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

        private void button1_Click(object sender, EventArgs e)
        {
            FormA fm = new FormA();
            fm.change+=new FormA.ChangeMainForm(ChangeLable);
            fm.ShowDialog();
        }

        private void ChangeLable(string str)
        {
            label1.Text = str;
        }
    }
}

namespace TestSelect
{
    public partial class FormA : Form
    {
        public FormA()
        {
            InitializeComponent();
        }
        public delegate void ChangeMainForm(string str);
        public event ChangeMainForm change;
        private void button1_Click(object sender, EventArgs e)
        {
            change(textBox1.Text);
        }
    }
}