Is ConcurrentDictionary always add item by order in C#?

Question

I tried BlockingCollection and ConcurrentDictionary. It continues by adding BlockingCollection items as soon as the thread is completed, but ConcurrentDictionary is always adding thread order. Is the ConcurrentDictionary waiting for the previous action to complete to add an item? Or does it order after adding? If ConcurrentDictionary adds item by thread order, is there any point in using parallel? Why does ConcurrentDictionary work this way while BlockingCollection works properly?

Codes and output is below:

BlockingCollection<string> blockingCollection = new BlockingCollection<string>();
ConcurrentDictionary<int, string> concurrentDictionary =
    new ConcurrentDictionary<int, string>();

var rand = new Random();

Parallel.For(0, 10, i =>
{
    int wait = rand.Next(10, 500);
    Thread.Sleep(wait);
    blockingCollection.Add($"t:{i} waittime:{wait}");
});

Parallel.For(0, 10, i =>
{
    int wait = rand.Next(10, 500);
    Thread.Sleep(wait);
    concurrentDictionary.TryAdd(i, $"t:{i} waittime:{wait}");
});

Console.WriteLine("--BlockingCollection--");
Console.WriteLine(string.Join("\n", blockingCollection.ToArray()));

Console.WriteLine("--ConcurrentDictionary--");
Console.WriteLine(string.Join("\n", concurrentDictionary
    .Select(x => $"Key:{x.Key} Value:{x.Value}").ToArray()));

Output:

I suspect the reason your output is like this might be because the implementation of ConcurrentDictionary you're using is iterating over its items in a sorted manner. After all, it's a dictionary, it works with hash codes, which are numbers and pretty easy to sort. — dimitar.bogdanov, Jan 24 '23 at 20:26
I'm not sure I understand your question or if it is sane. `waittime` is a misnomer: it is a randomly generated value. What do you mean with "in thread order"? Items in dictionaries don't have an order, but it looks like that `ConcurrentDictionary` iterates the keys in ascending order (that might be a coincedence). But iteration order usually has nothing to do with insertion order. — knittl, Jan 24 '23 at 20:27
Should be easy to test: `concurrentDictionary.TryAdd(5, "five"); concurrentDictionary.TryAdd(1, "one");` and then print it. Which order will the output be? (1, 5, …) — knittl, Jan 24 '23 at 20:34
Actually, I expected `ConcurrentDictionary` to give result like `BlockingCollection`. @knittle I tried and it prints sorted. @dimitar.bogdanov and can we say that `ConcurrentDictionary` always sorts by key (regardless of insertion time)? — Akif T, Jan 24 '23 at 20:50
Please [edit] the question to clarify instead of adding comments. Please make sure to highlight the fact you understand that dictionaries don't have order and listing items in particular order has no correlation to order of adding. — Alexei Levenkov, Jan 24 '23 at 21:07

Theodor Zoulias · Accepted Answer · 2023-01-25T07:23:08.207

The ConcurrentDictionary<TKey,TValue> has no notion of order. When you enumerate it, you can't make any assumption regarding the order in which the keys will be emitted. You are not even allowed to assume that a single enumeration will yield unique keys. You could study the source code of the class and try to get a better understanding of how the keys are stored, but it will be a daunting task. Even worse, the value of this knowledge will evaporate as soon as the next version of the .NET runtime is released. Microsoft reserves the right to change the behavior of any API, as long as the affected behavior is not documented. And making the correctness of your program depend on implementation details of code that you don't control, is not something that most developers are comfortable with, and for good reasons.

Just for fun I wrote the demo below, that shows how the collection behaves for shuffled sequences that contains elements incremented by different steps:

for (int step = 1; step <= 10; step++)
{
    Random random = new(0);
    Console.WriteLine($"Step: {step}");
    int[] source = Enumerable.Range(1, 15)
        .OrderBy(_ => random.Next())
        .Select(x => x * step)
        .ToArray();
    Console.WriteLine($"Source shuffled: [{String.Join(", ", source)}]");
    Console.WriteLine($"Source ordered:  [{String.Join(", ", source.Order())}]");
    ConcurrentDictionary<int, string> dictionary = new();
    foreach (int i in source) dictionary.TryAdd(i, null);
    Console.WriteLine($"dictionary.Keys: [{String.Join(", ", dictionary.Keys)}]");
    Console.WriteLine();
}

Output:

Step: 1
Source shuffled: [5, 10, 11, 8, 12, 14, 4, 6, 13, 1, 3, 2, 7, 9, 15]
Source ordered:  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
dictionary.Keys: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

Step: 2
Source shuffled: [10, 20, 22, 16, 24, 28, 8, 12, 26, 2, 6, 4, 14, 18, 30]
Source ordered:  [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
dictionary.Keys: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]

Step: 3
Source shuffled: [15, 30, 33, 24, 36, 42, 12, 18, 39, 3, 9, 6, 21, 27, 45]
Source ordered:  [3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45]
dictionary.Keys: [33, 3, 36, 6, 39, 9, 42, 12, 45, 15, 18, 21, 24, 27, 30]

Step: 4
Source shuffled: [20, 40, 44, 32, 48, 56, 16, 24, 52, 4, 12, 8, 28, 36, 60]
Source ordered:  [4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60]
dictionary.Keys: [4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60]

Step: 5
Source shuffled: [25, 50, 55, 40, 60, 70, 20, 30, 65, 5, 15, 10, 35, 45, 75]
Source ordered:  [5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75]
dictionary.Keys: [65, 35, 5, 70, 40, 10, 75, 45, 15, 50, 20, 55, 25, 60, 30]

Step: 6
Source shuffled: [30, 60, 66, 48, 72, 84, 24, 36, 78, 6, 18, 12, 42, 54, 90]
Source ordered:  [6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90]
dictionary.Keys: [66, 36, 6, 72, 42, 12, 78, 48, 18, 84, 54, 24, 90, 60, 30]

Step: 7
Source shuffled: [35, 70, 77, 56, 84, 98, 28, 42, 91, 7, 21, 14, 49, 63, 105]
Source ordered:  [7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105]
dictionary.Keys: [63, 35, 98, 7, 70, 42, 105, 14, 77, 49, 21, 84, 56, 28, 91]

Step: 8
Source shuffled: [40, 80, 88, 64, 96, 112, 32, 48, 104, 8, 24, 16, 56, 72, 120]
Source ordered:  [8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120]
dictionary.Keys: [32, 64, 96, 8, 40, 72, 104, 16, 48, 80, 112, 24, 56, 88, 120]

Step: 9
Source shuffled: [45, 90, 99, 72, 108, 126, 36, 54, 117, 9, 27, 18, 63, 81, 135]
Source ordered:  [9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99, 108, 117, 126, 135]
dictionary.Keys: [63, 126, 36, 99, 9, 72, 135, 45, 108, 18, 81, 54, 117, 27, 90]

Step: 10
Source shuffled: [50, 100, 110, 80, 120, 140, 40, 60, 130, 10, 30, 20, 70, 90, 150]
Source ordered:  [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150]
dictionary.Keys: [130, 100, 70, 40, 10, 140, 110, 80, 50, 20, 150, 120, 90, 60, 30]

Online demo.

So apparently the ConcurrentDictionary<TKey,TValue> emits the keys in a sorted order when the step is 1, 2 or 4, otherwise there is no obvious order. Which is a useless knowledge, but there you have it!

Is ConcurrentDictionary always add item by order in C#?

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