如何使用实时按钮访问另一个表单的属性？

Question

I'm trying to code a theme changer to my gui but it doesnt work that well, I've tried everything I know. I have 2 forms MainUI and Themes and I'm trying to press a button under themes form and then it will fire the code under MainUi live, what I mean with live is that it will happend directly so I don't need to close Themes for it to take effect for an example. .

My Main Ui's code for Themes is this:

private void button7_Click(object sender, EventArgs e)
    {
        bool Isopen = false;
        foreach(Form f in Application.OpenForms)
        {
            if (f.Text == "Themes")
            {
                Isopen = true;
                f.BringToFront();
                break;
            }
        }

        if (Isopen == false)
        { 
            Themes theme = new Themes();
            theme.Show();
        }
    }

    public void FireEvent()
    { //Example
        BackColor = Color.FromArgb(255, 255, 255);

    }

Themes:

private void button4_Click(object sender, EventArgs e)
    {
        MainUI main = new MainUI();
        main.FireEvent();
    }

Answer 1

You're creating a new Instance of MainUI each time a theme is selected, so you call FireEvent on the wrong instance of the form. You need to pass a reference to the Themes form. For example create a constructor that recieves a MainUI instance.

class Themes : Form
{
    private readonly MainUI _main;

    public Themes(MainUI main) : this()
    {
        _main = main;
    }

    private void button4_Click(object sender, EventArgs e)
    {
        _main.FireEvent();
    }
}

In the main UI use the following code:

private Themes _theme;
private void button7_Click(object sender, EventArgs e)
{
   if(_theme == null)
       _theme = new Themes(this);

   _theme.Show();
}