Queryable.Count throws exception when used with GroupBy and Select with constructor arguments

[niroula-kushal]

Queryable.Count throws exception when used with GroupBy and Select with constructor arguments

Include your code

Result.cs
public class Result
{
    public long Id { get; set; }

    public Result(long id)
    {
        Id = id;
    }

    public Result()
    {
        
    }
}
....
// Error Here
 var x = _context.Products
            .GroupBy(x => x.Id)
            .Select(x => new Result(x.Key))
            .Count();

The above code throws an exception. The problem seems to be occuring when using GroupBy followed by Select where we are selecting using Constructor arguments as in above code and then performing a Count operation.
The workaround for now is to use object initializer syntax. That seems to be functioning correctly.

This worked perfectly in Ef 5 and should be working in ef 6 as well.

Repro: https://github.com/RehmatFalcon/ef-err-group-by

You can view the HomeController#Index to see the error.

Include stack traces

Include the full exception message and stack trace for any exception you encounter.

Use triple-tick fences for stack traces. For example:

System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at System.Linq.Expressions.ExpressionExtensions.GetConstantValue[T](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.GetProjectionIndex(ProjectionBindingExpression projectionBindingExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   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.RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.ProcessShaper(Expression shaperExpression, RelationalCommandCache& relationalCommandCache, LambdaExpression& relatedDataLoaders, Int32& collectionId)
   at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.VisitShapedQuery(ShapedQueryExpression shapedQueryExpression)
   at Microsoft.EntityFrameworkCore.Query.ShapedQueryCompilingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
   at Repro.Controllers.HomeController.Index() in D:\Experiments\Repro\Repro\Controllers\HomeController.cs:line 36
   at lambda_method1(Closure , Object , Object[] )
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|25_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Include provider and version information

EF Core version: 6
Database provider: (e.g. npgsql) --tested with NPGSQL and SQLITE
Target framework: (e.g. .NET 6.0)

[roji]

In your specific query above, the Select is superfluous and can be removed, since you immediately apply Count to the results. Also, if Id is unique (e.g. a primary key), the above can be replaced by a simple row count (without GroupBy), since every group is guaranteed to have just one member.

But more importantly, your Result type is opaque to EF Core - it cannot know what the constructor actually does, and therefore shouldn't translate new Result(x.Key). I can see that this query did indeed work in EF Core 5.0, but it seems more correct for it to fail translation because, just like we do for other similar cases (@smitpatel can provide more input here).

The exception message could be improved, though.

[niroula-kushal]

@roji Sorry for the confusion, the code above was just for demonstration. The actual code that is failing in our system is much more complex than that and a group by is actually required there. Our code was using record type to get the results, which is why we were using the constructor syntax.

Your explanation does make sense and we have updated our codes to initializer syntax. You are right, the exception message could be better.

Thank you for your help 😊.