/// <summary>
/// 排序次项
/// 选择升序或降序
/// </summary>
/// <typeparam name="T">实体类型</typeparam>
/// <param name="source">数据源</param>
/// <param name="orderByKey">排序键(不分大小写)</param>
/// <param name="ascending">是否升序</param>
/// <exception cref="ArgumentNullException"></exception>
/// <exception cref="ArgumentException"></exception>
/// <returns></returns>
public static IOrderedQueryable <T> ThenBy <T>(this IOrderedQueryable <T> source, string orderByKey, bool ascending) where T : class
{
var methodName = ascending ? "ThenBy" : "ThenByDescending";
return(source.OrderByInternal(orderByKey, methodName));
}