Extension methods in C#

C# got very interesting feature allowing you to add a method to existing class. Here is what it looks like:

namespace ConsoleApp
{
    class A
    {
        public double value { get; set; }

        public A()
        {
            value = 7.0;
        }
    }

    static class ExtensionMethods
    {
        public static double ComputeReciprocal(this A a)
        {
            return 1.0 / a.value;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var a = new A();
            var value = a.value;
            var reciprocal = a.ComputeReciprocal();
        }
    }
}

C# successfully recognized ComputeReciprocal as A class extension method and let us call it like if it belonged to the class. Please pay attention to extension method parameter list, the first parameter always has to be: this ClassName parameterName, after the first parameter you can add as many additional parameters as you want:

namespace ConsoleApp
{
    class Person
    {
        public string FirstName { get; set; }

        public Person()
        {
            FirstName = "Alex";
        }
    }

    static class ExtensionMethods
    {
        public static string GetFullName(this Person person, string lastName)
        {
            return person.FirstName + " " + lastName;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var person = new Person();
            var fullName = person.GetFullName("Peters");
        }
    }
}

By the way, it doesn't matter what you call static class which contains your extension methods.