C# in Depth

262 CHAPTER 10 Extension methodsnamespace for the sake of making types available using only their simple names, butwithout making the extension methods within that namespace available at the sametime. I recommend using a namespace that solely contains static classes with extensionmethods to mitigate this problem.There’s one aspect of extension methods that can be quite surprising when youfirst encounter it but is also useful in some situations. It’s all about null references—let’s take a look.10.2.4 Calling a method on a null referenceI’d be amazed if I ever encountered anyone who’d done a significant amount of .NETprogramming without seeing a NullReferenceException due to calling a method witha variable whose value turned out to be a null reference. You can’t call instance methodson null references in C# (although IL itself supports it for nonvirtual calls)—but you cancall extension methods with a null reference. This is demonstrated by listing 10.5. Notethat this isn’t a snippet since nested classes can’t contain extension methods.Listing 10.5Extension method being called on a null referenceusing System;public static class NullUtil{public static bool IsNull(this object x){return x==null;}}public class Test{static void Main(){object y = null;Console.WriteLine(y.IsNull());y = new object();Console.WriteLine(y.IsNull());}}The output of listing 10.5 is “True” then “False”—if IsNull had been a normalinstance method, an exception would have been thrown in the second line of Main.Instead, IsNull was called with null as the argument. Prior to the advent of extensionmethods, C# had no way of letting you write the more readable y.IsNull() formsafely, requiring NullUtil.IsNull(y) instead. There’s one particularly obvious examplein the framework where this could be useful: string.IsNullOrEmpty. C# 3 allowsyou to write an extension method that has the same signature (other than the “extra”parameter for the extended type) as an existing static method on the extended type.To save you reading through that sentence several times, here’s an example—eventhough the string class has a static, parameterless method IsNullOrEmpty, you canstill create and use the following extension method:Licensed to Rhona Hadida