通用Func 对不同类型的集合进行排序(Generic Func to sort collections of different types)

我有许多类型的集合，我想按不同的属性对每个集合进行排序。例如， IEnumerable<Employee>将按Name和Age属性排序， IEnumerable<Department>将按NumberOfEmployees和DepartmentName属性排序。我在排序后使用PaginatedList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)); } }

注意第4个参数，它是将传递给OrderBy扩展方法的排序委托。

我正在使用通用方法来生成第4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>'

我也尝试将输出decalre为Func<Object,Object>但我得到了同样的错误。

我犯了什么错，怎么做这样的方法。

I have many collections for many types I want to sort each collection by different properties. For example IEnumerable<Employee> will be sorted by Name and Age properties,and IEnumerable<Department> will be sorted by NumberOfEmployees and DepartmentName properties. I use PaginatedList to paginating the collection after sorting it.

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)); } }

Note the 4th parameter which is the sorting delegate that will be passed to OrderBy extension method.

I am using a generic method to generate this 4th element

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; } }

but it gives me a compilation ErrorCannot implicitly convert type 'System.Func<Employee,String>' to 'System.Func<T,object>'

I also tried to decalre the output as Func<Object,Object> but I got the same error.

What is the fault I made and how do such method.

最满意答案

说我理解得很好

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"); }

Say I understood well

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; } }

And if I misunderstood, I think the simple usage of GetSortExpression method can help you to avoid your error

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

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