Extension methods

An extension method is a static method in a non-nested, non-generic static class whose first parameter carries this: public static bool IsEven(this int n) => n % 2 == 0;.

Call it like an instance method: 4.IsEven(). The compiler rewrites it to IntExt.IsEven(4).

The namespace holding the static class must be imported with using, or the method is invisible.

A real instance method with the same signature always wins over an extension, so extensions can't override behavior.

LINQ itself is a set of extension methods on IEnumerable<T>; write your own the same way, e.g. this IEnumerable<T> src with a generic <T> on the method.

Console.WriteLine(4.IsEven());
Console.WriteLine("hello world".WordCount());
int[] xs = [3, 1, 4, 1, 5];
Console.WriteLine(string.Join(",", xs.EveryOther()));

static class Ext
{
    public static bool IsEven(this int n) => n % 2 == 0;
    public static int WordCount(this string s) =>
        s.Split(' ', StringSplitOptions.RemoveEmptyEntries).Length;
    public static IEnumerable<T> EveryOther<T>(this IEnumerable<T> src)
    {
        int i = 0;
        foreach (var x in src)
            if (i++ % 2 == 0) yield return x;
    }
}