Extension methods

Even as an experienced software developer, it is amazing when you discover some really trivial things, or discover some interesting quirk of a programming languages.

I was looking at a Code Review the other week and I saw some code that looked really pointless. It was testing some code throws a ArgumentNullException.

[Fact]
public void LogWarningDetails_WithNullLogger_ThrowsArgumentNullException()
{
	ILogger logger = null;
	Assert.Throws<ArgumentNullException>(() => logger.LogWarning("Test Error Message"));
}

A NullReferenceException is an incredibly common mistake and probably the first problem new developers encounter. If you have a reference to an object, but the object is null, you cannot call instance methods on it.

Therefore if logger is null, then you cannot call LogWarning without an error being thrown.

So on first glance, this test looks like it is testing the basic fundamentals of the C# Programming language. However, this is testing for ArgumentNullException rather than NullReferenceException.

LogWarning was actually defined as an extension method, and this actually does allow you to call methods on null references. I’ve never realised this or even thought about it. It is the case because extension methods actually pass the reference in as a parameter.

So if you have an extension method (as indicated with the this keyword):

	public static bool IsNull(this object x) 
	{
		return x == null; 
	}

This can be called like this:

	static void Main() 
	{
		object y = null;
		Console.WriteLine(y.IsNull()); 
		y = new object(); 
		Console.WriteLine(y.IsNull());
	} 

Which would output true, then false. Which illustrates that the extension method does not crash if the reference to y is null, and the logic correctly works by returning true when y is null.

Conclusion:

Understanding NullReferenceExceptions is basically day 1 of learning to code in an Object Oriented Language like C# but I’ve never even considered there is an exception to the rule. A method call on a null reference won’t cause a NullReferenceException if the method is an Extension method!