How do I instantiate my class in a correct way ?

AnotherClass { MyClass _myClass {get; set} = new(); }

This is a property declaration with an auto-implemented property initialiser. _myClass is a property of type MyClass and it's initialised with a new instance of MyClass when AnotherClass is instantiated.

AnotherClass { MyClass _myClass = {get;set;} }

This is not valid C#. It looks like you are trying to declare a property. It should look like this MyClass _myClass { get; set; }

AnotherClass { MyClass _myClass = new(); }

This is a field declaration. _myClass is a field of type MyClass and it's initialised with a new instance of MyClass when AnotherClass is instantiated

Fields and properties typically have different access modifiers in C#.

AnotherClass { MyClass _myClass; }

This is a field declaration without initialisation. _myClass is a field of type MyClass, but it's not initialised, so its value will be null until it's explicitly set.


Essentially your question is about fields vs. properties and instantiation at the time of declaration (or not).

Fields are typically used for internal use while properties expose fields to the outside world.
Auto-implemented properties (like in the first example) are a shorthand syntax that automatically creates a private, anonymous backing field that can only be accessed through the property's get and set accessors.


With regards to your question and the clarification given in your comment below :

In Case a) I also wonder how I can hand over parameters to the constructor, when they do not yet exist at the time MyClass is instantiated.

That looks circular ? You seem to want to create an instance of MyClass that contains an instance of AnotherClass but AnotherClass also contains an instance of MyClass and since you are using the this keyword then it would seem that the 'two' instances of MyClass are actually one in the same. Are you trying to allow a nested class to use the properties, methods and functions of its parent? I would recommend that you ask a separate question detailing what you are trying to achieve and ask how you might go about it.

M

To add to what the others have said, part of the instantiation process (when you create an instance of a class) is a call to the constructor.
When you use new to create a class instance, the system does the following tasks in order:

1) Allocate a chunk of heap memory big enough to hold the class instance data.
2) Set all fields of the newly allocated memory to their default value: for example, a variable that contains a reference will be null, an integer will be zero and so forth.
3) The instance constructor is called passing a reference to the memory allocated as this^*
4) Object initializers are applied if any are specified.

So there are two ways to pass "parameters" to a constructor: via a constructor with a matching signature; or via an object initializer. In practice, you could do both:

public class Program
    {
    public static void Main()
        {
        MC mc = new MC(666) {Y = 333};
        Console.WriteLine($"{mc.X}:{mc.Y}");
        }
    }
public class MC
    {
    public int X {get; set;}
    public int Y {get; set;}
    public MC(int x) { X = x;}
    }

But that does look rather odd and should probably be avoided!

* Which constructor? Depends - a constructor is treated as a function, and the "calling rules" for a function are applied. See here: Expressions - C# language specification | Microsoft Learn[^]