public interface IForm
{
    void RefreshForm();
}

Form2 f = new Form2();
f.Owner = this;
f.ShowDialog();

Form form=new Form();
form.Show(this);

public delegate void sendmessage(string message); 
public event sendmessage SendTo ;

ChildForm frm = new ChildForm(); 
frm.SendTo += new ChildForm.sendmessage(SendArgs); 
frm.ShowDialog(this);
private void SendArgs(string Message)//主窗体接收消息 
{MessageBox.Show( "主窗体已收到消息： " + Message);}

public ChildForm(MainForm parent) 
{ 
InitializeComponent();
this.ParentFrom = parent;//引用 
}

ChildForm frm = new ChildForm(this); 
frm.ShowDialog(this);

void ...Click(....) 
{ 
this.Text = "测试引用 "; 
if (this.ParentFrom != null) this.ParentFrom.Text += "- " + this.Text;//....... 
}