How to make inline array initialization work like e.g. Dictionary initialization?

Question

Why is it possible to initialize a Dictionary like this:

var dict = new Dictionary() { 
    { \"key1\", 1 },
    { \"         


        

Answer 1

The collection initializer syntax is translated into calls to Add with the appropriate number of parameters:

var dict = new Dictionary();
dict.Add("key1", 1);
dict.Add("key2", 2);

This special initializer syntax will also work on other classes that have an Add method and implements IEnumerable. Let's create a completely crazy class just to prove that there's nothing special about Dictionary and that this syntax can work for any suitable class:

// Don't do this in production code!
class CrazyAdd : IEnumerable
{
    public void Add(int x, int y, int z)
    {
        Console.WriteLine(x + y + z); // Well it *does* add...
    }

    public IEnumerator GetEnumerator() { throw new NotImplementedException(); }
}

Now you can write this:

var crazyAdd = new CrazyAdd
{
    {1, 2, 3},
    {4, 5, 6}
};

Outputs:

6
15

See it working online: ideone

As for the other types you asked about:

• It doesn't work on an array because it has no Add method.
• List has an Add method but it has only one parameter.