How to access a variable from a child control that is added to another control?

Question

I have the following code (this is an excerpt which I tried to include all the important info in it)

public partial class MyTopScreen : Form
{
    public bool _myVariable ;
    public CommonForm _commonGridForm;
    public MyTopScreen(bool first_param, string second_param )
    {           
        CommonForm commonGridForm = new CommonForm();
        DataGridView dataGridView1 = commonGridForm.dataGridView1;
        _commonAttributForm = commonAttributForm;          
        InitializeComponent();
        this.TabPage1.Controls.Add(dataGridView1);
    }  
 }

and then when I am inside CommonForm which is as follows:

public partial class CommonForm : Form        
{
    public CommonForm()
    {
        //My Question is how do I access the _myVariable from this point in the code ?
        //this.parent      doesnot work or does not give me access to _myVaribale
    }
 }

Thanks in advance for your much appreciated help.

Pass it into the constructor. – Sriram Sakthivel Dec 12 '14 at 13:40 — Sriram Sakthivel, Dec 12 '14 at 13:40

score 3 · Answer 1 · answered Dec 12 '14 at 13:39

3

You have to cast the parent to appropriate type:

MyTopScreen myParent = this.parent as MyTopScreen;
if (myParent != null)
{
    bool myVariableFromParent = myParent._myVariable;
}

Only be careful with casting, because if the parent is a different control then you will get null reference exception when trying to access _myVariable. Also, don't try direct cast ((MyTopScreen)this.parent) because if the parent is a different control you will get cast exception.

answered Dec 12 '14 at 13:39

PiotrWolkowski

8,408
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I'll suggest direct cast as opposed to safe cast with `as` keyword. Because with `as` keyword you prevent `InvalidCastException` but that if block won't exceute which could produce more severe problem. IMO if you need something, ask it(dependency injection, take the dependency in constructor) – Sriram Sakthivel Dec 12 '14 at 13:46
I agree with injection, though, in general, if you pass parent's flag to a child then it's worth to rethink the design in the first place. But that's not the question asked... – PiotrWolkowski Dec 12 '14 at 13:51
Thanks for the responses and comments. Useful heads ups. – user1298925 Dec 12 '14 at 14:53
Happy to hear you find it useful. – PiotrWolkowski Dec 12 '14 at 14:58

score 3 · Accepted Answer · answered Dec 12 '14 at 13:40

One way, pass the instance of MyTopScreen to the other class' constructor if that relates on it. Store it in a field.

public partial class CommonForm : Form
{
    public CommonForm(MyTopScreen screen)
    {
        this.TopScreen = screen;
    }

    private MyTopScreen TopScreen{ get; set; }
}

Now you can access that variable in CommonForm via TopScreen._myVariable.

You have to pass it to the constructor:

CommonForm commonGridForm = new CommonForm(this);

score 3 · Answer 3 · edited May 23 '17 at 10:33

The reason this.Parent doesn't work is because you never set MyTopScreen as the parent. You need to add commonGridForm.Parent = this to your code. Then you can call ((MyTopScreen) this.Parent)._myVariable. But this is just to get your current code working; it's definitely not the best approach.

One improvement I would suggest is to not expose your fields publicly, but instead use them as backing fields exposed through properties (see this).

Also, you could pass a reference to the parent form into the child via the constructor. Here's what your code might look like:

public partial class MyTopScreen : Form
{
    //keep your fields private.

    private bool _myVariable;
    private CommonForm _commonGridForm;

    //expose fields through properties.

    public bool MyVariable
    {
        get { return _myVariable; }
    }

    public CommonForm CommonGridForm
    {
        get { return _commonGridForm; }
    }

    public MyTopScreen(bool first_param, string second_param)
    {
        CommonForm commonGridForm = new CommonForm(this);
        DataGridView dataGridView1 = commonGridForm.dataGridView1;
        _commonAttributForm = commonAttributForm;          
        InitializeComponent();
        this.TabPage1.Controls.Add(dataGridView1);

    }
}


public partial class CommonForm : Form
{
    private MyTopScreen _parent;

    //inject parent reference in child form's constructor.

    public CommonForm(MyTopScreen withParent)
    {
        _parent = withParent;
        //_parent.MyVariable <-- here's how you'd access your var.
    }
}

Thank you for your response. I got two simultaneous answers and to the best of my understanding I believe the other answer came first ?, but thank you greatly for a comprehensive answer. — user1298925, Dec 12 '14 at 14:55

score 2 · Answer 4 · answered Dec 12 '14 at 13:40

create a parameterized constructor (Constructor Overloading) of CommonForm Class and pass your bool value to it.

public partial class MyTopScreen : Form
{
    public bool _myVariable ;
    public CommonForm _commonGridForm;
    public MyTopScreen(bool first_param, string second_param )
    {           
        CommonForm commonGridForm = new CommonForm(_myVariable);
        DataGridView dataGridView1 = commonGridForm.dataGridView1;
        _commonAttributForm = commonAttributForm;          
        InitializeComponent();
        this.TabPage1.Controls.Add(dataGridView1);

    }  
 } 

public partial class CommonForm : Form
{
        public CommonForm(bool _myVariable)
        {
           //Here you access the variable
        }
 }

Thank you. This would work if my variable value is not changing. But in CommonForm the variable value may be changed. That is why in my case I need to pass the reference to the parent screen as opposed to the variable. — user1298925, Dec 12 '14 at 15:49

score 1 · Answer 5 · answered Dec 12 '14 at 13:42

There are a couple of ways of doing it. If the value won't change, you can pass it to the constructor of CommonForm (recommended):

public partial class CommonForm : Form
{
    public CommonForm(bool myVariable)
    {
       // available
    }
}

CommonForm commonGridForm = new CommonForm(_myVariable);

If it can change, you could pass an instance to the parent form in the constructor:

public partial class CommonForm : Form
{
    public CommonForm(MyTopScreen parent)
    {
       // available
       parent._myVariable;
    }
}

CommonForm commonGridForm = new CommonForm(this);

How to access a variable from a child control that is added to another control?

5 Answers5