Queryable.{Aggregate,Count}By accept optional argument values and cannot appear in an Expression tree

[jcracknell]

runtime/src/libraries/System.Linq.Queryable/src/System/Linq/Queryable.cs

Line 2449 in 32299f4

public static IQueryable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, TAccumulate seed, Expression<Func<TAccumulate, TSource, TAccumulate>> func, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull

Methods having optional (default) argument values cannot appear in an expression tree, where these should presumably be usable. As a (vacuous) example:

dbContext.Owner.SelectMany(o => dbContext.Cats.CountBy(c => c.Age))

will not compile.

dotnet/efcore#33177
dotnet/efcore#32741

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[teo-tsirpanis]

The C# language design team has decided to allow this in the language.

[jcracknell]

Well that's timely.

The observation here is that it should be possible to add an overload omitting the optional parameter (as is the case for every other Queryable method) without breaking binary compatibility. This will allow these methods to be used in expression trees immediately instead of waiting for whatever version lands support for optional parameters in expression trees.

[cston]

Compiler support added in dotnet/roslyn#77972.