新的C＃6对象初始化程序语法？

Question

I just noticed that the following is possible in C# written in Visual Studio 2015, but I've never seen it before:

public class X
{
    public int A { get; set; }

    public Y B { get; set; }
}

public class Y
{
    public int C {get; set; }
}

public void Foo()
{
    var x = new X { A = 1, B = { C = 3 } };
}

My expectation was for Foo to have to be implemented like this:

public void Foo()
{
    var x = new X { A = 1, B = new Y { C = 3 } };
}

Note that there is no need to call new Y .

Is this new in C# 6? I haven't seen any mention of this in the release notes , so maybe it's always been there?

Answer 1

You will get a NullReferenceException if you run this code.

It will not create an instance of Y , it will call the getter of XB property and try to assign value to property C.

It always worked like that. According to C# 5.0 language specification:

A member initializer that specifies an object initializer after the equals sign is a nested object initializer, ie an initialization of an embedded object. Instead of assigning a new value to the field or property, the assignments in the nested object initializer are treated as assignments to members of the field or property.

Answer 2

This feature was introduced in C# 3.0 as object initializers.

See example on p. 169 of C# Language 3.0 specification :

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

which has the same effect as

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