我如何在 .NET 中表示 URN（统一资源名称）以便 Equals 按预期工作

Question

RFC2141 提及：

6. Examples of lexical equivalence

   The following URN comparisons highlight the lexical equivalence
   definitions:

       1- URN:foo:a123,456
       2- urn:foo:a123,456
       3- urn:FOO:a123,456
       4- urn:foo:A123,456
       5- urn:foo:a123%2C456
       6- URN:FOO:a123%2c456
   

   URNs 1, 2, and 3 are all lexically equivalent.

随后的 RFC8141 保留了该等价性：

2.1. Namespace Identifier (NID)

NIDs are case insensitive (e.g., "ISBN" and "isbn" are equivalent).

我可以在 .NET 框架中轻松找到的最接近 URN 的表示是 URI class。但是，它似乎并没有完全遵守 RFC 对等价的定义：

    [TestMethod]
    public void TestEquivalentUrnsAreBroken()
    {
        Assert.AreEqual(
            new Uri("URN:foo:a123,456"),
            new Uri("urn:foo:a123,456"));

        Assert.AreEqual(
            new Uri("urn:foo:a123,456"),
            new Uri("urn:FOO:a123,456"));
    }

在上面的代码示例中，第一个断言按预期工作，而第二个断言失败。

有什么合理的方法可以让 URI class 遵守等价定义吗？我应该使用其他 class 吗？

请注意，我找到了 URN class，但文档中提到不应直接使用它。

Answer 1

Uri class does not support a specific parser for the urn: scheme out of the box. Maybe understandably so, because even if the comparison rules for the NID specify that it is case-insensitive, the rules for comparing two NSS would depend on rules as defined by the particular namespace, per RFC 8141.

对于快速而肮脏的方法，您可以尝试使用 Uri.Compare() 方法。对于两个 URI 相同的情况，它将 return 为零，否则为非零。

var u1 = new Uri("URN:foo:a123,456");
var u2 = new Uri("urn:foo:a123,456");
var u3 = new Uri("urn:FOO:a123,456");
var u4 = new Uri("urn:nope:a123,456");

Console.WriteLine(Uri.Compare(u1, u2, UriComponents.AbsoluteUri, UriFormat.SafeUnescaped, StringComparison.OrdinalIgnoreCase)); // 0
Console.WriteLine(Uri.Compare(u1, u3, UriComponents.AbsoluteUri, UriFormat.SafeUnescaped, StringComparison.OrdinalIgnoreCase)); // 0
Console.WriteLine(Uri.Compare(u2, u3, UriComponents.AbsoluteUri, UriFormat.SafeUnescaped, StringComparison.OrdinalIgnoreCase)); // 0
Console.WriteLine(Uri.Compare(u3, u4, UriComponents.AbsoluteUri, UriFormat.SafeUnescaped, StringComparison.OrdinalIgnoreCase)); // -8

对于更冒险的方法，您可以按照以下方式进行操作。这需要仔细思考才能正确实施。此代码不应按原样使用，而是作为起点。

using System;
using System.Text.RegularExpressions;
                    
public class Program
{
    public static void Main()
    {
        var u1 = new Urn("URN:foo:a123,456");
        var u2 = new Urn("urn:foo:a123,456");
        var u3 = new Urn("urn:foo:a123,456");
        var u4 = new Urn("urn:FOO:a123,456");
        var u5 = new Urn("urn:not-this-one:a123,456");
        Console.WriteLine(u1 == u2); // True
        Console.WriteLine(u3 == u4); // True
        Console.WriteLine(u4 == u5); // False
    }

    public class Urn : Uri
    {
        public const string UrnScheme = "urn";
        private const RegexOptions UrnRegexOptions = RegexOptions.Singleline | RegexOptions.CultureInvariant;
        private static Regex UrnRegex = new Regex("^urn:(?<NID>[a-z|A-Z][a-z|A-Z|-]{0,30}[a-z|A-Z]):(?<NSS>.*)$", UrnRegexOptions);

        public string NID { get; }
        public string NSS { get; }

        public Urn(string s) : base(s, UriKind.Absolute)
        {
            if (this.Scheme != UrnScheme) throw new FormatException($"URN scheme must be '{UrnScheme}'.");
            var match = UrnRegex.Match(this.AbsoluteUri);
            if (!match.Success) throw new FormatException("URN's NID is invalid.");
            NID = match.Groups["NID"].Value;
            NSS = match.Groups["NSS"].Value;
        }

        public override bool Equals(object other)
        {
            if (ReferenceEquals(other, this)) return true;
            return
                other is Urn u &&
                string.Equals(NID, u.NID, StringComparison.InvariantCultureIgnoreCase) &&
                string.Equals(NSS, u.NSS, StringComparison.Ordinal);
        }

        public override int GetHashCode() => base.GetHashCode();

        public static bool operator == (Urn u1, Urn u2)
        {
            if (ReferenceEquals(u1, u2)) return true;
            if (ReferenceEquals(u1, null) || ReferenceEquals(u2, null)) return false;
            return u1.Equals(u2);
        }

        public static bool operator != (Urn u1, Urn u2)
        {
            if (ReferenceEquals(u1, u2)) return false;
            if (ReferenceEquals(u1, null) || ReferenceEquals(u2, null)) return true;
            return !u1.Equals(u2);
        }
    }
}