[officialasishkumar]

Fixes #32741

.NET 9 added Queryable.CountBy, which EF didn't translate, so a query like context.Orders.CountBy(o => o.CustomerId) failed with a translation error.

Change

As suggested in the issue, QueryableMethodNormalizingExpressionVisitor now rewrites

source.CountBy(x => x.Prop)

to

source.GroupBy(x => x.Prop).Select(g => new KeyValuePair<TKey, int>(g.Key, g.Count()))

next to the existing MinBy/MaxBy normalization (#37573). The rest of the pipeline already handles this shape, so all providers that support GroupBy with aggregates get the translation:

SELECT [o].[ProductID] AS [Key], COUNT(*) AS [Value]
FROM [Order Details] AS [o]
GROUP BY [o].[ProductID]

• The KeyValuePair is created with its Key and Value members (as for an anonymous type), so operators composed after CountBy bind to the grouping key and the count. For example, CountBy(...).OrderByDescending(e => e.Value).Take(5) translates to ORDER BY COUNT(*) DESC with TOP, and CountBy(...).Where(e => e.Value > 40) to HAVING COUNT(*) > 40. Without the members, these member accesses can't be bound and the query fails translation.
• Enumerable.CountBy over a collection navigation is already converted to Queryable.CountBy by the same visitor, so c.Orders.CountBy(o => o.EmployeeID) in a projection also works (OUTER APPLY on SQL Server).
• CountBy with a key comparer isn't rewritten and fails translation, as GroupBy with a comparer does.
• QueryableMethods.CountBy is added (with its API baseline entry), like QueryableMethods.MaxBy/MinBy.

LINQ to Objects' CountBy throws for a null key because it uses a dictionary. The translated query instead returns a row for the null group, as GroupBy does.

Tests

New tests in NorthwindGroupByQueryTestBase: a property key, after a Where, an anonymous type key, Where on the count, OrderByDescending/ThenBy/Take over the result, Select of the key, over a collection navigation in a projection, and with a comparer (translation failure). They come with SQL Server and SQLite baselines. On SQLite the navigation case needs APPLY and asserts that it isn't supported, like the other correlated GroupBy tests there.

Without the change, all of them except the comparer test fail with a translation error. With it, I ran the whole NorthwindGroupByQuery test classes: SQLite (700 passed, 4 skipped), SQL Server against Azure SQL Edge (699 passed, 4 skipped) and InMemory (658 passed, 20 skipped). I only ran these test classes, not the full suite.

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