/// <summary>
/// 获取第一个或者null的实体
/// </summary>
/// <typeparam name="S">泛型类型</typeparam>
/// <param name="where">条件</param>
/// <param name="order">排序</param>
/// <param name="isAsc">是否升序</param>
/// <returns></returns>
public T First <S>(Expression <Func <T, bool> > where = null, Expression <Func <T, S> > order = null, bool isAsc = true)
{
return(_baseDal.First(where, order, isAsc));
}