using Windows.UI.Popups;
public async void MessageBoxShow(string content, string caption)
{
MessageDialog messageShow = new MessageDialog(content, caption);
await messageShow.ShowAsync();
}
方法要这样写才行！
Windows phone 8.1 MessageBox 变了哦！
原文：http://www.cnblogs.com/JiangLe/p/4214369.html