TryGetNonEnumeratedCount should return a count for IReadOnlyCollection<T>

[burkenyo]

Description

Users of Enumerable.TryGetNonEnumeratedCount() may expect the API to return true for a collection that only implements IReadOnlyCollection<T>. Instead, it returns false.

Reproduction Steps

create a new .NET 6 console app and run this code:

using System.Collections;

Console.WriteLine(Enumerable.TryGetNonEnumeratedCount(new A(), out _));

class A : IReadOnlyCollection<int>
{
    public int Count => 0;

    public IEnumerator<int> GetEnumerator()
    {
        yield break;
    }

    IEnumerator IEnumerable.GetEnumerator() =>
        GetEnumerator();
}

Expected behavior

True is written to the console, indicating the collection was able to return a count sans enumeration.

Actual behavior

False is written to the console, indicating the collection was not able to return a count sans enumeration.

Regression?

No, this is a new API.

Known Workarounds

Test the collection size manually.

Configuration

No response

Other information

If this was a conscious decision, feel free to close this issue.

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Issue Details

---

Description

Users of Enumerable.TryGetNonEnumeratedCount() may expect the API to return true for a collection that only implements IReadOnlyCollection. Instead, it returns false.

Reproduction Steps

create a new .NET 6 console app and run this code:

using System.Collections;

Console.WriteLine(Enumerable.TryGetNonEnumeratedCount(new A(), out _));

class A : IReadOnlyCollection<int>
{
    public int Count => 0;

    public IEnumerator<int> GetEnumerator()
    {
        yield break;
    }

    IEnumerator IEnumerable.GetEnumerator() =>
        GetEnumerator();
}

Expected behavior

True is written to the console, indicating the collection was able to return a count sans enumeration.

Actual behavior

False is written to the console, indicating the collection was not able to return a count sans enumeration.

Regression?

No, this is a new API.

Known Workarounds

Test the collection size manually.

Configuration

No response

Other information

If this was a conscious decision, feel free to close this issue.

Author:	burkenyo
Assignees:	-
Labels:	area-System.Linq, untriaged
Milestone:	-

[stephentoub]

This was by-design:
#48239 (comment)

Various issues have tracked revisiting IReadOnlyCollection-special-casing in LINQ all-up. The current one I believe is:
#42254

[burkenyo]

Thanks for the quick reply, closing.