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C# nested object initializer

C# 5.0 Language Specification 7.6.10.2 Object initializers states that
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. Nested object
initializers cannot be applied to properties with a value type, or to
read-only fields with a value type.
While I understand read-only fields cannot be modified by the initializers after the constructor is run, I do not have a clue about the restriction on properties.
The following is a code sample I used to test this restriction on properties:
using System;
namespace ObjectCollectionInitializerExample
{
struct MemberStruct
{
public int field1;
public double field2;
}
class ContainingClass
{
int field;
MemberStruct ms;
public int Field
{
get { return field; }
set { field = value; }
}
public MemberStruct MS
{
get { return ms; }
set { ms = value; }
}
}
class Program
{
static void Main(string[] args)
{
// Nested object initializer applied to a property of value type compiles!
ContainingClass cc = new ContainingClass { Field = 1, MS = new MemberStruct { field1 = 1, field2 = 1.2} };
Console.ReadKey();
}
}
}
I commented on the code where it was supposed to generate a compiler error based on the specification. But it compiles successfully. What am I missing here?
Thanks

What you have is not a nested object initializer because you explicitly create a new instance of MemberStruct. The inner object initializer doesn't directly follow the equals sign, but is an object initializer on its own associated with the call to the MemberStruct constructor.
This is how using a nested object initializer would look like:
ContainingClass cc = new ContainingClass { Field = 1, MS = { field1 = 1, field2 = 1.2} };
This will not compile when MS is a value type (struct), but it will compile when MS is a reference type (object).

Why does a combination of object and collection initializers use Add method?

The following combination of object and collection initializers does not give compilation error, but it is fundamentally wrong (https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/object-and-collection-initializers#examples), because the Add method will be used in the initialization:
public class Foo
{
public List<string> Bar { get; set; }
}
static void Main()
{
var foo = new Foo
{
Bar =
{
"one",
"two"
}
};
}
So you'll get NullReferenceException. What is the reason for making such an unsafe decision while developing the syntax of the language? Why not to use initialization of a new collection for example?

First, it's not only for combination of object and collection initializers. What you are referring here is called nested collection initializers, and the same rule (or issue by your opinion) applies to nested object initializers. So if you have the following classes:
public class Foo
{
public Bar Bar { get; set; }
}
public class Bar
{
public string Baz { get; set; }
}
and you use the following code
var foo = new Foo
{
Bar = { Baz = "one" }
};
you'll get the same NRE at runtime because no new Bar will be created, but attempt to set Baz property of the Foo.Bar.
In general the syntax for object/collection initializer is
target = source
where the source could be an expression, object initializer or collection initializer. Note that new List<Bar> { … } is not a collection initializer - it's an object creation expression (after all, everything is an object, including collection) combined with collection initializer. And here is the difference - the idea is not to omit the new, but give you a choice to either use creation expression + object/collection initializer or only initializers.
Unfortunately the C# documentation does not explain that concept, but C# specification does that in the Object Initializers section:
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. Nested object initializers cannot be applied to properties with a value type, or to read-only fields with a value type.
and
A member initializer that specifies a collection initializer after the equals sign is an initialization of an embedded collection. Instead of assigning a new collection to the target field, property or indexer, the elements given in the initializer are added to the collection referenced by the target.
So why is that? First, because it clearly does exactly what you are telling it to do. If you need new, then use new, otherwise it works as assignment (or add for collections).
Other reasons are - the target property could not be settable (already mentioned in other answers). But also it could be non creatable type (e.g. interface, abstract class), and even when it is a concrete class, except it is a struct, how it will decide that it should use new List<Bar> (or new Bar in my example) instead of new MyBarList, if we have
class MyBarList : List<Bar> { }
or new MyBar if we have
class MyBar : Bar { }
As you can see, the compiler cannot make such assumptions, so IMO the language feature is designed to work in the quite clear and logical way. The only confusing part probably is the usage of the = operator for something else, but I guess that was a tradeoff decision - use the same operator = and add new after that if needed.

Take a look at this code and the output of it due to the Debug.WriteLine():
public class Foo
{
public ObservableCollection<string> _bar = new ObservableCollection<string>();
public ObservableCollection<string> Bar
{
get
{
Debug.WriteLine("Bar property getter called");
return _bar;
}
set
{
Debug.WriteLine("Bar allocated");
_bar = value;
}
}
public Foo()
{
_bar.CollectionChanged += _bar_CollectionChanged;
}
private void _bar_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
Debug.WriteLine("Item added");
}
}
public MainWindow()
{
Debug.WriteLine("Starting..");
var foo = new Foo
{
Bar =
{
"one",
"two"
}
};
Debug.WriteLine("Ending..");
}
The output is:
Starting..
Bar property getter called
Item added
Bar property getter called
Item added
Ending..
For you questions:
What is the reason for making such an unsafe decision while developing the syntax of the language? Why not to use initialization of a new collection for example?
Answer:
As you can see the intention of the designer of that feature was not to reallocate the collection but rather to help you add items to it more easily considering that you manage your collection allocation by yourself.
Hope this clear things out ;)

