IQueryable不允许ThenBy

Question

我正在尝试按多个列对IQueryable进行排序，并且许多StackOverflow答案似乎表明我应该能够对Primary排序执行OrderBy，然后对其他排序执行ThenBy。OrderBy很好，但是不允许我使用ThenBy。它不会编译。我不明白为什么...

IQueryable<vMyView> contacts = db.vMyView;

var orderExpressions = new Dictionary<string, Expression<Func<vCRMAllContact, object>>>()

    {

         {"LastName", x => x.LastName},

         {"FirstName", x => x.FirstName},

         {"Email", x => x.Email},

         {"Telephone1", x => x.Telephone1}

     };

contacts = contacts.OrderBy(orderExpressions[sortExpression], ascending).ThenBy(orderExpressions["FirstName"]).Skip(pageIndex * pageSize).Take(pageSize);