Question

I wanted to see if I can write some c# code in a javascript-fashion way but I got an unexpected behaviour. I know c# is not broken but I don´t understand what is wrong and why it doesn´t do what I think it should.

Code:

class Program
{
    static void Main(string[] args)
    {
        var person = CreatePerson(new{
            FirstName = "Lucas",
            LastName = "Ontivero",
            Age = 34
        });

        Console.WriteLine("Name....... : {0}", person.FullName);
        Console.WriteLine("IsJuvenile. : {0}", person.IsJuvenile);

        person.SetFirstName("Pablo");
        Console.WriteLine("Name....... : {0}", person.FullName);

        Console.ReadKey();
    }

    static private dynamic CreatePerson(dynamic cfg)
    {
        string firstName = cfg.FirstName;
        string lastName = cfg.LastName;
        var age = cfg.Age;

        return new{
            FullName = string.Format("{0} {1}", firstName, lastName),
            FirstName = firstName,
            LastName = lastName,
            SetFirstName = new Action<string>(fn => firstName= fn),
            SetLastName = new Action<string>(ln => lastName = ln),
            IsJuvenile = age < 18 
        };
    }
}

Output:

Name....... : Lucas Ontivero
IsJuvenile. : False
Name....... : Lucas Ontivero

But I expected:

Name....... : Lucas Ontivero
IsJuvenile. : False
Name....... : Pablo Ontivero

Why firstName doesn´t change?

Answer 1

It does… but FullName does not. It is computed inside CreatePerson and stored as a string, not a property evaluating to a string later.

You could use lambdas to cause delayed evaluation, as you did with SetFirstName.