using System;

using System.Collections.Generic;

using System.Collections.Concurrent;

using System.Threading;

using System.Linq;

​

static class Program

{

    static void Main()

    {

        ConcurrentDictionary<int, int> dictionary = new();

​

        for (int i = 1; i <= 50_000; i++)

            dictionary.TryAdd(i, default);

​

        List<KeyValuePair<int, int>> list = new();

​

        Thread thread = new(() =>

        {

            for (int i = -1; i >= -50_000; i--)

                dictionary.TryAdd(i, default);

        });

        thread.Start();

​

        IEnumerable<KeyValuePair<int, int>> enumerable = dictionary;

        list.AddRange(enumerable.ToArray()); // Throws

​

        thread.Join();

        Console.WriteLine($"dictionary.Count: {dictionary.Count:#,0}, list.Count: {list.Count:#,0}");

    }

}

​