Is this a bug in the C# 4.0 compiler?

Question

This code compiles successfully, but I think it should fail to compile. Also, when you run it you get a NullReferenceException. The missing code is the "new Bar" in the initialization of the Bar property.

class Bar
{
    public string Name { get; set; }
}

class Foo
{
    public Bar Bar { get; set; }
}


class Program
{
    static void Main(string[] args)
    {
        var foo = new Foo
                      {
                          Bar = { Name = "Hello" }
                      };
    }
}

Is this a known bug?

Answer 1

Why do you think it should fail to compile? It is nested object initializer syntax, and it is the responsibility of the client code to provide a valid value for initialization.

From the documentation:

C# spec 7.5.10.2 "Object initializers"

A member initializer that specifies an object initializer after the equals sign is a nested object initializer, i.e. 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

No this is not a bug.

If you want it to run you either put a new before Bar (just like you did for Foo before the initializer) or you create the Bar object in Foo's constructor.

The object initializer is essentially just syntactic sugar.

This:

var foo = new Foo
            {
                Bar = { Name = "Hello" }
            };

Is exactly the same as this:

var foo = new Foo();
foo.Bar.Name = "Hello";