Question

So I started reading Jon Skeet’s 2nd edition of C# in depth and kind of confused about the following code in terms what it does and what is wrong with it (ch 13, section: Immutability and object initialization)

Message message = new Message(
"[e-mail]",
"[e-mail]",
"I hope you like the second edition")
{
       Subject = "A quick message"  // <=  {Subject = "A quick message"   }; what is it?
};

Elaboration on this topic, would help tremendously!

Would someone explain that?

Answer 1

This code:

Message message = new Message(
"[e-mail]",
"[e-mail]",
"I hope you like the second edition")
{
       Subject = "A quick message"
};

is equivalent to this:

Message tmp = new Message("[e-mail]", 
    "[e-mail]",
    "I hope you like the second edition");
tmp.Subject = "A quick message";
Message message = tmp;

That’s just object initializer syntax: see chapter 8 for more details.

The problems with this are:

• It’s not clear what all the string arguments mean – what’s “from”, what’s “to”, what’s the body etc
• It requires the type to be mutable (unfortunate for all kinds of reasons)
• If you want to also specify the subject in the constructor, you end up with a bunch
  of different overloads
• If you have cross-validation between (say) subject and body, you need to be careful about when you do it

With optional parameters and named arguments, all of this is solved:

// The argument names may be wrong here; I don't have the book with me.
// Intellisense will prompt you though :)
Message message = new Message(
    from: "[e-mail]",
    to: "[e-mail]",
    body: "I hope you like the second edition",
    subject: "A quick message);

It’s clear what means what, it’s all in a constructor call so the type can be immutable, you can have a single constructor with optional parameters (e.g. here we could have a byte[] attachment = null parameter specified in the constructor), and you can do all the validation in one place. Lovely.