這應該是一個基本的C#問題。假設我有一個Person
類,其中包含一個名爲Pets
屬性,它是一個List<Pet>
。通過將新對象分配給子實例來更新子對象
如果我想更新寵物,我可以將寵物作爲變量並操作屬性,但似乎無法創建新的寵物對象並將其分配給現有寵物對象。我得到了「分配的值不用於任何執行路徑」警告。我創建了一些非常簡單的代碼來概述這個問題。
在我真正的代碼中,我希望能夠使用新的子對象並替換現有的對象。如果你能告訴我如何用下面的樣本updatedCat做到這一點,我將不勝感激!
class Program
{
static void Main(string[] args)
{
var program = new Program();
program.RunMe();
}
public void RunMe()
{
var myPerson = new Person() { Name = "John Doe" };
var dog = new Pet() { Type = "Dog", Name = "Woofie" };
var cat = new Pet() { Type = "Cat", Name = "Chester" };
myPerson.Pets.Add(dog);
myPerson.Pets.Add(cat);
Console.WriteLine("Initial Pet Status:");
ListPets(myPerson);
var currentDog = myPerson.Pets.SingleOrDefault(p => p.Type == "Dog");
currentDog.Name = "Snoopie";
Console.WriteLine("\r\nPet Status After Updating Dog Directly (name should be 'Snoopie'):");
ListPets(myPerson);
var updatedCat = new Pet() { Type = "Cat", Name = "Felix" };
var currentCat = myPerson.Pets.SingleOrDefault(p => p.Type == "Cat");
currentCat = updatedCat;
//Resharper shows "Value assigned is not used in any execution path" for the currentCat
//and the current cat is never updated
Console.WriteLine("\r\nPet Status After Trying to Update Cat by Assigning a New Cat to existing Cat (name should be 'Felix' but it's not):");
ListPets(myPerson);
Console.ReadLine();
}
public void ListPets(Person person)
{
foreach (var pet in person.Pets)
{
Console.WriteLine(string.Format(" {0} has a {1} named {2}", person.Name, pet.Type, pet.Name));
}
}
}
public class Person
{
public string Name { get; set; }
public List<Pet> Pets { get; set; }
public Person()
{
Pets = new List<Pet>();
}
}
public class Pet
{
public string Type { get; set; }
public string Name { get; set; }
}
編輯ID添加到寵物和創建InsertOrUpdatePet方法 (注:我除去簡潔Console.WriteLine命令命令)
class Program
{
static void Main(string[] args)
{
var program = new Program();
program.RunMe();
}
public void RunMe()
{
var myPerson = new Person() { Name = "John Doe" };
var dog = new Pet() { Id = 1, Type = "Dog", Name = "Woofie" };
var cat = new Pet() { Id = 2, Type = "Cat", Name = "Chester" };
myPerson.Pets.Add(dog);
myPerson.Pets.Add(cat);
var updatedCat = new Pet() { Id = 2, Type = "Cat", Name = "Felix" };
InsertOrUpdatePet(myPerson, updatedCat);
var currentCat = myPerson.Pets.SingleOrDefault(p => p.Type == "Cat");
}
public void InsertOrUpdatePet(Person person, Pet pet)
{
var currentPet = person.Pets.SingleOrDefault(p => p.Id == pet.Id);
if(currentPet == null)
{
person.Pets.Add(pet);
}
else
{
currentPet = pet; // This doesn't work
person.Pets.SingleOrDefault(p => p.Id == pet.Id) = pet; //This throws an error
}
}
}
public class Person
{
public string Name { get; set; }
public List<Pet> Pets { get; set; }
public Person()
{
Pets = new List<Pet>();
}
}
public class Pet
{
public int Id { get; set; }
public string Type { get; set; }
public string Name { get; set; }
}
後者不起作用。 – jason 2012-03-04 18:56:32
看起來你還不瞭解對象和對象引用之間的區別。你可能想對此做一些研究。 – usr 2012-03-04 19:00:15