C#

public class mainForm : Form {

	FindDialog findDialog;	// reference to the find dialog

	public mainForm() {	// constructor
		// ...
	}
	
	// this could be called by clicking a "Open Find" button on mainForm
	public void btnOpenFind_Click(object sender, EventArgs e) {
		findDialog=new FindDialog();
		findDialog.btnFind.Click+=new EventHandler(btnFind_Click);
		findDialog.btnClose.Click+=new EventHandler(btnClose_Click);
		findDialog.ShowDialog();
	}
	private void btnFind_Click(object sender, EventArgs e) {
		// read findDialog.tbSearch.Text
		// and perform search in the text inside mainForm
	}
	private void btnClose_Click(object sender, EventArgs e) {
		// do postprocessing if needed (e.g. preserve current settings),
		// then
		findDialog.Close();
	}
}

public class FindDialog : Form {
	public Button btnFind;		// will start the search
	public Button btnClose;		// closes the dialog
	public TextBox tbSearch;		// holds what to search
}