Repository navigation
AsyncEnumerable.ThenBy with async key selector throws NullReferenceException: secondary keys not awaited #135343
Description
Description
EnumerableSorter<TElement, TKey>.ComputeKeysAsync doesn't await the next sorter's key computation (OrderBy.cs#L351):
_next?.ComputeKeysAsync(elements, count, cancellationToken);When the ThenBy key selector completes asynchronously, sorting starts before the secondary keys are computed. If the primary keys tie, Compare then reads a null _keys array.
Reproduction Steps
var sorted = await new[] { (A: 1, B: 2), (A: 1, B: 1) }
.ToAsyncEnumerable()
.OrderBy(x => x.A)
.ThenBy(async (x, _) => { await Task.Delay(10); return x.B; })
.ToListAsync();Expected behavior
(1, 1), (1, 2)
Actual behavior
System.InvalidOperationException: Failed to compare two elements in the array.
---> System.NullReferenceException: Object reference not set to an instance of an object.
at System.Linq.AsyncEnumerable.EnumerableSorter`2.Compare(Int32 index1, Int32 index2)
Suggested fix
if (_next is not null)
{
await _next.ComputeKeysAsync(elements, count, cancellationToken);
}Configuration
.NET 10.0.5, Linux x64 (bug still present on main)
Activity
dotnet-policy-service commentedon Oct 7, 2026
Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.
eiriktsarpalis commentedon Oct 7, 2026
Reproduced on .NET 10.0.12 and .NET 11.0.0-rc.1.26425.128 on Windows x64.
ThenByDescendingand third-level asynchronous ordering are also affected. A faulted secondary selector can additionally be silently ignored when primary keys differ.This should remain open as a bug, with regression coverage for selectors that actually suspend and for exception propagation. A tested workaround is to compute the asynchronous keys in
Select, then applyOrderBy/ThenBywith synchronous selectors.Await chained key computation in AsyncEnumerable ordering (#135356)