How to access an int from the parent class?

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In the code below, when I try to access int y; in the "son" class inside the parent class Form1, it tells me:

An object reference is required for the nonstatic field, method, or property Form1.y

When I hover over "y" in x = y;
To circumvent this, I did an awkward move by creating an instance in the class Program.

C#

public static Form1 f;

and

C#

Application.Run(f= new Form1());

and

C#

x = Program.f.y;

What I have tried:

public partial class Form1 : Form
    {

        class shape
        {
            int x,z;
            public test(int w)
            {
                x = y;
                z = w;
            }
        }
        List<shape> lstShapes= new List<shape>();

        int y = 1;

        public Form1()
        {
            InitializeComponent();
            lstShapes.Add(new Shape(8));
        }
    }

Posted 15-Feb-22 0:06am

FierceLion

Updated 15-Feb-22 7:17am

Deeksha Shenoy

v8

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Comments

Richard MacCutchan 15-Feb-22 6:15am

The variable y is a member of the Form1 class, so it cannot be accessed directly in the class. But why have you created the test class, as it is not used, and would seem to serve no purpose.

FierceLion 15-Feb-22 6:17am

It is a class for example in a game, the class' name is: Class shape {} and it has members for each shape like an int[,] array of its data and bool available; if it's available, I want the inside of class test to access the same instance of the class they're in at that time.

Richard MacCutchan 15-Feb-22 6:26am

You need to pass an instance of the parent to the inner class, probably via the constructor. Something like the following:

class test
{
    Form1 parent = null;
    int x;
    public test(Form1 outer)
    {
        parent = outer;
        x = outer.y;
    }
}

// in Form1 declare a test object
test myTest;

// and in the constructor add
myTest = new test(this);

FierceLion 15-Feb-22 6:31am

Thx, works, are you sure there's no simpler way?, please submit as an answer so I accept it.

Richard MacCutchan 15-Feb-22 8:55am

No, this is the only way, but it is still a bad design.

Richard MacCutchan 15-Feb-22 6:17am

Your update above just makes things more complicated. Get rid of the test class, it is not needed.

FierceLion 15-Feb-22 6:17am

See my other comment plz.

Pawel Wzietek 15-Feb-22 11:39am

The fact that shape is declared inside Form1 only means that private fields of Form1 appear as public to shape, otherwise it doesn't change anything in that you still need an instance reference to access a non-static class variable from a code outside the class. From this perspective, the code of test is in the shape class but outside the Form1 class. Look here for more examples:
https://www.geeksforgeeks.org/nested-classes-in-c-sharp/

FierceLion 15-Feb-22 11:55am

Thanks, very informative.

charles henington 15-Feb-22 16:37pm

You must have a reference to the class in which you are retrieving a value from as Richard MacCutchan explained. Personally, I prefer to make the field private and use reflection to retrieve the needed data. You can google how to use reflection and this will be a good learning lesson or you can simply mark the field internal.

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OriginalGriff · Accepted Answer · 2022-02-15T00:19:00

test is not derived from Form1, so it doesn't have access to Form1 data - and even if it did, it wouldn't "know" which of potentially hundreds of Form1 instances contained the right value of y.

Think about it: if you put your mobile in the glove box of your car, and then we go for a drive in my car, do you expect to be able to open the glove box in front of you and retrieve your mobile? Of course not - you know it is in the glove box of your car, not mine and that the two cars, and the two glove boxes are completely separate and do not share anything.
To access the glove box content you have to explicitly specify the car: my car, yoru care, this car, that car: you can access each glove box in turn.

It's the same with classes: to access a variable in Form1 you have to specify exactly which instance of the form class you are talking about. Inside the form class itself, that is done via an implicit or explicit use of this:

C#

public Form1()
{
    InitializeComponent();
    y = 666;
}

(Implicit)

C#

public Form1()
{
    InitializeComponent();
    this.y = 666;
}

(Explicit)

From outside the form class, you need to use an explicit reference to the actual instance of the form that you want to access.

And ... that's a bad idea in your case, because the test class should not know that the Form1 class even exists, let alone what variables it contains - it if does then you "lock" the two classes together, and you can no longer make a change to either without considering the effects on the other (which may be breaking changes if Form1 decides that y should be a display control instead of a simple variable).