c# - How to access a form control for another form?

Question

I have two Form classes, one of which has a ListBox. I need a setter for the SelectedIndex property of the ListBox, which I want to call from the second Form.

At the moment I am doing the following:

Form 1

public int MyListBoxSelectedIndex
{
     set { lsbMyList.SelectedIndex = value; }
}

Form 2

private ControlForm mainForm; // form 1

public AddNewObjForm()
{
     InitializeComponent();
     mainForm = new ControlForm();           
}

public void SomeMethod()
{
     mainForm.MyListBoxSelectedIndex = -1;
}

Is this the best way to do this?

Answer 1

Making them Singleton is not a completely bad idea, but personally I would not prefer to do it that way. I'd rather pass the reference of one to another form. Here's an example.

Form1 triggers Form2 to open. Form2 has overloaded constructor which takes calling form as argument and provides its reference to Form2 members. This solves the communication problem. For example I've exposed Label Property as public in Form1 which is modified in Form2.

With this approach you can do communication in different ways.

Download Link for Sample Project

//Your Form1

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

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

    public string LabelText
    {
        get { return Lbl.Text; }
        set { Lbl.Text = value; }
    }
}

//Your Form2

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

    private Form1 mainForm = null;
    public Form2(Form callingForm)
    {
        mainForm = callingForm as Form1; 
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {

    }

    private void button1_Click(object sender, EventArgs e)
    {
        this.mainForm.LabelText = txtMessage.Text;
    }
}

_{(source: ruchitsurati.net)}

_{(source: ruchitsurati.net)}