Why is a collection initializer without `new` allowed inside an object initializer but not outside?

Question

I noticed strange behaviour when initializing collection property.

Consider:

class X
{
    public IList<int> Ints { get; set; }
}

I can initialize Ints like that:

var theObject = new X
{
    Ints = { 12, 3, 4, 5, 6 }
};

But I cannot do that:

var x = new X();

x.Ints = { 12, 3, 4, 5, 6 }

Any ideas why? It seems pretty unintuitive.

Answer 1

new X ... is the start of an object creation expression. In this kind of expressions, an object or collection initializer is allowed:

object_creation_expression
    : 'new' type '(' argument_list? ')' object_or_collection_initializer?
    | 'new' type object_or_collection_initializer  // <--- here!
    ;

object_or_collection_initializer
    : object_initializer
    | collection_initializer
    ;

In your code, you have an object initialiser { Ints = ... }. Inside that, there is another collection initialiser { 12, 3, 4, 5, 6 }. This is allowed, as per the grammar:

object_initializer
    : '{' member_initializer_list? '}'
    | '{' member_initializer_list ',' '}'
    ;

member_initializer_list
    : member_initializer (',' member_initializer)*
    ;

member_initializer
    : initializer_target '=' initializer_value
    ;

initializer_target
    : identifier
    | '[' argument_list ']'
    ;

initializer_value
    : expression
    | object_or_collection_initializer // <---- here!
    ;

An initializer_value can either be an expression, or another object_or_collection_initializer. This also implies that, though they may look like it, object_or_collection_initializer, i.e. things like { 12, 3, 4, 5, 6 }, are not a kind of expression.

On the other hand, assignments don't allow this. Assignments only allow an expression to be on the right hand side:

assignment
    : unary_expression assignment_operator expression
    ;