Q:

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

Today I was surprised to find that in C# I can do:
List<int> a = new List<int> { 1, 2, 3 };

Why can I do this? What constructor is called? How can I do this with my own classes? I know that this is the way to initialize arrays but arrays are language items and Lists are simple objects ...

A:

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<T>)
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<int> a = new List<int> { 1, 2, 3 };

And
List<int> temp = new List<int>();
temp.Add(1);
temp.Add(2);
temp.Add(3);
List<int> a = temp;

You can call an alternate constructor if you want, for example to prevent over-sizing the List<T> during growing, etc:
// Notice, calls the List constructor that takes an int arg
// for initial capacity, then Add()'s three items.
List<int> a = new List<int>(3) { 1, 2, 3, }

Note that the Add() method need not take a single item, for example the Add() method for Dictionary<TKey, TValue> takes two items:
var grades = new Dictionary<string, int>
    {
        { "Suzy", 100 },
        { "David", 98 },
        { "Karen", 73 }
    };

Is roughly identical to:
var temp = new Dictionary<string, int>();
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<T>) and create one or more Add() methods:
public class SomeCollection<T> : IEnumerable<T>
{
    // implement Add() methods appropriate for your collection
    public void Add(T item)
    {
        // your add logic    
    }

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

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

Then you can use it just like the BCL collections do:
public class MyProgram
{
    private SomeCollection<int> _myCollection = new SomeCollection<int> { 13, 5, 7 };    

    // ...
}

(For more information, see the MSDN)

A:

It is so called syntactic sugar.
List<T> is the "simple" class, but compiler gives a special treatment to it in order to make your life easier.
This one is so called collection initializer. You need to implement IEnumerable<T> and Add method. 

A:

According to the C# Version 3.0 Specification "The collection object to which a collection initializer is applied must be of a type that implements System.Collections.Generic.ICollection for exactly one T."
However, this information appears to be inaccurate as of this writing; see Eric Lippert's clarification in the comments below.

