C# for the Java Developer: Extension Methods

After spending several years crafting Java code, I recently decided to dive back into C# and share what I learn in the process. In this blog post, I will talk about extensions methods. This concept, which exists in some JVM languages (like Kotlin) but not in Java, let the developer add methods to a class without touching its code (hence the name extension methods).

Let’s say you want to create a method that puts the first letter of a string in upper case and returns the new string. You cannot modify (or even inherit) the String class in Java to add this method. Since you don’t necessarily want to create your own custom flavour of the String class for your code base, you would probably create a static function that takes a String as an argument and return another String:

class StringExtensions {
    static String capitalize(String word) {
        return Character.toUpperCase(word.charAt(0)) + word.substring(1);

Now, you can simply invoke this function like this:

String word = "hello";
String capitalizedWord = StringExtensions.capitalize(word);

Of course, creating a static method is also possible in C#. However, C# has a feature that allows to add new methods to a class without having to modify its code. This feature is called extension methods. In order to create an extension method, you have to create a top-level static class and implement a static method. The first argument of this method specifies the type that the method operates on and should have the this modifier:

static class StringExtensions 
  static string Capitalize(this String word)
      // Note that this cannot access any private data in the String class. 
      return char.ToUpper(word[0]) + word.Substring(1);

In order to use it, the namespace that contains the class must be specified with a using directive. Afterwards, the method can be invoked as if it was an instance method of the type:

string word = "hello";
string capitalizedWord = word.Capitalize();

Extension methods are very powerful. Beside the fact that the code looks “cleaner”, it is also possible to seamlessly plug new behaviours in existing types by simply importing a namespace. The extension code can be in separate specific modules that can be imported only when needed.