Question

Is is possible to combine a List initializer and object initializer at the same time?
Given the following class definition:

class MyList : List<int>
{
    public string Text { get; set; }
}

// we can do this
var obj1 = new MyList() { Text="Hello" };

// we can also do that
var obj2 = new MyList() { 1, 2, 3 };

// but this one doesn't compile
//var obj3 = new MyList() { Text="Hello", 1, 2, 3 };

Is this by design or is it just a bug or missing feature of the c# compiler?

Answer 1

No, looking at the definitions from section 7.6.10 of the C# spec, an object-or-collection-initializer expression is either an object-initializer or a collection-initializer.

An object-initializer is composed of multiple member-initializers, each of which is of the form initializer = initializer-value whereas a collection-initializer is composed of multiple element-initializers, each of which is a non-assigment-expression.

So it looks like it’s by design – possibly for the sake of simplicity. I can’t say I’ve ever wanted to do this, to be honest. (I usually wouldn’t derive from List<int> to start with – I’d compose it instead.) I would really hate to see:

var obj3 = new MyList { 1, 2, Text = "Hello", 3, 4 };

EDIT: If you really, really want to enable this, you could put this in the class:

class MyList : List<int>
{
    public string Text { get; set; }
    public MyList Values { get { return this; } }
}

at which point you could write:

var obj3 = new MyList { Text = "Hello", Values = { 1, 2, 3, 4 } };