using System;

using System.Collections.Concurrent;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

// NOTE: If you replace Dictionary to 'ConcurrentDictionary' then the issue should go away

var collection = new Dictionary<int, string>();

​

var tasks = Enumerable.Range(1, 2000).Select(async num =>

{

    var rnd = new Random().Next(1, 5);

​

    await Task.Delay(TimeSpan.FromSeconds(rnd));

​

    collection[num] = $"index {num}";

});

​

await Task.WhenAll(tasks);

​

Console.WriteLine("All tasks complete");