C#中处理空值对象(null)的六种有效方法

string s; // 默认情况下，s 是 null

string s = GetSomeString();
if (s != null)
{
    Console.WriteLine(s.Length);
}
else
{
    Console.WriteLine("String is null");
}

// 如果 classroom 或 Students 或 索引器[0] 任何一个返回 null, 下面语句都会中断并返回 null
var firstStudentName = classroom?.Students?[0]?.Name; 

string s1 = null; 
string s2 = s1 ?? "nothing"; // 将返回 "nothing"

string s1 = null; 
// 下面语句，如果 s1 为 null，则将其设为 "nothing"
s1 ??= "nothing"; 

int length = data?.Length ?? -1;

int? age = GetAge();
if (age.HasValue)
{
    Console.WriteLine(age.Value);
}
else
{
    Console.WriteLine("Age is null");
}

// 1. 定义 IInventory 接口
public interface IInventory
{
    int Quantity { get; }
    bool IsInStock();
}

// 2. 实现 Inventory 类
public class Inventory : IInventory
{
    public int Quantity { get; private set; }

    public Inventory(int quantity)
    {
        Quantity = quantity;
    }

    public bool IsInStock()
    {
        return Quantity > 0;
    }
}

// 3. 空库存类
public class NullInventory : IInventory
{
    public int Quantity => 0;

    public bool IsInStock()
    {
        return false;
    }
}

// 4. 商品类
public class Product
{
    public string Name { get; set; }
    public IInventory Inventory { get; set; }

    public Product(string name, IInventory inventory)
    {
        Name = name;
        Inventory = inventory ?? new NullInventory();
    }

    public void CheckAvailability()
    {
        if (Inventory.IsInStock())
        {
            Console.WriteLine($"{Name} 在库存中有 {Inventory.Quantity} 件可用！");
        }
        else
        {
            Console.WriteLine($"{Name} 没有库存！");
        }
    }
}

var firstItem = items.FirstOrDefault(item => item.Id == 1);