C# complex type initializer compiles without new keyword

Question

I was recently working on some code, that has changed from using decimal to use a complex type that has the decimal number and a type to represent a fraction. I had to update some tests, and while typing I forgot to add the new keyword. The code compiled but the test kept failing, throwing a NullReferenceException. There I realized the missing new and that the property was not initialized. Has anybody an idea why this happening? I could not find anything in the C# lang specification that would explain this.

Here is the code sample:

public class Fraction 
{
    public int Numerator { get; set; }
    public int Denominator { get; set; }
}

public class MyDecimal
{
    public decimal? Decimal { get; set; }     
    public Fraction Fractional { get; set; }
}

public class ClassA 
{
    public MyDecimal Value { get; set; }
}

//...

var instance = new ClassA
{
     Value = // new MyDecimal is missing here
     {
         Decimal = 2.0m,
         Fractional = new Fraction 
         { 
               Numerator = 3,   
               Denominator = 4 
         }
     }
}

Please note that I'm using C# 6 and VS 2015, but I get the same result also in LINQPad.

If somebody could explain this (I'm looking in your direction Jon Skeet :) ) I would be glad.

Answer 1

The C# Specification 5.0 defines object initializer as (7.6.10.2 Object initializers):

An object initializer specifies values for zero or more fields or properties of an object.

object-initializer:
{   member-initializer-listopt   }
{   member-initializer-list   ,   }

And after the detailed explanation there is an example given which is very similar to your code:

If Rectangle’s constructor allocates the two embedded Point instances

public class Rectangle
{
  Point p1 = new Point();
  Point p2 = new Point();
  public Point P1 { get { return p1; } }
  public Point P2 { get { return p2; } }
}

the following construct can be used to initialize the embedded Point instances instead of assigning new instances:

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;

But there is only one difference, the Point instances here are initialized inside of the Rectangle class which occurs in the constructor of Rectangle.

So the syntax is valid by the specification, but you need to make sure Value is initialized before using the object initializer to initialize its properties in order to avoid NRE.