/// <summary>
/// 获取文件或目录最后的写入UTC时间。
/// </summary>
/// <param name="virtualPathProvider">虚拟路径提供者。</param>
/// <param name="virtualPath">虚拟路径。</param>
/// <returns>UTC时间。</returns>
public static DateTime?GetLastWriteTimeUtc(this IVirtualPathProvider virtualPathProvider, string virtualPath)
{
virtualPathProvider.NotNull("virtualPathProvider");
virtualPath.NotEmptyOrWhiteSpace("virtualPath");
var dateTime = virtualPathProvider.GetFileLastWriteTimeUtc(virtualPath);
return(dateTime.HasValue ? dateTime : virtualPathProvider.GetDirectoryLastWriteTimeUtc(virtualPath));
}