如何修复“字段 X 从未分配给并将其默认值为 null”？

Question

我正在尝试从控制台（街道、城市、国家/地区）输入输入，但字段带有下划线并显示消息（字段从未分配给并将其值为 null）。我还创建了一个不起作用的方法 SetFullAddress（如果是因为该消息，则为 idk）。

Address 类中的代码：

public class Address
{
    private string street;
    private string city;
    private string country;

    public Address()
    {
        this.Street = street;
        this.City = city;
        this.Country = country;
    }

    public string Street { get; set; }
    public string City { get; set; }
    public string Country { get; set; }

    public string SetFullAddress()
    {
        return ($"Full address: {street} {city} {country}");
    }

    public void DisplayAddress()
    {
        Console.WriteLine($"Street: {Street}");
        Console.WriteLine($"City: {City}");
        Console.WriteLine($"Country: {Country}");
        Console.WriteLine(SetFullAddress());


    }

}

在Main方法内部：

        Address address = new Address();
        Console.Write("Street: ");
        address.Street = Console.ReadLine();

        Console.Write("City: ");
        address.City = Console.ReadLine();

        Console.Write("Country: ");
        address.Country = Console.ReadLine();

        Console.WriteLine();
        address.DisplayAddress();

Answer 1

如果您没有遇到任何错误并且只想禁用该脚本的警告，您可以使用#pragma warning disable 0649

但不要滥用它，这些警告可能会在以后对您有所帮助。

Answer 2

这可能会对您有所帮助：

   public static void Main()
    {

        Console.Write("Street: ");
        string street = Console.ReadLine();

        Console.Write("City: ");
        string city = Console.ReadLine();

        Console.Write("Country: ");
        string country = Console.ReadLine();

        Address address = new Address(street, city, country);
        Console.WriteLine();
        address.DisplayAddress();
    }

    public class Address
    {
        private string street;
        private string city;
        private string country;

        public Address(string street, string city, string country)
        {
            this.street = street;
            this.city = city;
            this.country = country;
        }

        public string Street {
            get => street;
            set => street = value;
        }
        public string City {
            get => city;
            set => city = value;
        }
        public string Country {
            get => country;
            set => country = value;
        }

        public string SetFullAddress()
        {
            return ($"Full address: {street} {city} {country}");
        }

        public void DisplayAddress()
        {
            Console.WriteLine($"Street: {Street}");
            Console.WriteLine($"City: {City}");
            Console.WriteLine($"Country: {Country}");
            Console.WriteLine(SetFullAddress());


        }

    }

Answer 3

警告的原因是您从未使用（读取分配值）私有字段。

    private string street;
    private string city;
    private string country;

    public Address()
    {
        this.Street = street;
        this.City = city;
        this.Country = country;
    }

    public string Street { get; set; }
    public string City { get; set; }
    public string Country { get; set; }

你是，而不是使用自动实现的属性。您可以安全地删除它们并按照以下方式重写您的 SetFullAddress 方法（使用自动实现的属性）

public string SetFullAddress()
{
    return ($"Full address: {Street} {City} {Country}");
}

或者您可以使用隐式类型的私有支持字段创建属性

public string Street 
{ 
    get => street; 
    set => street = value; 
}
public string City
{ 
    get => city; 
    set => city = value; 
}
public string Country
{ 
    get => country; 
    set => country = value; 
}

请注意，当您使用自动实现的属性时，编译器会创建支持字段。你可以阅读更多关于自动实现属性here。

Answer 4

警告告诉你到底是什么问题，以下永远不会分配

private string street;
private string city;
private string country;

也许你想在构造函数中初始化实际的属性

//private string street;
//private string city;
//private string country;

public Address(string street, string city, string country)
{
    this.Street = street;
    this.City = city;
    this.Country = country;
}

Compiler Warning (level 4) CS0649

字段 'field' 永远不会分配给，并且将始终具有其默认值 价值'价值'

编译器检测到未初始化的私有或内部字段 从未赋值的声明。

以下示例生成 CS0649：

class MyClass  
{  
   Hashtable table;  // CS0649  
   // You may have intended to initialize the variable to null  
   // Hashtable table = null;  

   // Or you may have meant to create an object here  
   // Hashtable table = new Hashtable();  

   public void Func(object o, string p)  
   {  
      // Or here  
      // table = new Hashtable();  
      table[p] = o;  
   }  

   public static void Main()  
   {  
   }  
}