How to access a field in one class from another?

Question

Note that this is not an inheritance or interface.

I have a class HP

public class HP
{
    public int Base;

    public int Value
    {
        get
        {
            //Need to access Monster.Level to calculate total HP.
        }
    }
}

And a class Monster calling HP

public class Monster
{
    public HP hp;
    public int Level;
}

How can I access Monster.Level from HP?

I tried to pass Level by reference to HP upon instantiating.

public class HP
{
    private ref int Level;

    public A(ref i)
    {
        Level=i;
    }
}

But I want to keep A as simple and clean as possible.

The reason I don't want to pass the level upon creation is Level is a constantly changing valuable, I'll will then have to "update" the HP every time the monster gains a level or something, which is NOT a efficient way to do things.

And there are other things accessing Level, such as Attack and Damage, etc.

I currently has a method within HP, Attack, Damage, etc., taking the Level as a parameter.

public int Value(int lv)
{
    return Base+10*lv;
}

But according to the a little bit of SOLID principle I know, this is a VERY BAD coding!

Answer 1

It seems like what you called HP is more of a HPCalculator responsible of calculating Monster's HP based on this Moster's level and possibly other data in the future, I think it seem crucial for such HPCalculator to have access to the Monster of which health it calculates.

In C# classes are done by references not by values meaning you can in no problem have something like this:

public class Monster{
    public HP hp;
}

public class HP{
     public Monster monster;
}