Combining List initializer and object initializer

问题:

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?

回答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 } };

回答2:

No, it's a not a bug. It is by design of the language.

When you write

var obj1 = new MyList() { Text="Hello" };

this is effectively translated by the compiler to

MyList temp = new MyList();
temp.Text = "Hello";
MyList obj = temp;

When you write

var obj2 = new MyList() { 1, 2, 3 };

this is effectively translated by the compiler to

MyList temp = new MyList();
temp.Add(1);
temp.Add(2);
temp.Add(3);
MyList obj2 = temp;

Note that in the first case you are using an object initializer, but in the second case you are using a collection initializer. There is no such thing as an object-and-collection intializer. You are either initializing the properties of your object, or you are initializing the collection. You can not do both, this is by design.

Also, you shouldn't derive from List<T>. See: Inheriting List<T> to implement collections a bad idea?

回答3:

If you want to get something like this functionality, consider making a constructor argument:

var x = new CustomList("Hello World") { 1, 2, 3 }