strange C# object initialization notation

Question

Whilst working in a codebase I found a line of code that seemed to initialize an CustomObject (ObservableObject to be precise) as follows:

CustomObject someObject = {Prop1="1", Prop2="2", Prop3="3"}

where CustomObject has three string properties Prop1, Prop2, Propr3. The problem is that I have not been able to reproduce such an initialization in a sample project of my own. What is this notation called? To me it seems as though list notation is used to initialize a custom object...how is this possible? It's certainly not an anonymous type that is being created.

Exactly as written, that's just not legal C#. It would at the very least require a new (to make an anonymous type) or a new() (implicit type for constructor, with an initializer). If you omit the type (someObject = { Prop1 = ... }) it is legal as initialization syntax for a property or field, but only in the context of creating another object (i.e. new ContainingObject { someObject = ... }). Do you have the context in which this line occurred, or possibly a correction of what the exact line was? C# by now has acquired many such "initialization shorthands". — Jeroen Mostert
– Jeroen Mostert, Commented Mar 3, 2022 at 19:11
Yes, it's done as initialization syntax of a property, so my question should be: var parentObjecct = new ParentObject(){ SomeObject = { Prop1="1", Prop2="2", Prop3="3"}}; But still, SomeObject is an object, how can we initialize it with list notation? — Marcin
– Marcin, Commented Mar 3, 2022 at 19:24
That's not "list notation" (a collection initializer, which compiles to Add calls), that's a nested object initializer. It has been legal since C# 3 (so quite a while now). It's simply shorthand for var o = new ParentObject(); o.SomeObject.Prop1 = "1"; o.SomeObject.Prop2 = "2"... Note that this requires that ParentObject initialize SomeObject to an instance, otherwise it'll fail -- the sytax merely assigns, it doesn't create (in this it resembles collection initializers, which merely call .Add without creating). — Jeroen Mostert
– Jeroen Mostert, Commented Mar 3, 2022 at 19:36
Thanks @JeroenMostert. That answers it. But unfortunately I have no clue how to mark a comment as an answer. Cheers — Marcin
– Marcin, Commented Mar 3, 2022 at 19:57

DRapp · Accepted Answer · 2022-03-03 19:13:27Z

-1

What you are seeing is a default property settings construction of the class.

By default, you may see

var SomeObject = new SomeObjectClass();
SomeObject.Prop1 = "1";
SomeObject.Prop2 = "2";
SomeObject.Prop3 = "3";

By using the curly brackets does the implied same result. You are just stating... give me an instance of a given object, and by the way, please set the following properties to these default values I'm providing via...

var SomeObject = new SomeObjectClass{ 
                         SomeObject.Prop1 = "1",
                         SomeObject.Prop2 = "2",
                         SomeObject.Prop3 = "3" };

Now, in the sample context you have provided there is no "new" of a given class trying to be created.

answered Mar 3, 2022 at 19:13

DRapp

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