[Hatr828]

ImmutableHashSet.Builder.ExceptWith can leave the builder in an inconsistent state if the input enumerable throws during enumeration.

For example:

var builder = ImmutableHashSet.CreateBuilder<int>();
builder.UnionWith(new[] { 1, 2, 3, 4 });

try
{
    builder.ExceptWith(ThrowingEnumerable());
}
catch (InvalidOperationException)
{
}

int count = 0;
foreach (int item in builder) count++;

// Before the fix: count is 1, but builder.Count is 4.

IEnumerable<int> ThrowingEnumerable()
{
    yield return 1;
    throw new InvalidOperationException();
}

If enumeration throws, updates are skipped leaving the builder with an incorrect count and potentially losing unrelated elements. Added a regression test that failed before the fix and passed afterward.

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[Manishram-ai]

I reproduced this on .NET 8.0.8 with the example from the description. After the throw, Count is still 4 but the builder only contains 2. Removing items one at a time like the fix does keeps Count and the contents in sync.

UnionWith has the same problem though. If the enumerable throws after it yields a new item, the item ends up in the set but Count doesn't go up.

var builder = ImmutableHashSet.CreateBuilder<int>();
builder.UnionWith(new[] { 1, 2, 3, 4 });

try { builder.UnionWith(ThrowingEnumerable()); } catch (InvalidOperationException) { }

// builder.Contains(5) is true, but builder.Count is still 4

IEnumerable<int> ThrowingEnumerable()
{
    yield return 5;
    throw new InvalidOperationException();
}

ImmutableDictionary's builder AddRange does the same thing. Not sure if you'd rather fix those here or in a separate PR.

[Hatr828]

@Manishram-ai I checked ImmutableHashSet.Builder.UnionWith and issue is more severe than just an incorrect count. For example:

var builder = ImmutableHashSet.CreateBuilder<int>();
builder.UnionWith(new[] { 1, 2, 3, 4 });

try { builder.UnionWith(ThrowingEnumerable()); } catch (InvalidOperationException) { }

foreach (int item in builder)  Console.WriteLine(item); // prints only "1  2  3" on .NET 10.0.10

IEnumerable<int> ThrowingEnumerable()
{
    yield return 5;
    yield return 6;
    throw new InvalidOperationException();
}

The original element 4 is no longer enumerated and elements are skipped only if UnionWith has 2+ elements (in this example). I suspect this is related to tree rebalancing and the builder retaining the old root when the final update is skipped.

I think I will fix UnionWith and AddRange in separate PRs.

[Manishram-ai]

Thanks for digging into it. I see the same on .NET 8.0.8: it enumerates 1 2 3, Contains(4) is false, and Count is still 4. ToImmutable() returns that same broken set (3 items, Count 4), so the damage outlives the builder. Separate PRs for UnionWith and AddRange sound good, happy to review them when they're up.