協方差泛型C＃

Question

這是來自MSDN的示例，用於C＃中泛型的協方差。

我無法打印FullName，我可以知道為什么不打印輸出嗎？

// Simple hierarchy of classes.  
public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Employee : Person { }

public class Print
{
    // The method has a parameter of the IEnumerable<Person> type.  
    public static void PrintFullName(IEnumerable<Person> persons)
    {
        foreach (Person person in persons)
        {
            Console.WriteLine("Name: {0} {1}",
            person.FirstName, person.LastName);
        }
    }
}
class Program
{
    public static void Main()
    {
        IEnumerable<Person> employees = new List<Person>();
        Person person = new Person();
        person.FirstName = "nuli";
        person.LastName = "swathi";
        Print.PrintFullName(employees);
        Console.ReadKey();
    }
}

Answer 1

因為您的employees列表為空。

您應該將Person實例添加到employees列表，然后將其打印出來。

即

class Program
{
    public static void Main()
    {
        IList<Person> employees = new List<Person>(); //you need to declare this as a List/IList to get the `Add` method
        Person person = new Person();
        person.FirstName = "nuli";
        person.LastName = "swathi";

        //add this line
        employees.Add(person);

        Print.PrintFullName(employees);
        Console.ReadKey();
    }
}

Answer 2

您是否有可能忘記了string.format？ 嘗試使用此：

Console.WriteLine( String.format("Name: {0} {1}", person.FirstName, person.LastName))

代替這個：

Console.WriteLine("Name: {0} {1}", person.FirstName, person.LastName);

甚至更好：

Console.WriteLine($"Name: {person.FirstName} {person.LastName}");

參考： https : //docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/tokens/interpolated

編輯：由於其mdn示例其更有可能的列表為空。

Answer 3

僅使用List，List實現IEnumerable。 您的Program類中存在一些錯誤，您可以在下面找到更正的代碼：

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Employee : Person { }

class Print
{
    // The method has a parameter of the IEnumerable<Person> type.  
    public static void PrintFullName(IEnumerable<Person> persons)
    {
        foreach (Person person in persons) {
            Console.WriteLine(string.Format("Name: {0} {1}", person.FirstName, person.LastName)); // needs to format the string for it to work, hence string.format().....
        }
    }
}
public class Program
{
    public static void Main()
    {
        List<Person> employees = new List<Person>(); // no need to convert it to an IEnumerable object as List already implements IEnumerable<>
        Person person = new Person();
        person.FirstName = "nuli";
        person.LastName = "swathi";
        employees.Add(person);  // You have to populate the list first
        Print.PrintFullName(employees);   
        Console.ReadLine();     // Changed from Console.ReadKey()
    }
}