Why can I initialize a List like an array in C#?

Question

Today I was surprised to find that in C# I can do:

List a = new List { 1, 2, 3 };

Why can I do this? What constructor

Answer 1

This is part of the collection initializer syntax in .NET. You can use this syntax on any collection you create as long as:

• It implements IEnumerable (preferably IEnumerable)

• It has a method named Add(...)

What happens is the default constructor is called, and then Add(...) is called for each member of the initializer.

Thus, these two blocks are roughly identical:

List a = new List { 1, 2, 3 };

And

List temp = new List();
temp.Add(1);
temp.Add(2);
temp.Add(3);
List a = temp;

You can call an alternate constructor if you want, for example to prevent over-sizing the List during growing, etc:

// Notice, calls the List constructor that takes an int arg
// for initial capacity, then Add()'s three items.
List a = new List(3) { 1, 2, 3, }

Note that the Add() method need not take a single item, for example the Add() method for Dictionary takes two items:

var grades = new Dictionary
    {
        { "Suzy", 100 },
        { "David", 98 },
        { "Karen", 73 }
    };

Is roughly identical to:

var temp = new Dictionary();
temp.Add("Suzy", 100);
temp.Add("David", 98);
temp.Add("Karen", 73);
var grades = temp;

So, to add this to your own class, all you need do, as mentioned, is implement IEnumerable (again, preferably IEnumerable) and create one or more Add() methods:

public class SomeCollection : IEnumerable
{
    // implement Add() methods appropriate for your collection
    public void Add(T item)
    {
        // your add logic    
    }

    // implement your enumerators for IEnumerable (and IEnumerable)
    public IEnumerator GetEnumerator()
    {
        // your implementation
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Then you can use it just like the BCL collections do:

public class MyProgram
{
    private SomeCollection _myCollection = new SomeCollection { 13, 5, 7 };    

    // ...
}

(For more information, see the MSDN)