Question

On Form1, I want to make a button wich switches on/off a Form2’s label’s visibility (visible true or false). Thank you much, I’m beginner!

Answer 1

Well here is what you should do. The simpler but not recommended solution. I have written it just to show how simple it can be:

 public partial class Form1 : Form
 {
        Form2 form2;
        public Form1()
        {
            InitializeComponent();
            form2 = new Form2();
            form2.Show();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            bool labelVisible = form2.ToggleLabelVisibility();
        }
 }
public partial class Form2 : Form
{
     public Form2()
     {
        InitializeComponent();
     }

     public bool ToggleLabelVisibility()
     {
         label1.Visible = !label1.Visible;
         return label1.Visible;
     }
}

The better solution should Invoke delegate on the Form2:
In Form1 change only this:

private void button1_Click(object sender, EventArgs e)
{
      bool labelVisible = (bool)form2.Invoke(form2.ToggleVisibility);
}

Form2:

public partial class Form2 : Form
{
    public delegate bool visibilityToggler();
    public visibilityToggler ToggleVisibility;

    public Form2()
    {
        InitializeComponent();
        ToggleVisibility = new visibilityToggler(ToggleLabelVisibility);
    }
    public bool ToggleLabelVisibility()
    {
        label1.Visible = !label1.Visible;
        return label1.Visible;
    }
}

I must say that this is old school, I am sure that there are many other ways to do this especially from C# 4.0 but this is how I did it for years.