请问下我这样写，为什么不行 啊，
DialogResult dr = MessageBox.Show("内容已被修改过，是否保存？", "写字板", MessageBoxButtons.YesNoCancel);

            if (dr == DialogResult.Yes)
            {
                SaveFile();
                this.Close();

            }
            else if (dr == DialogResult.Cancel)
            {
                return;

            }
            else
                this.Close();