ReadOnlyDictionary Values and Keys collections are out of sync with KeyValuePairs

[ante-maric]

Description

When underlying dictionary passed to a constructor of ReadOnlyDictionary is ConcurrentDictionary, all KeyValuePairs that are added to the ConcurrentDictionary after ReadOnlyDictionary is created are not visible through ReadOnlyDictionary.Values and ReadOnlyDictionary.Keys collection. However, ReadOnlyDictionary indexer and ContainsKey work correctly.

All of this is not a problem if regular Dictionary is used.

Reproduction Steps

        var dictionary = new ConcurrentDictionary<string, int>();
        dictionary["1"] = 1;
        dictionary["2"] = 2;
        dictionary["3"] = 3;
        var readOnlyDictionary = new ReadOnlyDictionary<string, int>(dictionary);
        readOnlyDictionary.Values.Count.ShouldBe(dictionary.Count); // ok, count is 3

        dictionary["4"] = 4;
        readOnlyDictionary.ContainsKey("4").ShouldBeTrue(); // ok, key is present
        readOnlyDictionary["4"].ShouldBe(dictionary["4"]); // ok, value is 4
        readOnlyDictionary.Values.Count.ShouldBe(dictionary.Count); // fails, count is 3

Expected behavior

Every KeyValuePair that is retrievable by ReadOnlyDictionary indexer. ContainsKey, TryGetValue... should also be present in Values and Keys collections. Everything should be in sync with underlying ConcurrentDictionary.

Actual behavior

ReadOnlyDictionary's Values and Keys collections do not contain items that are added to underlying dictionary after creation of ReadOnlyDictionary if underlying dictionary is of type ConcurrentDictionary.

Regression?

No response

Known Workarounds

No response

Configuration

.NET version is 6.0.400 running on Windows 10 21H1.

Other information

No response

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Issue Details

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Description

When underlying dictionary passed to a constructor of ReadOnlyDictionary is ConcurrentDictionary, all KeyValuePairs that are added to the ConcurrentDictionary after ReadOnlyDictionary is created are not visible through ReadOnlyDictionary.Values and ReadOnlyDictionary.Keys collection. However, ReadOnlyDictionary indexer and ContainsKey work correctly.

All of this is not a problem if regular Dictionary is used.

Reproduction Steps

        var dictionary = new ConcurrentDictionary<string, int>();
        dictionary["1"] = 1;
        dictionary["2"] = 2;
        dictionary["3"] = 3;
        var readOnlyDictionary = new ReadOnlyDictionary<string, int>(dictionary);
        readOnlyDictionary.Values.Count.ShouldBe(dictionary.Count); // ok, count is 3

        dictionary["4"] = 4;
        readOnlyDictionary.ContainsKey("4").ShouldBeTrue(); // ok, key is present
        readOnlyDictionary["4"].ShouldBe(dictionary["4"]); // ok, value is 4
        readOnlyDictionary.Values.Count.ShouldBe(dictionary.Count); // fails, count is 3

Expected behavior

Every KeyValuePair that is retrievable by ReadOnlyDictionary indexer. ContainsKey, TryGetValue... should also be present in Values and Keys collections. Everything should be in sync with underlying ConcurrentDictionary.

Actual behavior

ReadOnlyDictionary's Values and Keys collections do not contain items that are added to underlying dictionary after creation of ReadOnlyDictionary if underlying dictionary is of type ConcurrentDictionary.

Regression?

No response

Known Workarounds

No response

Configuration

.NET version is 6.0.400 running on Windows 10 21H1.

Other information

No response

Author:	ante-maric
Assignees:	-
Labels:	area-System.Collections
Milestone:	-

[svick]

This is not really about ReadOnlyDictionary, but about the behavior of ConcurrentDictionary.Values (and Keys):

var dictionary = new ConcurrentDictionary<string, int>();
dictionary["1"] = 1;
dictionary["2"] = 2;
dictionary["3"] = 3;
var values = dictionary.Values;
values.Count.ShouldBe(dictionary.Count); // ok, count is 3

dictionary["4"] = 4;
dictionary.ContainsKey("4").ShouldBeTrue(); // ok, key is present
values.Count.ShouldBe(dictionary.Count); // fails, count is 3

This happens because accessing Values creates a snapshot of the dictionary (and ReadOnlyDictionary only accesses Values of the wrapped dictionary once).

I suspect that this is an intentional design decision that's not going to change. But at least it should be documented.

[ante-maric]

Since accessing the Values only once is the behavior of ReadOnlyDictionary, I would argue this is about ReadOnlyDictionary. You can't observe this issue with ConcurrentDictionary alone. But we could agree it's potato-potato. What I would suggest that warning should be part of ReadOnlyDictionary documentation and adding the same to ConcurrentDictionary documentation would be a bonus, maximizing the chance for that to be seen by those who really need to.

[eiriktsarpalis]

Since accessing the Values only once is the behavior of ReadOnlyDictionary, I would argue this is about ReadOnlyDictionary.

Generally speaking, most IDictionary.Keys/Values collection implementations are kept in sync with their parent dictionary, so I suspect this is more an issue of ConcurrentDictionary being special rather than ReadOnlyDictionary requiring documentation.