c# - 通用 Func 对不同类型的集合进行排序

Question

我有很多类型的集合，我想按不同的属性对每个集合进行排序。例如IEnumerable<Employee>将按 Name 排序和 Age属性，和 IEnumerable<Department>将按 NumberOfEmployees 排序和 DepartmentName特性。我用 PaginatedList排序后对集合进行分页。

public class PaginatedList<T> : List<T>
{
  public PaginatedList(IEnumerable<T> source, Int32 pageIndex, Int32 pageSize , Func<T,Object> orderBy)
  {
    this.AddRange(source.OrderBy(orderBy).Skip((PageIndex - 1) * PageSize).Take(PageSize));
  }
}

请注意第 4 个参数，它是将传递给 OrderBy 扩展方法的排序委托(delegate)。

我正在使用通用方法生成这第四个元素

public Func<T, Object> SortingFactory<T>(String sortby) 
{
  switch (typeof(T).ToString())
  {
    case "Employee":
      switch(sortby)
      {
        case "Name":
           return new Func<Employee,String>(delegate(Employee e) { return e.Name; });
           break;                            
        case "Age":
           return new Func<Employee,Int32>(delegate(Employee e) { return e.Age; });
           break;
      }
      break;
    case "Department":
      switch(sortby)
      {
        case "NumberOfEmployees":
           return new Func<Department,Int32>(delegate(Department d) { return d.NumberOfEmployees; });
           break;                            
        case "DepartmentName":
           return new Func<Department,String>(delegate(Department d) { return d.DepartmentName; });
           break;
      }
      break;
  }
}

但它给了我一个编译错误 Cannot implicitly convert type 'System.Func<Employee,String>' to 'System.Func<T,object>'

我还尝试将输出贴标为 Func<Object,Object>但我得到了同样的错误。

我犯了什么错，怎么办。

Answer 1

说我理解的很好

public class PaginatedList<T> : List<T>
{
  public PaginatedList(IEnumerable<T> source, Int32 pageIndex, Int32 pageSize )
  {
    this.AddRange(GetOrderFor<T>().Skip((PageIndex - 1) * PageSize).Take(PageSize));
  }
}

public static class Helpers
    {

        public static Func<T, object> GetSortExpression<T>(string sortExpressionStr)
        {
            var param = Expression.Parameter(typeof (T), "x");
            var sortExpression = Expression.Lambda<Func<T, object>>(Expression.Convert(Expression.Property(param, sortExpressionStr), typeof(object)), param);
            return sortExpression.Compile();
        }

        public static IOrderedEnumerable<T> GetOrderFor<T>(this IEnumerable<T> list)
        {
            switch (typeof (T).Name)
            {
                case "Employee":
                    return list.OrderBy(GetSortExpression<T>("Name")).ThenBy(GetSortExpression<T>("Age"));
                case "Category":
                    return list.OrderBy(GetSortExpression<T>("Name")).ThenBy(GetSortExpression <T> ("Id"));
            }
            return null;
        }
    }

如果我误解了，我认为 GetSortExpression 方法的简单用法可以帮助您避免错误

case "Employee":
      switch(sortby)
      {
        case "Name":
           return Helpers.GetSortExpression<T>("Name");                           
        case "Age":
           return Helpers.GetSortExpression<T>("Age");
      }