Question

I am trying to understand whats the difference between a static and public properties. But when I tried to access my public property ‘Test’ in other form it says ‘null’.

Heres Form1:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private string _test;

    public string Test
    {
        get { return _test; }
        set { _test = value; }
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        _test = "This is a test";
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 frm2 = new Form2();
        frm2.Show();
    }
}

Here’s Form2:

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        Form1 frm1 = new Form1();
        label1.Text = frm1.Test;
    }
}

To check the value of ‘Test’ in Form1, I put a breakpoint to this line:

label1.Text = frm1.Test;

But the value is ‘null’.

Please help me how can I access public properties to other forms.

And BTW I tried to make this public property be a ‘public static’. I can access this using this:

Form1.Test

But I noticed that I can change ‘Test’ value from Form2 which I don’t want to happen. That’s why I am trying to use public property but with no luck. Can somebody clarify me these things. Thanks for all your help guys!

EDIT: (For follow up question) 

Sir John Koerner’s answer is the best answer for my question. But I have a follow up question, I tried to make these ‘test’ properties to be a ‘static’, and I noticed that even if I make this property a static or public property, it still can be edit in Form2. To make myself clear here’s a sample:

public partial class Form2 : Form
{
    private Form1 f1;
    public Form2(Form1 ParentForm)
    {
        InitializeComponent();
        f1 = ParentForm;
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        label1.Text = f1.Test;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        f1.Test = "This test has been changed!";

        this.Close();
    }
}

After Form2 closed, I tried to break again in Form1_Load to check value of ‘Test’, and it was changed! How can I make a public property in Form1 to readOnly in Form2 and cannot be editted? Please clarify to me. Thanks a lot guys!

Answer 1

Your property is an instance variable, so the value can be different across different instances of Form1.

If you are trying to access instance variables from a parent form, the easiest way to do that is to pass Form1 in to the constructor of Form2.

public partial class Form2 : Form
{
    private Form1 f1;
    public Form2(Form1 ParentForm)
    {
        InitializeComponent();
        f1 = ParentForm;
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        label1.Text = f1.Test;
    }
}

Then when you create a new Form2 from Form1, you can do this:

Form2 frm2 = new Form2(this);

If you want your property to be read only, you can simply not specify a setter:

public string Test
{
    get { return _test; }
}