子窗体(Win_Set):

MainWindow m;
                m = (MainWindow)this.Owner;
                m.showText("获取文件成功\r\n");

主窗体(MainWindow):

Win_Set  ws= new Win_Set();
                   ws.Owner = this;
                    ws.Show();

要调用的函数:

public void showText(String msg)
        {
            messagePanel.addMessage(DateTime.Now.ToString(),msg);
        }

实现的功能就是子窗体调用主窗体的函数来向主窗体的控件中添加文本信息。