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

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f = new Form2(() => this.Visible = ! this.Visible);
            f.ShowDialog();
        }
    }

    
    //Диалоговая форма
    public delegate void MinimizeMainForm();

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

        public Form2(MinimizeMainForm minimizeMainForm) : this()
        {
            this.minimizeMainForm = minimizeMainForm;
        }

        private void Form2_SizeChanged(object sender, EventArgs e)
        {
            this.minimizeMainForm();
        }
    }