/// <summary>
/// 递归获取文件夹及其子文件夹下的。
/// </summary>
/// <param name="folder"></param>
/// <returns></returns>
public static async Task <List <StorageFile> > GetAllFilesAsync(this StorageFolder folder)
{
List <StorageFile> allFiles = new List <StorageFile>();
var list = await folder.ArraryAllFilesAsync(allFiles);
return(list);
}