using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

public class Program

{

    public static void Main()

    {

        // Difference between the two strings is the first e is a combination of a unicode character and a

        // combining character, and the 2nd e is a single unicode character.  Ugh.

        // https://unicodelookup.com/#ể/1

        // https://unicodelookup.com/#ể/1

        Console.WriteLine(string.Equals("Hiển", "Hiển", StringComparison.InvariantCultureIgnoreCase));

        Console.WriteLine(string.Equals("Hiển", "Hiển", StringComparison.OrdinalIgnoreCase));

        // Reproduction of BUG-12825

        var d = new KeyedCache(StringComparer.InvariantCultureIgnoreCase);

        d.Add("Hiển"); // 1st version of string

        d.Add("Hiển"); // 2nd version of string

    }

}

​

class KeyedCache : KeyedCollection<string, string>

{

    public KeyedCache(IEqualityComparer<string> comparer) : base(comparer)

    {

    }

​

    protected override void InsertItem(int index, string item)

    {

        base.InsertItem(index, item);

    }

    protected override string GetKeyForItem(string item)

    {

        return item;

    }

}