c# – 为什么没有strings.Cast演员列表到列表？

在 this question中建议,我可以使用.Cast< object>将一个泛型集合向上转换为一组对象.在 reading up a bit on .Cast<>之后,我仍然无法将它作为一个通用集合投射到另一个通用集合中.以下为什么不工作？

using System.Collections.Generic;
using System.Linq;
using System;

namespace TestCast2343
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> strings = new List<string> { "one","two","three" };

            //gives error: cannot convert from 'System.Collections.Generic.List<string>'
            //to 'System.Collections.Generic.List<object>'
            //IEnumerable<string> items = strings.Cast<object>();

            //this works
            strings.Cast<object>();

            //but they are still strings:
            foreach (var item in strings)
            {
                System.Console.WriteLine(item.GetType().Name);
            }

            //gives error: cannot convert from 'System.Collections.Generic.List<string>'
            //to 'System.Collections.Generic.List<object>'
            ProcessCollectionDynamicallyWithReflection(strings);

            Console.ReadLine();
        }

        static void ProcessCollectionDynamicallyWithReflection(List<object> items)
        {
            //...
        }
    }

}

回答：

谢谢里德,这是我开始工作的代码：

using System.Collections.Generic;
using System.Linq;
using System;

namespace TestCast2343
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> strings = new List<string> { "one","three" };
            List<int> ints = new List<int> { 34,35,36 };
            List<Customer> customers = Customer.GetCustomers();

            ProcessCollectionDynamicallyWithReflection(strings.Cast<object>().ToList());
            ProcessCollectionDynamicallyWithReflection(ints.Cast<object>().ToList());
            ProcessCollectionDynamicallyWithReflection(customers.Cast<object>().ToList());

            Console.ReadLine();
        }

        static void ProcessCollectionDynamicallyWithReflection(List<object> items)
        {
            foreach (var item in items)
            {
                Console.WriteLine(item.GetType().Name);
            }
        }
    }

    public class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Street { get; set; }
        public string Location { get; set; }
        public string ZipCode { get; set; }

        public static List<Customer> GetCustomers()
        {
            List<Customer> customers = new List<Customer>();
            customers.Add(new Customer { FirstName = "Jim",LastName = "Jones",ZipCode = "23434" });
            customers.Add(new Customer { FirstName = "Joe",LastName = "Adams",ZipCode = "12312" });
            customers.Add(new Customer { FirstName = "Jake",LastName = "Johnson",ZipCode = "23111" });
            customers.Add(new Customer { FirstName = "Angie",LastName = "Reckar",ZipCode = "54343" });
            customers.Add(new Customer { FirstName = "Jean",LastName = "Anderson",ZipCode = "16623" });
            return customers;
        }
    }
}

解决方法

你在滥用 Cast<T>.

首先,这里：

IEnumerable<string> items = strings.Cast<object>();

当你调用strings.Cast< object>()时,这将返回IEnumerable< object>,而不是IEnumerable< string>.但是,集合中的项仍然是字符串,但保留在对象的引用中.

稍后,当您想将此传递给一个采用List< object>的方法时,您需要转换您的IEnumerable< T>进入IList< T>.这很容易就像这样：

// Cast to IEnumerabe<object> then convert to List<object>
ProcessCollectionDynamicallyWithReflection(strings.Cast<object>().ToList());