Consider the following code:
class Program
{
static void Main()
{
var foo = new Foo
{
Bar =
{
"one",
"two"
}
};
}
}
public class Foo
{
public List<string> Bar { get; set; } = new List<string>();
}
The compiler does not know whether you already created a new list instance within the class constructor (or in another method).
Recall that collection initializer is a series of calls to Add method on an existing collection!
See also:
Custom Collection Initializers
Also note that this initializer applies to a collection that was exposed as a property. Hence the collection initializer is possible as part of the outer object initializer (the Foo object in your example).
However, if it was a simple variable, the compiler would not let you to intialize the collection this way. Here is an example:
List<string> list =
{
"one",
"two"
};
This will throws a compilation error.
As last example, the output of the following code will be: "one, two, three, four, ". I think that now you understand why.
Pay attention to the list static instance, as well as to the private modifier in the "set" of the Bar property, which does not matters because the initializer just calls the Add method, which is accessible even when the Bar "set" is private.
class Program
{
static void Main()
{
var foo1 = new Foo
{
Bar =
{
"one",
"two"
}
};
var foo2 = new Foo
{
Bar =
{
"three",
"four"
}
};
PrintList(foo1.Bar);
}
public static void PrintList(List<string> list)
{
foreach (var item in list)
{
Console.Write(item + ", ");
}
Console.WriteLine();
}
}
public class Foo
{
private static readonly List<string> _bar = new List<string>();
public List<string> Bar { get; private set; } = _bar;
}

I believe the key thing to understand here is that there are two syntactic sugar flavors at play (or at least, there should be):
Object Initialization
Collection Initialization
Take away the List for a moment and look at the field as an object:
public class Foo
{
public object Bar { get; set; }
}
When using Object Initialization, you assign an object (or null):
var foo = new Foo()
{
Bar = new object(); //or Bar = null
}
Now, let's go back to your original example and slap Collection Initialization on top of this. This time around, the compiler realizes this property implements IEnumerable and the array you have provided is of the right type, so it attempts to call the Add method of the interface. It must first go looking for the object, which in your case is null because you haven't initialized it internally. If you debug through this, you will find that the getter gets called and returns null, hence the error.
The correct way of mixing both features then would be for you to assign a new object that you initialize with your values:
var foo = new Foo()
{
Bar = new List<string>(){ "one", "two" }
};
If you debug this version, you will find that the setter is called instead, with the new instance you initialized.
Alternatively, you can initialize your property internally:
public List<string> Bar { get; set; } = new List<string>();
If you debug this version, you will find that the property is first initialized with a value and your version of the code then executes without error (by calling the getter first):
var foo = new Foo()
{
Bar = {"one", "two"}
};
To illustrate the syntactic sugar aspect, Collection Initialization only works within the confines of a constructor calling statement:
List<string> bar = {"one", "two" }; //ERROR: Can only use array initializer expressions to assign to array types. Try using a new expression instead.
List<string> bar = new[] { "one", "two" }; //ERROR: Cannot implicitly convert type 'string[]' to 'System.Collections.Generic.List<string>'
List<string> bar = new List<string>() { "one", "two" }; //This works!
If you wish to allow initialization like in your original example, then the expectation is that the variable will be set to an instance before the Add method can be called. This is true whether you use syntactic sugar or not. I could just as well run into the same error by doing this:
var foo = new Foo();
foo.Bar.Add("one");
So you may want to initialize the variable in order to cover all bases, unless of course a null value has a semantic meaning in your application.

Why does this wrong object initialisation with curly braces even compile? [duplicate]

This question already has an answer here:
Nested object initializer syntax
(1 answer)
Closed 4 years ago.
Whille creating some dummy data for a collection for a WPF/MVVM project, I produced the following wrong code, which compiles fine, but throws an exception at runtime.
There's a nested structure of data objects, which I wrongly instantiate with only the curly braces (looks like writing JavaScript does cause permanent damage to the brain).
using System.Collections.ObjectModel;
namespace testapp
{
class Program
{
static void Main(string[] args)
{
var collection = new ObservableCollection<TopLevelDataObject>();
collection.Add(new TopLevelDataObject{Nested = {Bar = 5}}); // NullReferenceException
}
class TopLevelDataObject
{
public NestedDataObject Nested { get; set; }
public string Foo { get; set; }
}
class NestedDataObject
{
public double Bar { get; set; }
}
}
}
Why does that compile?
If I create an annonymous type, like Nested = new {Bar = 5}, I get the error message during compilation (which thus fails):
Cannot implicitly convert type '<anonymous type: int Bar>' to 'testapp.Program.NestedDataObject'
Why do I not get such an error when ommitting the new operator?
It even gives me a code hint for the property:
My guess would be that {Bar = 5} is simply a code block, which on its own is a valid thing to have.
But why is it valid to assign a code block to anything (in this case, the Nested property)?

