为什么我无法通过List< Customer>作为接受List< object>的方法的参数？ [英] Why can't I pass List<Customer> as a parameter to a method that accepts List<object>?

问题描述

以下代码给了我这个错误：

无法从
转换System.Collections.Generic.List'
到
'System.Collections.Generic.List'。

如何向编译器，客户确实从对象继承？或者它只是没有继承与泛型集合对象（发送 List< string> 获取相同的错误）。

  using System.Collections.Generic; 
使用System.Windows; 
使用System.Windows.Documents; 
 
命名空间TestControl3423 
 {
 public partial class Window2：Window 
 {
 public Window2（）
 {
 InitializeComponent（） ; 
 
列表< Customer> customers = Customer.GetCustomers（）; 
 FillSmartGrid（客户）; 
 
 //列表< CorporateCustomer> corporateCustomers = CorporateCustomer.GetCorporateCustomers（）; 
 // FillSmartGrid（corporateCustomers）; 
 
 
 $ b public void FillSmartGrid（List< object> items）
 {
 //对物品进行反射并动态显示
} 
} 
 
公共类客户
 {
公共字符串名字{get;组; } 
 public string LastName {get;组; } 
 public string Street {get;组; } 
 public string Location {get;组; } 
 public string ZipCode {get;组; } 
 
 public static List< Customer> GetCustomers（）
 {
列表< Customer> customers = new List< Customer>（）; 
 customers.Add（新客户{FirstName =Jim，LastName =Jones，ZipCode =23434}）; 
 customers.Add（新客户{FirstName =Joe，LastName =Adams，ZipCode =12312}）; 
 customers.Add（新客户{FirstName =Jake，LastName =Johnson，ZipCode =23111}）; 
 customers.Add（新客户{FirstName =Angie，LastName =Reckar，ZipCode =54343}）; 
 customers.Add（new Customer {FirstName =Jean，LastName =Anderson，ZipCode =16623}）; 
回报客户; 
 
 
 
  

解决方案

.NET没有协方差和反差（还）。

B从A派生并不意味着 List< B> 从 List< A> 派生。它没有。它们是两种完全不同的类型。

.NET 4.0将获得有限的协方差和反方差。

The following code gives me this error:

Cannot convert from 'System.Collections.Generic.List' to 'System.Collections.Generic.List'.

How can I indicate to the compiler that Customer indeed inherits from object? Or does it just not do inheritance with generic collection objects (sending a List<string> gets the same error).

using System.Collections.Generic;
using System.Windows;
using System.Windows.Documents;

namespace TestControl3423
{
    public partial class Window2 : Window
    {
        public Window2()
        {
            InitializeComponent();

            List<Customer> customers = Customer.GetCustomers();
            FillSmartGrid(customers);

            //List<CorporateCustomer> corporateCustomers = CorporateCustomer.GetCorporateCustomers();
            //FillSmartGrid(corporateCustomers);
        }


        public void FillSmartGrid(List<object> items)
        {
            //do reflection on items and display dynamically
        }
    }

    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;
        }
    }
}

解决方案

.NET does not have co-variance and contra-variance (yet).

That B derives from A doesn't imply that List<B> derives from List<A>. It doesn't. They are two totally different types.

.NET 4.0 will get limited co-variance and contra-variance.

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