Query: read nullable value-type GroupBy keys as nullable on the client

[QHarshil]

Fixes #33663

Summary

When groupings are assembled on the client, a NULL key of nullable value type throws. From #33663:

var group = context.Companies.GroupBy(x => new { x.CountryId }); // CountryId is int?
var groupRes = group.ToList();

System.InvalidOperationException: Nullable object must have a value.
   at Microsoft.EntityFrameworkCore.Query.Internal.GroupBySingleQueryingEnumerable`2.Enumerator.MoveNext()

This happens with a final GroupBy on 8.0, 9.0 and 10.0. On main it also happens for projections that stream a grouping's elements: #38963 moved them onto the same client-side grouping, so the issue's group.Select(x => new { x.Key, C = x.ToArray() }), which works on 10.0, now throws too. #38963 isn't on release/11.0, so without a fix this ships in 12.0.

The issue's code on each build (SQLite, one company with no country):

	10.0.12	main	this PR
group.ToList()	throws	throws	2 groups
group1.ToList() (new { x.CountryId, x.Id })	throws ArgumentNullException	throws	3 groups
select2 (new { x.Key, C = x.ToArray() })	2 groups	throws	2 groups
select4	3 groups	throws	3 groups

The Sum cases at the end of the issue already work on main. The generated SQL doesn't change for any query.

Implementation

For GroupBy(i => i.Group) over an int? column, the key translates to Convert(column, int?) over an int-typed SQL expression, and AddGroupByKeySelectorToProjection read the operand as int before converting it back. From the compiled query:

keySelector: (queryContext, dataReader) => (int?)(int)dataReader.IsDBNull(0) ? default(int?) : (int?)dataReader.GetInt32(0),
keyIdentifier: (queryContext, dataReader) => new object[]{ (object)(int)dataReader.IsDBNull(0) ? default(int?) : (int?)dataReader.GetInt32(0) },

Key column binding. The binding recorded for a key column is now the nullable read. The key selector still converts it to the column's type, so a key that is non-nullable in C# behaves as before. The key identifier boxes the nullable read as is; its value comparers already compare null to null. This matches how the identifier already reads columns that aren't part of the key.

Convert over a key column. A Convert to the nullable form of its SQL operand returns that nullable read, and a Convert to a reference type converts it. The second covers keys typed object, such as GroupBy(e => (object?)e.ReportsTo). A Convert with a user-defined conversion method is left as it was.

Scope

A key read as a nullable or reference type now gets the value EF projects for it. GroupBy(e => (uint?)e.Manager!.EmployeeID) over an optional navigation gives a NULL group, as Select(e => (uint?)e.Manager!.EmployeeID) gives null; on main it threw. A key that stays non-nullable, such as GroupBy(e => e.Manager!.EmployeeID), still throws "Nullable object must have a value", as projecting it does.

Not covered here:

• No items when projecting out groupings with a null key #32026's shape, Select(g => new { g.Key, Count = g.Count(), Items = g.ToList() }), joins the grouping back to its source on key = key, so the NULL group still comes back empty. GroupBy nullable column followed by First() over group returns null record #29240 is the same join for First(). I'm looking at that separately.
• For a source with no row identifier, such as Concat ('Value cannot be null. (Parameter 'collection')' after apply groupBy #33494), client-side grouping is already wrong on main for any key: Stream GroupBy elements instead of joining the source back to itself #38963 turned the crash in 'Value cannot be null. (Parameter 'collection')' after apply groupBy #33494 into a single grouping holding every row. Nullable keys over those sources now behave the way non-nullable keys already do there.