Why does that compile?
Because when that code is compiled, it is compiled as just a set of assignment operations. It doesn't all have to be new instances you create.
If you construct a new instance of Nested from the constructor, you can assign a value to Nested.Bar.
Change public NestedDataObject Nested { get; set; } to this to see how it works:
public NestedDataObject Nested { get; } = new NestedDataObject();
(Note you can never assign a value to Nested outside the constructor in the above code!)

Why does that compile?
Because var x = new SomeType{Property = value}; is the same as:
var x = new SomeType();
x.Property = value;
Indeed we can even leave in the () to have var x = new SomeType(){Property = value}; or even var x = new SomeType(argument){Property = value}; combining passing an argument to the constructor and setting a value.
As such you can see that there is always a constructor called, and if you leave out the parentheses that make that explicit its always the nullary (no-argument) constructor.
Meanwhile a type with no explicit constructor always has a public nullary constructor (the "default constructor").
Hence new TopLevelDataObject{Nested = {Bar = 5}} is the same as:
var temp = new TopLevelDataObject();
temp.Nested.Bar = 5; // NRE on temp.Nested
Because TopLevelDataObject could have a constructor that sets `Nestedt then the code you have could work, so it should compile. Of course, because it doesn't have such a constructor it doesn't work.
(Note that initialisers don't operate quite the same with anonymous types, in that case it gets rewritten to call a hidden constructor hence allowing the properties to be read-only even though initialisers cannot be used with read-only properties of non-anonymous types. The syntax allows them to look the same and hence be easily understood as similar but the result is not the same).

Why does this nested object initializer throw a null reference exception?

The following testcase throws a null-reference exception, when it tries to assign Id to an object which is null, since the code is missing the "new R" before the object initializer.
Why is this not caught by the compiler? Why is it allowed, in which use-cases would this be a meaningful construct?
[TestClass]
public class ThrowAway
{
public class H
{
public int Id { get; set; }
}
public class R
{
public H Header { get; set; }
}
[TestMethod]
public void ThrowsException()
{
var request = new R
{
Header =
{
Id = 1
},
};
}
}

The compiler doesn't give a warning because you could have:
public class R
{
public H Header { get; set; }
public R()
{
Header = new H();
}
}
so Header could be initialized by someone/something. Solving if someone/something will initialize Header is a complex problem (probably similar to the Halting problem)... Not something that a compiler wants to solve for you :-)
From the C# specifications:
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. Nested object initializers cannot be applied to properties with a value type, or to read-only fields with a value type.
We are in the case of nested initializer, and see the bolded part. I didn't know it.
Now, note that new R { } is, by C# spec, an 7.6.10.1 Object creation expressions followed by an object-initializer, while the Header = { } is a "pure" 7.6.10.2 Object initializers.

Assignment to readonly property in initializer list

Can one tell me, why the heck does it compile?
namespace ManagedConsoleSketchbook
{
public interface IMyInterface
{
int IntfProp
{
get;
set;
}
}
public class MyClass
{
private IMyInterface field = null;
public IMyInterface Property
{
get
{
return field;
}
}
}
public class Program
{
public static void Method(MyClass #class)
{
Console.WriteLine(#class.Property.IntfProp.ToString());
}
public static void Main(string[] args)
{
// ************
// *** Here ***
// ************
// Assignment to read-only property? wth?
Method(new MyClass { Property = { IntfProp = 5 }});
}
}
}

This is a nested object initializer. It's described in the C# 4 spec like this:
A member initializer that specifies an object initializer after the equals sign is a nested object initializer - that is, 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. Nested object initializers cannot be applied to properties with a value type, or to read-only fields with a value type.
So this code:
MyClass foo = new MyClass { Property = { IntfProp = 5 }};
would be equivalent to:
MyClass tmp = new MyClass();
// Call the *getter* of Property, but the *setter* of IntfProp
tmp.Property.IntfProp = 5;
MyClass foo = tmp;

Because you are using the initializer which uses the setter of ItfProp, not the setter of Property.
So it will throw a NullReferenceException at runtime, since Property will still be null.

Because
int IntfProp {
get;
set;
}
is not readonly.
You did not invoke setter of MyClass.Property, just getter.