C# – Nested object initializer syntax


Resharper has just suggested the following refactoring to me:

// Constructor initializes InitializedProperty but
// the UninitializedSubproperty is uninitialized.
var myInstance = new MyClass();
myInstance.InitializedProperty.UninitializedSubproperty = new MyOtherClass();

// becomes

var myInstance = new MyClass
        InitializedProperty = { UninitializedSubproperty = new MyOtherClass() }

I've never seen this kind of object initialization before. In particular I don't see how

InitializedProperty = { UninitializedSubproperty = new MyOtherClass() }

makes any sense – it's not assigning anything to InitializedProperty.

Is this behaviour specified anywhere?

Best Solution

This syntax is called Object Initialization. C# specification clearly gives a lot of examples on this subject: Object initializers

An object initializer consists of a sequence of member initializers, enclosed by { and } tokens and separated by commas. Each member initializer must name an accessible field or property of the object being initialized, followed by an equals sign and an expression or an object initializer or collection initializer. It is an error for an object initializer to include more than one member initializer for the same field or property. It is not possible for the object initializer to refer to the newly created object it is initializing.

Examples are:

Rectangle r = new Rectangle
                P1 = { X = 0, Y = 1 },
                P2 = { X = 2, Y = 3 }

Compiles down to:

Rectangle r = new Rectangle();
r.P1.X = 0;
r.P1.Y = 1;
r.P2.X = 2;
r.P2.Y = 3;


public class Rectangle
    public Rectangle()
        P1 = new Point(); //default Point for demo purpose
        P2 = new Point(); //default Point for demo purpose

    public Point P1 { get; set; }
    public Point P2 { get; set; }


public class Point
    public int X { get; set; }
    public int Y { get; set; }

Also consider reading a great chapter 8.3 Simplified initialization in C# in depth book. Jon Skeet provides another look at advantages of using this kind of syntax for initializing tree-like structures.