MinBy MaxBy support

[henriquewr]

fixes #25566

MinBy(x => x.Id) is lowered to OrderBy(x => x.Id).First/OrDefault()

MaxBy(x => x.Id) is lowered to OrderByDesceding(x => x.Id).First/OrDefault()

To comply with LINQ contract, the First method is determined by the nullability of the source, if source is not nullable, First would be selected, otherwise FirstOrDefault

new List<int>().MaxBy(x => x); // throws System.InvalidOperationException: 'Sequence contains no elements'

new List<int?>().MaxBy(x => x); // returns null

I followed the proposal of #25566 (comment)

Including implementing support for anonymous types in MaxBy/MinBy, that isn't supported in LINQ to Objects (Is supporting this really a great idea?)

new List<int>() { 1, 2, 3 }.MaxBy(x => new { x }); // throws System.ArgumentException: 'At least one object must implement IComparable.'

[Copilot]

Pull request overview

This pull request adds support for SQL translation of .NET 6's MinBy/MaxBy LINQ methods to Entity Framework Core, addressing issue #25566. The implementation lowers MinBy/MaxBy operations to OrderBy/OrderByDescending followed by First/FirstOrDefault, with special handling for complex selectors including anonymous types.

Changes:

• Added MinBy/MaxBy query method normalization that transforms them into OrderBy/OrderByDescending + First/FirstOrDefault patterns
• Implemented async extension methods (MinByAsync/MaxByAsync) for IQueryable
• Added comprehensive test coverage across SQL Server, InMemory, and Cosmos providers
• Included support for anonymous type selectors by expanding them into ThenBy/ThenByDescending chains

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/EFCore/Query/Internal/QueryableMethodNormalizingExpressionVisitor.cs	Core implementation that transforms MinBy/MaxBy calls into OrderBy+First patterns with special handling for anonymous type selectors
src/EFCore/Query/Internal/NavigationExpandingExpressionVisitor.ExpressionVisitors.cs	Minor optimization to pre-allocate list capacity
src/EFCore/Extensions/EntityFrameworkQueryableExtensions.cs	Added MinByAsync and MaxByAsync extension methods with XML documentation
src/EFCore/Query/QueryableMethods.cs	Added MinBy and MaxBy MethodInfo properties
src/Shared/EnumerableMethods.cs	Added MinBy and MaxBy MethodInfo properties
test/EFCore.Specification.Tests/TestUtilities/QueryAsserter.cs	Added AssertMinBy and AssertMaxBy test assertion helpers (contains dead code issue)
test/EFCore.Specification.Tests/TestUtilities/ExpectedQueryRewritingVisitor.cs	Added query rewriting logic for expected results with anonymous type selectors
test/EFCore.Specification.Tests/Query/QueryTestBase.cs	Added AssertMinBy and AssertMaxBy helper methods
test/EFCore.Specification.Tests/Query/NorthwindGroupByQueryTestBase.cs	Added specification tests for MinBy/MaxBy in GroupBy scenarios and fixed #region formatting
test/EFCore.Specification.Tests/Query/NorthwindAggregateOperatorsQueryTestBase.cs	Added comprehensive specification tests for MinBy/MaxBy including edge cases
test/EFCore.SqlServer.FunctionalTests/Query/NorthwindGroupByQuerySqlServerTest.cs	Added SQL Server specific tests and SQL baselines (contains test method call bug)
test/EFCore.SqlServer.FunctionalTests/Query/NorthwindAggregateOperatorsQuerySqlServerTest.cs	Added SQL Server specific tests and SQL baselines (contains test method call bug)
test/EFCore.InMemory.FunctionalTests/Query/NorthwindAggregateOperatorsQueryInMemoryTest.cs	Added InMemory provider specific tests
test/EFCore.Cosmos.FunctionalTests/Query/NorthwindAggregateOperatorsQueryCosmosTest.cs	Added Cosmos provider specific tests (contains test method call bug)

[roji]

@henriquewr can you explain the First() vs. FirstOrDefault() logic here, and why we wouldn't just always do FIrstOrDefault()? In other words, why the special case for nullable value types (and why it doesn't apply to reference types, which are also nullable).

[henriquewr]

@roji

The First/OrDefault is selected just to mimic the behavior of LINQ:

For value types empty collections always throws:
new List<int>().MaxBy(x => x); // throws System.InvalidOperationException: 'Sequence contains no elements'

So to mimic the same behavior the First method is selected:
new List<int>().OrderBy(x => x).First(); // throws System.InvalidOperationException: 'Sequence contains no elements';

new List<int>().OrderBy(x => x).FirstOrDefault(); // returns null

For nullable value types:

new List<int?>().MaxBy(x => x); // returns null
For nullable types the method FirstOrDefault is selected:

new List<int?>().OrderBy(x => x).First(); // throws System.InvalidOperationException: 'Sequence contains no elements'
new List<int?>().OrderBy(x => x).FirstOrDefault(); // returns null

(and why it doesn't apply to reference types, which are also nullable)

Yep, I forgot about that, reference types returns null too for empty collections:

new List<Type/*just any reference type*/>().MaxBy(x => x); // returns null