method, coupled with a null object.

Take a simple class, and add a Log property.

public class LoggingClass
{
    public ILogger Log { get; set; }
}

Then, create a simple extension method on the ILogger, and return a null object

        public static ILogger Safe(this ILogger logger)
        {
            if (logger == null)
                return NullLogger.Instance;
            else
                return logger;
        }

Now all the logging code can simply call the Safe() method to ensure that, in the absence of a logger, a null object will be used instead. The code using it looks like the following.

        public void DoSomething()
        {
            Log.Safe().WriteDebug("I did something!");
        }

Voila!