Enumerating concurrent collections with snapshots in C#

In standard collections in C#, it is not allowed to alter collections you iterate upon using foreach for example, since it throws InvalidOperationException - Collection was modified; enumeration operation may not execute. Concurrent collections can be altered while being iterated. This is the default behavior, allow concurrent behavior while iterating - as locking the entire concurrent collection is costly. You can however enforce a consistent way of iterating the concurrent collection by making a snapshot of it. For concurrent dictionaries, we use the ToArray method.

	var capitals = new ConcurrentDictionary<string, string>{
		["Norway"] = "Oslo",
		["Denmark"] = "Copenhagen",
		["Sweden"] = "Stockholm",
		["Faroe Islands"] = "Torshamn",
		["Finland"] = "Helsinki",
		["Iceland"] = "Reykjavik"
	};

	//make a snapshot of the concurrent dictionary first 
	
	var capitalsSnapshot = capitals.ToArray();
	
	//do some modifications
	
	foreach (var capital in capitals){
		capitals[capital.Key] = capital.Value.ToUpper();
	}

	foreach (var capital in capitalsSnapshot)
	{
		Console.WriteLine($"The capital in {capital.Key} is {capital.Value}");
	}

This outputs:

The capital in Denmark is Copenhagen
The capital in Sweden is Stockholm
The capital in Faroe Islands is Torshamn
The capital in Norway is Oslo
The capital in Finland is Helsinki
The capital in Iceland is Reykjavik  

The snapshot of the concurrent collection was not modified by the modifications done. Let's look at the concurrent collection again and iterate upon it.

	foreach (var capital in capitals)
	{
		Console.WriteLine($"The capital in {capital.Key} is {capital.Value}");
	}

This outputs:

Enumerate capitals in concurrent array - just enumerating with ToArray() - elements can be changed while enumerating. Faster, but more unpredictable
The capital in Denmark is COPENHAGEN
The capital in Sweden is STOCKHOLM
The capital in Faroe Islands is TORSHAMN
The capital in Norway is OSLO
The capital in Finland is HELSINKI
The capital in Iceland is REYKJAVIK

As we can see, the concurrent dictionary has modified its contents and this shows that we can get modifications upon iterating collections. If you do want to get consistent results, using a snapshot should be desired. But note that this will lock the entire collection and involve costly operations of copying the contents. If you do do concurrent collection snapshots, keep the number of snapshots to a minimum and iterate upon these snapshots, preferable only doing one snapshot in one single place in the method for the specific concurrent dictionary.

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