Test plan

New tests, each over a NULL key:

Test	Where
Final_GroupBy_nullable_value_type_key	NorthwindGroupByQueryTestBase, FinalGroupBy
Final_GroupBy_nullable_cast_over_optional_navigation	same
Final_GroupBy_anonymous_key_with_nullable_value_type	same
Final_GroupBy_composite_key_with_nullable_value_type	same
Final_GroupBy_nullable_value_type_key_as_object	same
GroupBy_nullable_value_type_key_Select_ToList	NorthwindGroupByQueryTestBase, GroupByWithoutAggregate
GroupBy_nullable_value_type_key_Select_element_projection	same
GroupBy_nullable_value_type_key_with_element_selector_Select_ToList	same
Final_GroupBy_nullable_value_type_key_with_split_Include	NorthwindGroupByQueryRelationalTestBase
GroupBy_nullable_foreign_key_in_anonymous_key (the issue's code)	AdHocMiscellaneousQueryRelationalTestBase, region 33663

The Northwind tests use Employee.ReportsTo, which is NULL for the one employee with no manager, and for the split query the highest EmployeeID over a customer's orders, which is NULL for customers with no orders. All 10 fail on main on SQL Server and SQLite (20 of 20 executions each) and pass with the change. InMemory doesn't support a final GroupBy, so its overrides assert the existing translation failure; the other tests pass there on both builds. No existing SQL baseline changes.

Reverting the key column binding, or the nullable Convert case, fails all 10. Reverting only the reference-type condition fails Final_GroupBy_nullable_value_type_key_as_object.

Test results

• SQL Server 2025 (an x64 container on Apple Silicon): the full functional suite, 50,994 tests, 445 skipped. 14 failed in the batched run. Two are GroupBy_ResultSelector_Entire_Entity_Where, which fails the same way on main here: two LACOR orders share the latest OrderDate, so which one First() picks depends on the plan. The other 12 pass when their classes run on their own, on this branch and on main (connection-pool timeouts and a server crash under load).
• SQLite functional: 38,406 tests, 286 skipped, 0 failed.
• InMemory functional: 28,362 tests, 263 skipped, 0 failed.
• EFCore.Tests 6,990, EFCore.Relational.Tests 1,500, EFCore.SqlServer.Tests 1,406, EFCore.Sqlite.Tests 896, EFCore.InMemory.Tests 39, EFCore.ApiBaseline.Tests 14: all passed.

To run the new tests:

./restore.sh && ./build.sh
source ./activate.sh
cd artifacts/bin/EFCore.Sqlite.FunctionalTests/Debug/net11.0
dotnet exec Microsoft.EntityFrameworkCore.Sqlite.FunctionalTests.dll --filter-method '*nullable_value_type*' --filter-method '*nullable_cast_over_optional_navigation*' --filter-method '*nullable_foreign_key_in_anonymous_key*'

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[Copilot]

Copilot review overview

🟢 Approval recommended

The reviewed changes have no unresolved blocking issues and include comprehensive regression coverage.

Review effort: Lite
Findings: None

What changed in this PR

Fixes client-side GroupBy failures when nullable value-type keys contain SQL NULL values.

Changes:

• Preserves nullable reads for grouping keys and identifiers.
• Handles nullable and reference-type conversions safely.
• Adds provider-specific regression and compatibility tests.

File	Summary
test/​EFCore.SqlServer.FunctionalTests/​Query/​NorthwindGroupByQuerySqlServerTest.cs	Adds SQL Server GroupBy coverage.
test/​EFCore.SqlServer.FunctionalTests/​Query/​AdHocMiscellaneousQuerySqlServerTest.cs	Adds SQL Server regression coverage.
test/​EFCore.Sqlite.FunctionalTests/​Query/​AdHocMiscellaneousQuerySqliteTest.cs	Adds SQLite regression coverage.
test/​EFCore.Specification.Tests/​Query/​NorthwindGroupByQueryTestBase.cs	Adds nullable-key GroupBy tests.
test/​EFCore.Relational.Specification.Tests/​Query/​NorthwindGroupByQueryRelationalTestBase.cs	Adds split-query Include coverage.
test/​EFCore.Relational.Specification.Tests/​Query/​AdHocMiscellaneousQueryRelationalTestBase.cs	Adds issue #33663 regression coverage.
test/​EFCore.InMemory.FunctionalTests/​Query/​NorthwindGroupByQueryInMemoryTest.cs	Records expected InMemory translation failures.
src/​EFCore.Relational/​Query/​SqlExpressions/​SelectExpression.cs	Implements null-safe GroupBy key projection and conversion handling.

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