Csharp 当我将`ConcurentDictionary`转换为`IDictionary`时，出现了奇怪的并发行为

我有以下测试:

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"));
    }
}

如果我在.NET FIDELL中运行它，有时会得到以下异常:

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.

at System.Collections.Concurrent.ConcurrentDictionary`2.System.Collections.Generic.IDictionary<TKey,TValue>.Add(TKey key, TValue value)
at System.Collections.Generic.CollectionExtensions.TryAdd[TKey,TValue](IDictionary`2 dictionary, TKey key, TValue value)
at Program.<>c__DisplayClass0_0.b__0(Int32 i)
at System.Threading.Tasks.Parallel.<>c__DisplayClass19_0`2.b__1(RangeWorker& currentWorker, Int64 timeout, Boolean& replicationDelegateYieldedBeforeCompletion)
--- End of stack trace from previous location ---
at System.Threading.Tasks.Parallel.<>c__DisplayClass19_0`2.b__1(RangeWorker& currentWorker, Int64 timeout, Boolean& replicationDelegateYieldedBeforeCompletion)
at System.Threading.Tasks.TaskReplicator.Replica.Execute()
--- End of inner exception stack trace ---

然而，如果我不施展ConcurrentDictionary，它似乎总是有效的.

到底怎么回事？