尝试这样做将涵盖大多数情况，尽管可能不是所有边缘情况：

using System;
using System.Linq;

public class Program
{
    static public string GetHostnameWithoutSubdomain(string url)
    {
        Uri uri = new Uri(url);
        if (uri.ToString().Contains("localhost"))
        {
            return uri.Authority;
        }
        string[] uriParts = uri.Host.Split('.');
        int lastIndex = uriParts.Length - 1;
        // Ensure that the URI isn't an IP address by checking whether the last part is a number or (as it should be) a top-level domain:
        if (uriParts[uriParts.Length - 1].All(char.IsDigit))
        {
            return uri.Host;
        }
        // If the URI has more than 3 parts and the last part has just two characters, e.g. "uk" in example.co.uk or "cn" in moh.gov.cn, it's probably a top-level domain:
        if (uriParts.Length > 3 && uriParts[lastIndex].Length <= 2)
        {
            return uriParts[lastIndex - 2] + "." + uriParts[lastIndex - 1] + "." + uriParts[lastIndex];
        }
        if (uriParts.Length > 2)
        {
            return uriParts[lastIndex - 1] + "." + uriParts[lastIndex];
        }
        return uri.Host;
    }

    public static void Main()
    {
        Console.WriteLine(GetHostnameWithoutSubdomain("https://localhost:123/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://example.com/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://www.example.com/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://test.www.example.com/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://255.255.255.255/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://test.www.example.co.uk/some/page"));
        Console.WriteLine(GetHostnameWithoutSubdomain("https://www.moh.gov.cn/some/page"));
    }
}

这也适用于测试环境与生产域不同的开发设置，因此可以处理localhost边缘情况。
输出：

localhost:123
example.com
example.com
example.com
255.255.255.255
example.co.uk
moh.gov.cn