Issues

• Test failure: OutOfProcessTest.Loader/CollectibleAssemblies/Statics/CollectibleTLSStaticCollection/CollectibleTLSStaticCollection.cmd

  area-System.Collectionsblocking-clean-ci-optionalKnown Build Erroruntriaged
  dotnet/runtime#136813 · Filter by author VincentBu opened on Nov 2, 2026
  1

• Fix ImmutableHashSet<T>.Builder.ExceptWith when enumeration throws

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#136790·Hatr828 opened Nov 1, 2026 · Updated Nov 2, 2026
  4

• Fix ImmutableHashSet<T>.Builder.ExceptWith when passed itself

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#136617·Hatr828 opened Oct 29, 2026 · Updated Oct 30, 2026
  2

• Skip clearing reference-free Dictionary entries

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#136605·tedd opened Oct 28, 2026 · Updated Nov 2, 2026
  6

• [mono][wasm][AOT] CollectionsMarshal dictionary value-ref write is lost across GC

  area-System.Collectionsuntriaged
  dotnet/runtime#136544 · Filter by author SeyTi01 opened on Oct 27, 2026
  1

• [API Proposal]: Make ByteArrayOrdinalComparer public

  api-suggestionarea-System.Collections
  dotnet/runtime#136531 · Filter by author ladeak opened on Oct 27, 2026 · Future
  5

• Fix ImmutableSortedSet<T>.Builder.ExceptWith when passed itself

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#136251·Hatr828 opened Oct 21, 2026 · Updated Oct 21, 2026
  3

• Fix ImmutableArray builder self-insertion corruption

  area-System.Collections
  Unread
  dotnet/runtime#136064·eiriktsarpalis with CopilotAI opened Oct 19, 2026 · Updated Oct 19, 2026
  11

  

• ImmutableArray<T>.Builder.InsertRange returns unexpected values when it use the same instance

  area-System.Collectionsuntriaged
  dotnet/runtime#136026 · Filter by author prozolic opened on Oct 18, 2026
  1

  

• Clean-up: Reduce repeated checks for ICollection<T>.Count, Iterator.GetCount and ICollection.Count

  area-System.Collectionsuntriaged
  dotnet/runtime#135269 · Filter by author MikeBarker-MSFT opened on Oct 6, 2026
  4

• Try optimize without allocation

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#135189·Advitalitum opened Oct 4, 2026 · Updated Oct 4, 2026
  6

• Add fast path for Overlaps when other is ISet with fast lookup and same equality comparer #132763

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#134726·Advitalitum opened Sep 26, 2026 · Updated Oct 11, 2026
  13

• [Perf] Linux/x64: 1 Regression on 9/15/2026 9:27:19 PM +00:00

  arch-x64area-System.Collectionsos-linuxruntime-coreclrtenet-performancetenet-performance-benchmarksuntriaged
  dotnet/runtime#134436 · Filter by author performanceautofiler opened on Sep 22, 2026
  4

• [Frozen] Document duplicate handling in summaries of all FrozenDictionary factory methods

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#134318·priyadarshnisundararajan opened Sep 21, 2026 · Updated Sep 21, 2026
  4

• Improve ConcurrentStack.TryPopRange performance under contention.

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#134306·aw0lid opened Sep 20, 2026 · Updated Oct 29, 2026
  12

• Add System.Collections.Concurrent.ConcurrentHashSet<T>

  api-suggestionarea-System.Collectionsuntriaged
  dotnet/runtime#133985 · Filter by author DanSamek opened on Sep 15, 2026
  2

• Add IEnumerable constructors for BitArray

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#133831·DanSamek opened Sep 13, 2026 · Updated Sep 14, 2026
  4

• Track the actual item count in BlockingCollection

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#133637·aw0lid opened Sep 10, 2026 · Updated Oct 29, 2026
  8

• Fix SortedSet.IsSubsetOf et al

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#133116·lilinus opened Sep 2, 2026 · Updated Sep 8, 2026
  2

• [API Proposal]: Add additional capacity overloads to ToBuilder for immutable collections.

  api-suggestionarea-System.Collections
  dotnet/runtime#132929 · Filter by author aw0lid opened on Aug 29, 2026 · Future
  8

• Optimize ImmutableHashSet<T>.IsSubsetOf to avoid unnecessary allocations

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#131063·aw0lid opened Jul 20, 2026 · Updated Aug 17, 2026
  6

• Simplify collision ratio computation loop in FrozenHashTable.CalcNumBuckets

  area-System.Collectionscommunity-contribution
  Unread
  dotnet/runtime#131062·prozolic opened Jul 20, 2026 · Updated Oct 31, 2026
  11

• Allow SortedDictionary overwrites while enumerating

  area-System.Collections
  dotnet/runtime#129746 · Filter by author Youssef1313 opened on Jun 23, 2026 · Future
  4

• Batched + software-prefetch lookups and enumeration for large Dictionary / ImmutableDictionary

  api-suggestionarea-System.Collections
  dotnet/runtime#129513 · Filter by author touitou-dan opened on Jun 17, 2026 · 12.0.0
  2

• ToFrozenDictionary overloads differ in duplicate handling

  area-System.Collectionshelp wanted
  dotnet/runtime#127798 · Filter by author Timovzl opened on May 5, 2026 · Future
  4