Form1——主窗体：

namespace FirstDlg
{
    public partial class Form1 : Form
    {
        private Form2 f;
        public Form1()
        {
            InitializeComponent();
        }
        public string TextStored
        {
            get { return tbTest.Text; }
            set { tbTest.Text = value; }
        }
        private void button1_Click(object sender, EventArgs e)
        {
            tbTest.Text = f.TextStored;//属性传值
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            f = new Form2();
            f.Show();
        }
    }
}

Form2——对话框窗体：

namespace FirstDlg
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        public string TextStored
        {
            get { return tbTest.Text; }
            set { tbTest.Text = value; }
        }
        private void btnOK_Click(object sender, EventArgs e)
        {
            this.DialogResult = DialogResult.OK;
            this.Close();
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            this.DialogResult = DialogResult.Cancel;
            this.Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            foreach (Form form in Application.OpenForms)//搜索窗体名称
            {
                if (form.Name == "Form1")
                {
                    Form1 f = (Form1)form;
                    tbTest.Text = f.TextStored;
                }
            }
        }
    }
}