Why does casting to IDictionary cause problems?

[Ygg01]

Hi all,

When I run this program several times on dotnet fiddle (net 6.0 and above):

using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading.Tasks;               

public class Program
{
    public static void Main()
    {
        IDictionary<string, string> dictionary = new ConcurrentDictionary<string, string>(); // FAILS sometimes
        // ConcurrentDictionary<string, string> dictionary = new ConcurrentDictionary<string, string>(); // WORKS always

        Parallel.For(0, 10, i => dictionary.TryAdd("x1", "y1"));
    }
}

I sometimes get Unhandled exception. System.AggregateException: One or more errors occurred. (The key already existed in the dictionary) System.ArgumentException: The key already existed in the dictionary. exception

But not casting to IDictionary seems to make code work if not all the time, then substantially more often.

What's happening?

[HaloFour]

With IDictionary you're likely invoking the TryAdd extension method which, unless it explicitly detects ConcurrentDictionary, likely isn't a threadsafe implementation.