打开文件，保存文件

  private void ShowNewForm(object sender, EventArgs e)
        {
            Form childForm = new Form();
            childForm.MdiParent = this;
            childForm.Text = "窗口 " + childFormNumber++;
            childForm.Show();
        }

        private void OpenFile(object sender, EventArgs e)
        {
            OpenFileDialog openFileDialog = new OpenFileDialog();
            openFileDialog.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
            openFileDialog.Filter = "文本文件(*.txt)|*.txt|所有文件(*.*)|*.*";
            if (openFileDialog.ShowDialog(this) == DialogResult.OK)
            {
                string FileName = openFileDialog.FileName;
            }
        }

        private void SaveAsToolStripMenuItem_Click(object sender, EventArgs e)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            saveFileDialog.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
            saveFileDialog.Filter = "文本文件(*.txt)|*.txt|所有文件(*.*)|*.*";
            if (saveFileDialog.ShowDialog(this) == DialogResult.OK)
            {
                string FileName = saveFileDialog.FileName;
            }
        }