Extension methods

You can’t edit the string class or a third-party library. But you can extend them: an extension method is a static method that you call as if it were an instance method of the type.

1public static class StringExtensions2{3    public static string ToTitleCase(this string s)4        => CultureInfo.InvariantCulture.TextInfo.ToTitleCase(s.ToLower());5}

Writing one

1public static class DoubleExtensions          // 1. a static class2{3    // 2. a static method  3. 'this' before the first parameter4    public static double MmToM(this double mm) => mm / 1000.0;5 6    public static bool IsBetween(this double value, double min, double max)7        => value >= min && value <= max;8}9 10double depth = 300;11Console.WriteLine(depth.MmToM());              // reads naturally12Console.WriteLine(4.5.IsBetween(3, 6));

0.3
True

• The class must be static and not nested; the method must be static.
• The first parameter, marked this, is the object the method is “called on”.
• The namespace containing the class must be imported with using to see the methods.
• Extension methods can only use the type’s public members: they don’t break encapsulation.

Extending interfaces

1public static class MemberExtensions2{3    public static double TotalWeight(this IEnumerable<Beam> beams)4    {5        double sum = 0;6        foreach (var b in beams) sum += b.Kg;7        return sum;8    }9}10 11List<Beam> level1 = LoadLevel(1);12Beam[] picked = GetSelection();13Console.WriteLine(level1.TotalWeight());    // works on a List…14Console.WriteLine(picked.TotalWeight());    // …and an array: both are IEnumerable<Beam>

This is exactly how LINQ works: Where, Select, OrderBy and friends are extension methods on IEnumerable<T> in the System.Linq namespace. That is why every collection gets them for free.

Rules of thumb

• If a type has an instance method with the same signature, the instance method wins.
• Use extensions for small, general helpers. Don’t hide important business logic in them.
• Group them in classes named after what they extend: StringExtensions, GeometryExtensions.

1public static class StringExtensions2{3    public static string Repeat(this string s, int times)4        => string.Concat(Enumerable.Repeat(s, times));5 6    public static bool IsPalindrome(this string s)7    {8        string lower = s.ToLowerInvariant();9        for (int i = 0, j = lower.Length - 1; i < j; i++, j--)10            if (lower[i] != lower[j]) return false;11        return true;12    }13}14 15Console.WriteLine("ab".Repeat(3));16Console.WriteLine("Level".IsPalindrome());

ababab
True