IQueryable.Count() throws InvalidOperationException

[vlkchris]

I am running the following code

var query = _myDb.Persons.AsQueryable();

query = query.OrderBy(m => m.PersonId);

// filters here
// removed for brevity

// DistinctBy(p => p.PersonId) does not translate - work with GroupBy here.
query = query.GroupBy(mp => mp.PersonId).Select(g => g.First());

//count results before skip and take
var count = query.Count(); // <== here it throws

// pagination 
if (request.Offset > 0) query = query.Skip(request.Offset.Value);
if (request.Limit > 0) query = query.Take(request.Limit.Value);

var searchResult = await query.ToListAsync();

I have cloned EFCore repo to find out what is happening.
The Exception is thrown here:

private sealed class ClientProjectionRemappingExpressionVisitor : ExpressionVisitor
{
    private readonly List<object> _clientProjectionIndexMap;

    public ClientProjectionRemappingExpressionVisitor(List<object> clientProjectionIndexMap)
    {
        _clientProjectionIndexMap = clientProjectionIndexMap;
    }

    [return: NotNullIfNotNull("expression")]
    public override Expression? Visit(Expression? expression)
    {
        if (expression is ProjectionBindingExpression projectionBindingExpression)
        {
            var value = _clientProjectionIndexMap[projectionBindingExpression.Index!.Value]; <== throws

At this point the projectionBindingExpression.Index is null, and it throws
System.InvalidOperationException {"Nullable object must have a value."}

Full stack trace:

   at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue()
   at System.Nullable`1.get_Value()
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ClientProjectionRemappingExpressionVisitor.Visit(Expression expression) in C:\Users\unchro\source\repos\efcore\src\EFCore.Relational\Query\SqlExpressions\SelectExpression.Helper.cs:line 576
   at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)
   at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ClientProjectionRemappingExpressionVisitor.Visit(Expression expression) in C:\Users\unchro\source\repos\efcore\src\EFCore.Relational\Query\SqlExpressions\SelectExpression.Helper.cs:line 625
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ApplyProjection(Expression shaperExpression, ResultCardinality resultCardinality, QuerySplittingBehavior querySplittingBehavior) in C:\Users\unchro\source\repos\efcore\src\EFCore.Relational\Query\SqlExpressions\SelectExpression.cs:line 829
   at Microsoft.EntityFrameworkCore.Query.Internal.SelectExpressionProjectionApplyingExpressionVisitor.VisitExtension(Expression extensionExpression) in C:\Users\unchro\source\repos\efcore\src\EFCore.Relational\Query\Internal\SelectExpressionProjectionApplyingExpressionVisitor.cs:line 41
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPostprocessor.Process(Expression query) in C:\Users\unchro\source\repos\efcore\src\EFCore.Relational\Query\RelationalQueryTranslationPostprocessor.cs:line 49
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query) in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\QueryCompilationContext.cs:line 190
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async) in C:\Users\unchro\source\repos\efcore\src\EFCore\Storage\Database.cs:line 76
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async) in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\Internal\QueryCompiler.cs:line 111
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0() in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\Internal\QueryCompiler.cs:line 95
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler) in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\Internal\CompiledQueryCache.cs:line 74
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query) in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\Internal\QueryCompiler.cs:line 91
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression) in C:\Users\unchro\source\repos\efcore\src\EFCore\Query\Internal\EntityQueryProvider.cs:line 78
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
   at FyndooService.DataAccess.Providers.MatrixDataProvider.<SearchCrmEntitiesAsync2>d__9.MoveNext() in C:\Users\unchro\Source\Repos\FyndooService2\src\DataAccess\Providers\MatrixDataProvider.cs:line 115

I noticed the following:

• If I skip the Count() statement the code runs fine and I get desired results.
• If I skip the GroupBy() statement, the Count() works fine (but with unwanted result as duplicate PersonIds are all counted.)

Include provider and version information

EF Core version: 6.0.1
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: net6.0
Operating system: Windows 10
IDE: Visual Studio Professional 2022 17.0.2

[ajcvickers]

Possible duplicate of #26748.