javascript - C# 有符号类型吗？

Question

JavaScript (ES2015) 引入了新的 Symbol type ，它与字符串的不同之处在于符号是通过引用进行比较，而字符串是通过值进行比较。

C# 有什么可比较的吗？

Answer 1

C# 中的

object 满足 ECMAScript 6 中 symbol 的许多相同属性。

考虑 JavaScript 代码:

const MY_KEY = Symbol();
let obj = {};

obj[MY_KEY] = 123;
console.log(obj[MY_KEY]); // 123

在 C# 中，这可以类似地表示为:

readonly object MyKey = new object();
// ...
var obj = new Dictionary<object, int>() {{MyKey, 123}};
Console.WriteLine(obj[MyKey]); // 123

但是，Symbol 类型包含其他功能:全局注册表、众所周知的 key 以及使用描述标记符号的功能(ES2015 规范，§ 19.4)。此功能可以在 C# 6 中作为一个简单的包装器大量复制:

class Symbol : IEquatable<Symbol>
{
    private object o = new object();
    public string Description { get; }

    public Symbol(string description)
    {
        Description = description;
    }
    public Symbol() : this(null) { }

    public bool Equals(Symbol other) => other?.o == o;
    public static bool operator == (Symbol o1, Symbol o2) => 
        EqualityComparer<Symbol>.Default.Equals(o1, o2);
    public static bool operator !=(Symbol o1, Symbol o2) => !(o1 == o2);

    public override int GetHashCode()
    {
        return o.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return Equals(obj as Symbol);
    }
    public override string ToString() => $"symbol({Description})";

    // static methods to support symbol registry
    private static Dictionary<string, Symbol> GlobalSymbols = 
          new Dictionary<string, Symbol>(StringComparer.Ordinal);

    public static Symbol For(string key) => 
          GlobalSymbols.ContainsKey(key) ? 
                GlobalSymbols[key] :
                GlobalSymbols[key] = new Symbol(key);

    public static string KeyFor(Symbol s) =>
          GlobalSymbols.FirstOrDefault(a => a.Value == s).Key; // returns null if s not defined

    // Well-known ECMAScript symbols
    private const string ns = "Symbol.";
    public static Symbol HasInstance => For(ns + "hasInstance");
    public static Symbol IsConcatSpreadable => For(ns + "isConcatSpreadable");
    public static Symbol Iterator => For(ns + "iterator");
    public static Symbol Match => For(ns + "match");
    public static Symbol Replace => For(ns + "replace");
    public static Symbol Search => For(ns + "search");
    public static Symbol Species => For(ns + "species");
    public static Symbol Split => For(ns + "split");
    public static Symbol ToPrimitive => For(ns + "toPrimitive");
    public static Symbol ToStringTag => For(ns + "toStringTag");
    public static Symbol Unscopables => For(ns + "unscopables");
}

结果行为符合预期的 ECMAScript 行为:

Symbol a = new Symbol();
Symbol b = new Symbol("A");
Symbol c = new Symbol("A");
Symbol d = c;
var dict = new Dictionary<object, int>() { { c, 42 } };
Symbol e = Symbol.For("X");
Symbol f = Symbol.For("X");
Symbol g = Symbol.For("Y");
Console.WriteLine(a == b); // false
Console.WriteLine(b == c); // false
Console.WriteLine(c == d); // true
Console.WriteLine(dict[d]); // 42
Console.WriteLine(e == f); // true
Console.WriteLine(f == g); // false
Console.WriteLine(Symbol.For("X") == e); // true
Console.WriteLine(Symbol.KeyFor(e) == "X"); // true
Console.WriteLine(Symbol.Unscopables.Description); // Symbol.unscopables