Why is my class hidden in inspector with C# and not JS?

I used to code in JS (with Unity at least) and I recently decided to go for C#. In JS, when you create a class and use it in a monobehaviour, it's shown in the inspector, with a little arrow to unfold, but apparently not in C#. The code below explain the matter :

JS code (I can see "classVar" in the inspector, and "varNotHidden" within it)

class SecondClass    
    var varNotHidden : int = 0;    
    var varHidden : int = 0;    

class MainClass extends MonoBehaviour    
    var classVar : SecondClass;    

C# code (I can't see "classVar" in the inspector)

using UnityEngine;

public class SecondClass 
    public int varNotHidden = 0;
    public int varHidden = 0;

public class MainClass : MonoBehaviour 
    public SecondClass classVar;

So I wonder why "classVar" remain invisible in C#. Both variable and class are public, I'm not using [HideInInspector] for it. I'm kind of lost ...

Is there something like [PrettyPleaseDontHideInInspector] ? I really hope I don't have to use a CustomEditor for each class ...

You have to add the `[System.Serializable]` attribute to your class in C#.

class someClass {
      public int thisValueWillBeDisplayed;