How to make Form1 label.text change when checkbox on form2 is checked?

Question

I\'m very new to c# and am trying my first experiments with 2 different forms.

I\'d like to make it so you have a label1 and a button1 on Form1, and a checkbox1 on F

Answer 1

Using a Controller and Events to decouple the forms

The correct way to do this kind of thing is to decouple the two forms from each other by introducing a Controller class, and using events to signal state changes.

Here's an example.

Firstly, create a new default Windows Forms app called WindowsFormsApplication1 and add two forms, form1 and form2.

Then add to form1 a button called "button1" and a label called "label1".

Then add to form2 a checkbox called "checkbox1".

In the form1 designer, double-click the button to add a click handler for it.

In the form2 designer, double-click the checkbox to add a change handler for it.

Now add a new class called "Controller" and add to it the following code:

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    internal sealed class Controller
    {
        public void RunForm1()
        {
            _form1 = new Form1();
            // The next line defines our response to the button being pressed in Form1
            _form1.ButtonClicked += (sender, args) => showForm2();
            Application.Run(_form1);
        }

        private void showForm2()
        {
            var form2 = new Form2();
            // The next line defines our response to the checkbox changing in Form2.
            form2.CheckBoxChanged += 
                (sender, args) => 
                _form1.SetLabel("Checkbox = " + ((CheckBox)sender).Checked);

            form2.ShowDialog(_form1);
        }

        private Form1 _form1 ;
    }
}

Now change Form1.cs to this:

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1: Form
    {
        // Here's where we announce our event handler to the world:
        public event EventHandler ButtonClicked;

        public Form1()
        {
            InitializeComponent();
        }

        public void SetLabel(string text)
        {
            label1.Text = text;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // We make a copy of ButtonClicked before checking it for null because
            // in a multithreaded environment some other thread could change it to null
            // just after we checked it for nullness but before we call it, which would
            // cause a null reference exception.
            // A copy cannot be changed by another thread, so that's safe to use:

            var handler = ButtonClicked;

            if (handler != null)
                handler(sender, e);
        }
    }
}

And change Form2.cs to this:

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form2: Form
    {
        // Here's the event handler for the check box:
        public event EventHandler CheckBoxChanged;

        public Form2()
        {
            InitializeComponent();
        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {
            var handler = CheckBoxChanged;

            if (handler != null)
                handler(sender, e);
        }
    }
}

Finally, change Program.cs to this:

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            Controller controller = new Controller();
            controller.RunForm1();
        }
    }
}

Now run the program and click the button, then click the checkbox a few times. You will see the label in Form1 changing.

In this way, you have completely decoupled Form1 from Form2 and placed the control logic into a separate Controller class.