C# Extension Methods - How far is too far?

Question

Rails introduced some core extensions to Ruby like 3.days.from_now which returns, as you'd expect a date three days in the future. With extension methods in C# we can now do something similar:

static class Extensions
{
    public static TimeSpan Days(this int i)
    {
        return new TimeSpan(i, 0, 0, 0, 0);
    }

    public static DateTime FromNow(this TimeSpan ts)
    {
        return DateTime.Now.Add(ts);
    }
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(
            3.Days().FromNow()
        );
    }
}

Or how about:

static class Extensions
{
    public static IEnumerable<int> To(this int from, int to)
    {
        return Enumerable.Range(from, to - from + 1);
    }
}

class Program
{
    static void Main(string[] args)
    {
        foreach (var i in 10.To(20))
        {
            Console.WriteLine(i);
        }
    }
}

Is this fundamentally wrong, or are there times when it is a good idea, like in a framework like Rails?

Answer 1

I like extension methods a lot but I do feel that when they are used outside of LINQ that they improve readability at the expense of maintainability.

Take 3.Days().FromNow() as an example. This is wonderfully expressive and anyone could read this code and tell you exactly what it does. That is a truly beautiful thing. As coders it is our joy to write code that is self-describing and expressive so that it requires almost no comments and is a pleasure to read. This code is paramount in that respect.

However, as coders we are also responsible to posterity, and those who come after us will spend most of their time trying to comprehend how this code works. We must be careful not to be so expressive that debugging our code requires leaping around amongst a myriad of extension methods.

Extension methods veil the "how" to better express the "what". I guess that makes them a double edged sword that is best used (like all things) in moderation.