linq orderby name thenby childrencollection.name

似乎您正在尝试获取按“组”和“部门名称”排序的所有部门的列表。如果是这样,那么您可能想这样做:

var res = from c in _repository.GetActiveDepartmentGroupsWithDepartments()
          from d in c.Departments
          orderby c.DepartmentGroupName, d.DepartmentName
          select d;

或在方法语法中:

var res = _repository.GetActiveDepartmentGroupsWithDepartments()
                     .SelectMany(c => c.Departments, (c,d) => new { c, d })
                     .OrderBy(x => x.c.DepartmentGroupName)
                     .ThenBy(x => x.d.DepartmentName)
                     .Select(x => x.d);

因为deparment是一个集合,所以必须将其转换为标量才能用于排序。

一个选项是在集合中选择一个实体,例如第一个部门的名称:

_repository.GetActiveDepartmentGroupsWithDepartments()
   .OrderBy(c => c.DepartmentGroupName)
   .ThenBy(c => c.Departments
       .OrderBy(d => d.DepartmentName)
       .FirstOrDefault()
       .DepartmentName
    )

另一种选择是按集合本身的属性排序,例如部门数:

_repository.GetActiveDepartmentGroupsWithDepartments()
   .OrderBy(c => c.DepartmentGroupName)
   .ThenBy(c => c.Departments.Count())