/// <summary>
/// 获取指定类型的第一个实体或null
/// </summary>
/// <param name="where">条件</param>
/// <param name="order">排序</param>
/// <param name="isAsc">是否升序，默认升序</param>
/// <param name="selector">要返回对象的表达式</param>
/// <returns></returns>
public S GetFirstOrDefaultEntity <S, K>(Expression <Func <T, bool> > where = null, Expression <Func <T, K> > order = null,
bool isAsc = true, Expression <Func <T, S> > selector = null) where S : class
{
return(_baseDal.GetFirstOrDefaultEntity(where, order, isAsc, selector));
}