如何将一个类用作另一个类中的属性，然后实例化它？ [C＃]

Question

我试图更好地理解类，类'地址'被用作'Person'类属性'ShippingAddress'中的类型，但我想知道如何将值分配给地址属性，因为它下面的方式是给我一个错误：

上找到.NET Tutorials

**型“System.NullReferenceException”未处理的异常发生在Classes_and_Objects.exe

其他信息：对象未将参考设置为对象的实例。**

using System; 
    using System.Collections.Generic; 
    using System.Linq; 
    using System.Text; 
    using System.Threading.Tasks; 

    namespace Classes_and_Objects 
    { 
     class Program 
     { 
     static void Main(string[] args) 
     { 
      Person john = new Person(); 
      john.FirstName = "John"; 
      john.LastName = "Doe"; 
      john.ShippingAddress.StreetAddress = "78 Fake Street" ; 
      john.ShippingAddress.City = "Queens"; 
      john.ShippingAddress.State = "NY"; 
      john.ShippingAddress.PostalCode = "345643"; 
      john.ShippingAddress.Country = "United States"; 
      } 

     } 

     public class Address 
     { 
      public string StreetAddress { get; set; } 
      public string City { get; set; } 
      public string State { get; set; } 
      public string PostalCode { get; set; } 
      public string Country { get; set; } 
     } 
     public class Person 
     { 
      public string FirstName { get; set; } 
      public string LastName { get; set; } 
      public Address ShippingAddress { get; set; } 
     } 

    } 
    } 

Answer 1

你也需要实例化地址。以下是您应如何编写代码：

static void Main(string[] args) 
{ 
    Person john = new Person(); 
    john.FirstName = "John"; 
    john.LastName = "Doe"; 
    john.ShippingAddress = new Address(); 
    john.ShippingAddress.StreetAddress = "78 Fake Street" ; 
    john.ShippingAddress.City = "Queens"; 
    john.ShippingAddress.State = "NY"; 
    john.ShippingAddress.PostalCode = "345643"; 
    john.ShippingAddress.Country = "United States"; 
} 

Answer 2

Jhon.shippingadress = new Address();