Why is an Add method required for { } initialization?

问题

To use initialization syntax like this:

var contacts = new ContactList
{
    { "Dan", "dan.tao@email.com" },
    { "Eric", "ceo@google.com" }
};

...my understanding is that my ContactList type would need to define an Add method that takes two string parameters:

public void Add(string name, string email);

What's a bit confusing to me about this is that the { } initializer syntax seems most useful when creating read-only or fixed-size collections. After all it is meant to mimic the initialization syntax for an array, right? (OK, so arrays are not read-only; but they are fixed size.) And naturally it can only be used when the collection's contents are known (at least the number of elements) at compile-time.

So it would almost seem that the main requirement for using this collection initializer syntax (having an Add method and therefore a mutable collection) is at odds with the typical case in which it would be most useful.

I'm sure I haven't put as much thought into this matter as the C# design team; it just seems that there could have been different rules for this syntax that would have meshed better with its typical usage scenarios.

Am I way off base here? Is the desire to use the { } syntax to initialize fixed-size collections not as common as I think? What other factors might have influenced the formulation of the requirements for this syntax that I'm simply not thinking of?

回答1:

The reason for this is that it was retrofitted. I agree with you that using a constructor taking a collection would make vastly more sense, but not all of the existing collection classes implemented this and the change should (1) work with all existing collections, (2) not change the existing classes in any way.

It’s a compromise.

回答2:

I'm sure I haven't put as much thought into this matter as the C# design team; it just seems that there could have been different rules for this syntax that would have meshed better with its typical usage scenarios.

Your analysis is very good; the key problem is the last three words in the statement above. What are the actual typical usage scenarios?

The by-design goal motivated by typical usage scenarios for collection initializers was to make initialization of existing collection types possible in an expression syntax so that collection initializers could be embedded in query comprehensions or converted to expression trees.

Every other scenario was lower priority; the feature exists at all because it helps make LINQ work.

The C# 3 compiler team was the "long pole" for that release of Visual Studio / .NET - we had the most days of work on the schedule of any team, which meant that every day we delayed, the product would be delayed. We wanted to ship a quality product on time for all of you guys, and the perfect is the enemy of the good. Yes, this feature is slightly clunky and doesn't do absolutely everything you might want it to, but it was more important to get it solid and tested for LINQ than to make it work for a bunch of immutable collection types that largely didn't even exist.

Had this feature been designed into the language from day one, while the frameworks types were still evolving, I'm sure that things would have gone differently. As we've discussed elsewhere on this site, I would dearly love to have a write-once-read-many fixed size array of values. It would be nice to define a common pattern for proffering up a bunch of state to initialize an arbitrary immutable collection. You are right that the collection initializer syntax would be ideal for such a thing.

Features like that are on the list for potential future hyptothetical language versions, but not real high on the list. In other words, let's get async/await right first before we think too hard about syntactic sugars for immutable collection initialization.

回答3:

It's because the initialization statement is shorthand for the CLR. When it gets compiled into bytecode, it will call the Add method you've defined.

So you can make the case that this initialization statement is not really a "first class" feature, because it doesn't have a counterpart in IL. But that's the case for quite a lot of what we use, the "using" statement for example.

回答4:

The main reason is Syntactic Sugar.

The initializer syntax only makes writing programing in C# a bit easier. It doesn't actually add any expressive power to the language.

If the initializer didn't require an Add() method, then it would be a much different feature than it is now. Basically, it's just not how C# works. There is no literal form for creating general collections.

回答5:

Not an answer, strictly speaking, but if you want to know what sort of things influenced the design of collection initialisers then you'll probably find this interesting:

• What is a collection? [straight from the Horse's mouth Mads Torgersen]

回答6:

What should the initialization syntax use, if not an Add method? The initialization syntax is 'run' after the constructor of the collection is run, and the collection fully created. There must be some way of adding items to the collection after it's been created.

If you want to initialize a read-only collection, do it in the constructor (taking a T[] items argument or similar)

回答7:

As far as I understand it, the collection initializer syntax is just syntactic sugar with no special tricks in it. It was designed in part to support initializing collections inside Linq queries:

from a in somewhere
select new {
   Something = a.Something
   Collection = new List<object>() {
      a.Item1,
      a.Item2,
      ...
   }
}

Before there was no way to do this inline and you'd have to do it after the case, which was annoying.

回答8:

I'd love to have the initializer syntax for immutable types(both collections and normal types). I think this could be implemented with a special constructor overload using a syntax similar to params.

For example something like this:

MyClass(initializer KeyValuePair<K,V>[] initialValues)

But unfortunately the C# team didn't implement such a thing yet :(

So we need to use a workaround like

MyClass(new KeyValuePair<K,V>[]{...})

for now

回答9:

Collection initializers are expressions, so they can be used where only expression are valid, such as a field initializer or LINQ query. This makes their existence very useful.

I also think the curly-bracketed { } kind of initialization, smells more like a fixed size collection, but it's just a syntax choice.

来源：https://stackoverflow.com/questions/4681265/why-is-an-add-method-required-for-initialization