Why do C# collection initializers work this way?

Question

I was looking at C# collection initializers and found the implementation to be very pragmatic but also very unlike anything else in C#

I am able to create code like this:

using System;
using System.Collections;

class Program
{
    static void Main()
    {
        Test test = new Test { 1, 2, 3 };
    }
}

class Test : IEnumerable
{
    public IEnumerator GetEnumerator()
    {
        throw new NotImplementedException();
    }

    public void Add(int i) { }
}

Since I have satisfied the minimum requirements for the compiler (implemented IEnumerable and a public void Add) this works but obviously has no value.

I was wondering what prevented the C# team from creating a more strict set of requirements? In other words why, in order for this syntax to compile, does the compiler not require that the type implement ICollection? That seems more in the spirit of other C# features.

Answer 1

Your observation is spot on - in fact, it mirrors one made by Mads Torgersen, a Microsoft C# Language PM.

Mads made a post in October 2006 on this subject titled What Is a Collection? in which he wrote:

Admitted, we blew it in the first version of the framework with System.Collections.ICollection, which is next to useless. But we fixed it up pretty well when generics came along in .NET framework 2.0: System.Collections.Generic.ICollection<T> lets you Add and Remove elements, enumerate them, Count them and check for membership.

Obviously from then on, everyone would implement ICollection<T> every time they make a collection, right? Not so. Here is how we used LINQ to learn about what collections really are, and how that made us change our language design in C# 3.0.

It turns out that there are only 14 implementations of ICollection<T> in the framework, but 189 classes that implement IEnumerable and have a public Add() method.

There's a hidden benefit to this approach - if they had based it on the ICollection<T> interface, there would have been exactly one supported Add() method.

In contrast, the approach they did take means that the initializers for the collection just form sets of arguments for the Add() methods.

To illustrate, let's extend your code slightly:

class Test : IEnumerable
{
    public IEnumerator GetEnumerator()
    {
        throw new NotImplementedException();
    }

    public void Add(int i) { }

    public void Add(int i, string s) { }
}

You can now write this:

class Program
{
    static void Main()
    {
        Test test 
            = new Test 
            {
                1, 
                { 2, "two" },
                3 
            };
    }
}