C# 返回上一个界面

https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&tn=baidu&wd=C%23 返回上一个界面&oq=%E4%B8%89%E5%8F%B0%E8%B7%AF%E7%94%B1%E5%99%A8%E4%BA%92%E8%BF%9E%20%E9%9D%99%E6%80%81%E8%B7%AF%E7%94%B1&rsv_pq=b446ef37000179a5&rsv_t=f0capP9BNESaXubu0jE6PsLuxEkfRz98u%2F33hNcHjGQDT3fvuN3BdCcq6vo&rqlang=cn&rsv_enter=1&inputT=9616&rsv_sug3=20&rsv_sug1=6&rsv_sug7=100&rsv_sug2=0&rsv_sug4=9616