NullReferenceException with object initializer suggested by resharper

Question

I have a strange issue with the object initializer syntax.

Here are my sample classes:

public class Foo
{
    public Bah BahProp { get; set; }
}

public class Bah
{
    public int Id { get; set; }
}

Consider following three ways to initialize an object:

The old, verbose but explicit way, working correctly:

var foo1 = new Foo();
foo1.BahProp = new Bah();
foo1.BahProp.Id = 1;
// correctly initialized

The second way i'm always using, using object initializer syntax:

var foo2 = new Foo
{
    BahProp = new Bah { Id = 1 }
}; // correctly initialized

A third way resharper has suggested my collegue with a different resharper version(is it a bug?):

var foo3 = new Foo
{
    BahProp = { Id = 1 }
};  // NullReferenceException

What is the last approach doing differently?

My resharper version is 2016.1.1, my colleague was on 10.02. My resharper suggested the second way. But what does the third way do and when is it useful?

Update: So it seems that it was a bad resharper sugestion to use the last way, that's why they have changed it meanwhile to use the second way.

You can avoid the NullReferenceException if you want to use the third way by initializing all properties/fields that are reference types inline or in the constructor.

I will definitely not use this strange property assignment syntax.

I would check the compiled .dll with ILSpy or any other decompiler to see what the compiler did differently — Waescher
– Waescher, Commented Jul 14, 2016 at 9:10
"I will definitely not use this strange property assignment syntax". — Sinatr
– Sinatr, Commented Jan 12, 2018 at 15:42

Dennis_E · Accepted Answer · 2017-09-08 12:56:11Z

7

new Foo { BahProp = { Id = 1 } }

compiles to:

new Foo().BahProp.Id = 1;

or, little more verbose:

var foo3 = new Foo();
foo3.BahProp.Id = 1;

So BahProp is null. You're not constructing it.
(This is perhaps the most confusing syntax in all of C#)
Option 2 works because you're calling the constructor of Bah.

Option 3 would also work if you initialize BahProp inside the constructor of Foo. It will have been constructed by the time BahProp = { Id = 1 } is called.

The same is seen with collection initializers:

public class Foo {
    public List<int> Numbers { get; set; }
}

var foo = new Foo { Numbers = { 1, 2, 3 } };

This does not initialize the List. It only calls Add on it.

You really must see new MyObject() { X = 1, Y = 2 } as two distinct parts:
new MyObject() constructs a new object and
{ X = 1, Y = 2 } sets the values of its properties (and that's all it does).
Object and collection initializers can be nested. The top-level initializer must follow a constructor, but a nested initializer does not.

edited Sep 8, 2017 at 12:56

answered Jul 14, 2016 at 9:24

Dennis_E

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12 Comments

Tim Schmelter Over a year ago

Thanks. Note that i've modified my bah class since the type of the property doesn't matter. It doesn't need to be a reference to each other. If { Id = 1 } does not call the default constructor of Bah implicitely, when is it useful at all? Do you have a link to MSDN?

Dennis_E Over a year ago

@TimSchmelter I don't think you can find this particular case on MSDN. I just tried it out on tryroslyn.azurewebsites.net

Tim Schmelter Over a year ago

but this initializer syntax must be documented somewhere: BahProp = { Id = 1 }. Or is is not an initializer syntax but only a strange property assignment. I'm afraid it's still not clear.

Dennis_E Over a year ago

All { SomeProperty = someValue } does is set the value of SomeProperty. It just allows you to do it all in one statement. It does not call any constructor implicitly.

Tim Schmelter Over a year ago

So the correct way was to initialize all fields/properties which are reference types in the constructor.

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