如何从基于 OrderBy“属性”的集合中返回 IOrderedEnumerable

Question

class Employee
{
    public string Name {get;set;}
    public int employee_id {get;set}
    public int Age {get;set}
}

Class EmployeeCollection : IEnumerable, IEnumerable<Employee>, IOrderedEnumerable<Employee>
{
    public Expression<Func<Employee, dynamic>> SortOrder {get;set;}
    protected Dictionary<int,Employee> EmployeeById = new Dictionary<int,Employee>();
    public void AddEmployee(Employee ePassed)
    {
        EmployeeById[ePassed.employee_id] = ePassed;
    }
    public IEnumerator<Employee> GetEnumerator()
    {
      foreach (int id in EmployeeByID.Keys)
      {
        yield return EmployeeById[id];
      }
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
      return this.GetEnumerator();
    }
    public IOrderedEnumerable<Employee> CreateOrderedEnumerable<TKey>(Func<Employee, TKey> KeySelector, IComparer<TKey> comparer, bool descending)
    {
      if (descending)
          return this.OrderByDescending(KeySelector, comparer);
      else
          return this.OrderBy(KeySelector, comparer);
    }
    public IEnumerable<Employee> OrderedObjects
    {
        if (this.SortOrder == null)
            return (IEnumerable<Employee>)this;  // No Sort Order applied
        else
        {
          // How do I get the "parameters" from SortOrder to pass into CreateOrderedEnumerable?
          throw new NotImplementedException();
        }
    }
}

我希望能够使用类似于以下的语法...

EmployeeCollection.SortOrder = (x => x.Name);
foreach (Employee e in EmployeeCollection.OrderedObjects)
{
  // Do something in the selected order
}

有数千个示例说明如何将排序、过滤等结果放入新的列表、集合、ObservableCollection 等，但如果您现有的集合已经响应事件，则会自动添加新对象以响应通知、用户操作，来自服务器的新数据等，然后所有这些功能要么“丢失”，要么必须“添加”，以使新的 List、Collection、ObservableCollection 等监听原始集合，以便以某种方式与原始集合已经知道和处理的所有各种更新、属性等......我希望能够让原始的“EmployeeCollection”简单地在请求的 SortOrder 中分发“Employee”对象......

我对“SortOrder”属性的语法做了一个“疯狂的猜测”，因为我想让 SortOrder 属性的语法类似于团队中其他开发人员习惯使用的 lambda 表达式的 orderby 部分通过查看 System.Linq.Enumerable 中类似于以下内容的扩展方法： public static IOrderedEnumerable<TSource> OrderBy<ToSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector);

我是 Linq、lambda 等的新手，如果我不知何故错过了 Linq/表达式树、谓词、匿名代表等其他人认为显而易见的关键方面，请提前道歉。

Answer 1

当您不使用ThenBy-type 操作（这些都是IOrderedEnumerable<T> 接口真正添加的）时，此方法有效。请参阅 C#: How to implement IOrderedEnumerable<T> 以获得支持该解决方案的解决方案。

public class Employee
{
    public string Name {get;set;}
    public int employee_id {get;set;}
    public int Age {get;set;}
}

public class EmployeeCollection : IEnumerable, IEnumerable<Employee>, IOrderedEnumerable<Employee>
{
    public Func<Employee, object> SortOrder {get;set;}
    public Func<Employee, bool> Filter {get;set;}
    protected Dictionary<int,Employee> EmployeeById = new Dictionary<int,Employee>();
    public void Add(Employee ePassed)
    {
        EmployeeById[ePassed.employee_id] = ePassed;
    }
    public IEnumerator<Employee> GetEnumerator()
    {
        var employees = EmployeeById.Keys.Select(id => this.GetEmployee(id));
        if (Filter != null)
            employees = employees.Where(Filter);
        if (SortOrder != null)
            employees = employees.OrderBy(SortOrder);
        return employees.GetEnumerator();
    }
    public Employee GetEmployee(int id)
    {
        return EmployeeById[id];
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
      return this.GetEnumerator();
    }
    public IOrderedEnumerable<Employee> CreateOrderedEnumerable<TKey>(Func<Employee, TKey> KeySelector, IComparer<TKey> comparer, bool descending)
    {
      throw new NotImplementedException();
    }
}

// this is some code you might use to test this:
var EmployeeCollection = new EmployeeCollection
{
    new Employee { employee_id = 1, Age = 20, Name = "Joe" },
    new Employee { employee_id = 2, Age = 30, Name = "Thomas" },
    new Employee { employee_id = 3, Age = 25, Name = "Robert" },
};
EmployeeCollection.SortOrder = x => x.Age;
EmployeeCollection.Filter = x => x.Name.Length > 4;
foreach (Employee e in EmployeeCollection)
{
    // do something
}