How to send a "message" to parent form from child form?

Question

I have two forms: Form1: Parent: Login Screen Form2: Child: Application

When the application starts Form1 is shown. User enters password, and if this password matches with a constant, Form1 is hidden and Form2 shows up. Now when I close Form2 using a "log off" button, I shall send a "message" to Form1, so it can show up, again. Think this as a messenger application. When you log off, it will send you back to log in screen.

I am searching for two days, but can't find anything that worked for me. Sorry if this question is a duplicate.

what i usally do is create a class with a static forms instances , and switch between them — lagripe, Jan 19 '19 at 18:49
You can create a project with 2 forms. Also, create a static property in a static class. Use this property to store common info between the 2 forms. An alternative approach is explained here: https://stackoverflow.com/questions/43219637/sharing-informations-between-form-class and here: https://social.msdn.microsoft.com/Forums/en-US/4a7b97de-c86d-4773-96ed-753af585de3c/sharing-of-data-between-forms?forum=Vsexpressvcs — NoChance, Jan 19 '19 at 19:02
In object languages, a "message" sent from object A to object B is usually implemented as a method call, the A calls a method of B. The only question is whether you want to introduce yet another mediating object, C, between the two, or rather have a direct dependency between them without any extra entity. — Wiktor Zychla, Jan 19 '19 at 19:09

Olivier Jacot-Descombes · Answer 1 · 2021-12-26T12:15:29.150

2

In Form1, you can subscribe to the FormClosed event of the second form before opening it:

var form2 = new Form2();
form2.FormClosed += Form2_FormClosed;
form2.Show();

Then you can do anything useful in the event handler in Form1:

private void Form2_FormClosed(object sender, FormClosedEventArgs e)
{
    MessageBox.Show("Form 2 closed");
}

edited Dec 26 '21 at 12:15

answered Jan 20 '19 at 18:31

Olivier Jacot-Descombes

104,806
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score 1 · Accepted Answer · answered Jan 22 '19 at 13:27

I suggest that you use the Main/Application Form as Parent and Show the Login Form as Child. Show the Login Form with ShowDialog() in the "Shown" Event of the Main/Parent Form (for exemple). Depending on how smart your Form classes are you might have to change some code below. The code sample below implies, that the Login form does all the login logic. It then signals its parent if the login was successful or not with the DialogResult property upon closing.

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

    private void ButtonLogoutClick(object sender, EventArgs e)
    {
        LoginUser();
    }

    private void MainFormShown(object sender, EventArgs e)
    {
        LoginUser();
    }

    private void LoginUser()
    {
        using (var loginForm = new LoginForm())
        {
            var loginResult = loginForm.ShowDialog();
            if (loginResult == DialogResult.OK)
            {
                //Login Success
                var userId = loginForm.User.ID; //Query user ID from Login Form for example
            }
            else
            {
                //Login Failed
                Close(); //Close Program for example
            }
        }
    }
}

score 1 · Answer 3 · answered Dec 26 '21 at 12:30

Apparently LoginForm is a form that creates and shows your ChildForm. While ChildForm is shown, LoginForm is not visible, nor can it have the focus. As soon as ChildForm is closed, LoginForm should become visible again and have the focus.

This looks like standard Modal Dialog behaviour to me, except the invisiblity.

class LoginForm : Form
{
    private void ShowChild()
    {
        using (var childForm = new ChildForm(...))
        {
            // if needed: set ChildForm properties
            ...

            // just before showing the childForm, hide this loginForm
            this.Visible = false;

            // show the childForm until it is closed
            DialogResult dlgResult = childForm.ShowDialog(this);

            // interpret the results of the childForm,
            // for instance something like this:
            switch (dlgResult)
            {
                // operator pressed OK; process the result and show the login
                case DialogResult.OK:
                    string savedFileName = childForm.FileName;
                    this.Process(savedFileName);
                    break;

                // operator indicates that program can close
                case DialogResult.Cancel:
                    this.Close();
                    break;

                // all other solutions: do nothing but show the login screen
                default:
                     break;
            }

            // show the login screen
            this.Visible = true;
        }
    }
}

score 0 · Answer 4 · answered Jan 21 '19 at 06:07

0

Form1:

create static and public variable

static public string message = "Message from Form1";

Form2

use this variable

MessageBox.Show(Form1.Message, " Message");

answered Jan 21 '19 at 06:07

Ivan

1

How to send a "message" to parent form from child form?

4 Answers4