orderby或OrderByDescending在表达列上不工作的LINQ

Question

var q4 = await dbentity.tbltest.Where(v => v.Nvcode == vcode)
                               .OrderByDescending(o => Convert.ToInt32(o.Period.Substring(5, 4)))
                               .Select(a => new { Period = a.Period, Year = Convert.ToInt32(a.Period.Substring(5, 4)) })
                               .Distinct()
                               .ToListAsync();

上面的查询返回

Period     Year
2012-2013, 2013
2019-2020, 2020 
2013-2018, 2018
2007-2008, 2008

我想在一年中排序IE，请参见下面的

Period     Year
2019-2020, 2020 
2013-2018, 2018
2012-2013, 2013
2007-2008, 2008

orderby和orderbyDescending在LINQ中不起作用以实现上述输出。