Forms not communicating correctly

Question

I've been trying to make multiform programs for quite some time now, but I run into the same 2 problems again and again:

1. Form2 (Or however you call it) treats already set variables as null
2. Form1 doesn't accept, when Form2 changes a variable from Form1

It's been so annoying lately, that I've decided to ask here, what I may (or may not) be doing wrong. Here's my quick reproductuion - A program, where you can set a given value using another form:

valueForm

public partial class valueForm : Form
{
    public int value;
    private changeValueForm change;
    public valueForm()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        value = 2;
        valueLabel.Text = value.ToString();
        change = new changeValueForm();
    }

    private void change_Click(object sender, EventArgs e)
    {
        change.Show();
    }

    private void updatin_Tick(object sender, EventArgs e)
    {
        valueLabel.Text = change.thisValue.ToString();
    }
}

changeValueForm

public partial class changeValueForm : Form
{
    private valueForm valueForm;
    public int thisValue;
    private string[] digits = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };
    public changeValueForm()
    {
        InitializeComponent();
    }

    private void changeValueForm_Load(object sender, EventArgs e)
    {
        valueForm = new valueForm();
        thisValue = valueForm.value;
        valueBox.Text = thisValue.ToString();
    }

    private void closeButton_Click(object sender, EventArgs e)
    {
        if (digits.Contains(valueBox.Text.ToString()))
        {
            thisValue = Int32.Parse(valueBox.Text);
            valueForm.value = thisValue;
        }
    }
}

So, summa summarum, what is wrong with this code?

P.S. How to pass values between forms in c# windows application? Might have hepled with the 1. problem, but valueForm still doesn't accept changes made in changeValueForm - It just stays as it was.

Answer 1

Instead of creating new instance of valueForm here:

private void changeValueForm_Load(object sender, EventArgs e)
{
    valueForm = new valueForm();

You should pass to your changeValueForm instance reference to your existed valueForm and use it.

For example this can be done when you're creating your changeValueForm, like change = new changeValueForm(this);.

In this case you also need to and changeValueForm constructor to

public changeValueForm(valueForm f)
{
    InitializeComponent();

    valueForm = f;
}

Also it it unclear from the code you've posted - when it it suposed to call updatin_Tick method and who should call it? I can guess from the method name - it should be some timer - but there is not any timer declared in your code.

Answer 2

It is a common situation. In your changeValueForm you create a new instance of the valueForm. This new instance has nothing in common (a part from being the same class) with the initial form. It is a totally different form. So, when you set properties to the instance created inside the changeValueForm you are setting the properties of a form that is not visible. (Just try to call Show() on that instance).

The easiest way to fix the problem is passing the instance of the initial valueForm to the changeValueForm created by your code. Usually this is done in the constructor, but could also passed using a public custom property of the valueChangeForm class.

USING THE CONSTRUCTOR

public partial class changeValueForm : Form
{
    private valueForm valueForm;
    ....

    public changeValueForm(valueForm currentInstance)
    {
        InitializeComponent();
        valueForm = currentInstance;
    }

Of course when you build your changeValueForm you call

public partial class valueForm : Form
{
    public int value;
    private changeValueForm change;
    public valueForm()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        value = 2;
        valueLabel.Text = value.ToString();

        // pass this instance to the changeValueForm
        change = new changeValueForm(this);
    }

USING A PROPERTY

public partial class changeValueForm : Form
{
    private valueForm valueForm;

    public changeValueForm ClientCaller 
    { get { return valueForm}; 
      set { valueForm = value};
    }....


public partial class valueForm : Form
{
    ...
    private void Form1_Load(object sender, EventArgs e)
    {
        ....
        change = new changeValueForm();
        change.ClientCaller = this;
    }