爲什麼沒有strings.Cast <object> cast列表<string>到列表<object>？

Question

在this question中建議我可以將一個通用集合向上投射到具有.Cast<object>的對象集合。在reading up a bit on .Cast<>之後，我仍然無法將它變成一個通用集合以投射到另一個通用集合中。 爲什麼以下不工作？

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) 
     { 
      //... 
     } 
    } 

} 

答：

謝謝里德，這裏是我工作的代碼：

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" }; 
      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; 
     } 
    } 
} 

Answer 1

您錯用Cast<T>。

首先，在這裏：

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

當你調用strings.Cast<object>()，這將返回IEnumerable<object>，不IEnumerable<string>。但是，集合中的項目仍然是字符串，但被保存在對象的引用中。

後來，當您想要將此傳遞到採用List<object>的方法時，您需要將您的IEnumerable<T>變成IList<T>。這很容易做到，像這樣：

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