What is the best way to pass data between a MainFrame (or Main Dialog) and a Modal Dialog?

public string UserName
{
    get { return userNameTextBox.Text; }
}

Dialog dialog = new Dialog();
if (dialog.ShowDialog() == DialogResult.OK)
{
    string username = dialog.UserName;
}

myGetter.SetTransferObject(dataStructInstance)
myGetter.GoGetTheData()
// do stuff with dataStructInstance now that myGetter set it up.