ArgumentException during query translation: ToString not declared on Nullable<int>

[ixtreon]

Describe what is not working as expected:

Calling ToString() on a related object's int-typed column causes an ArgumentException. Doing the same for a Guid-typed column works as expected.

The primary object is an event log entry, and the related object is a (nullable) user with two identifiers - an int and a Guid. Retrieving FromUser.ID.ToString() causes the exception; calling FromUser.Guid.ToString() does not.

Full exception details (message and stack trace):

Exception message: 

    System.ArgumentException : Method 'System.String ToString()' declared on type 'System.Int32' cannot be called with instance of type 'System.Nullable\`1[System.Int32]'

Stack trace:

   at System.Linq.Expressions.Expression.ValidateCallInstanceType(Type instanceType, MethodInfo method)
   at System.Linq.Expressions.Expression.ValidateStaticOrInstanceMethod(Expression instance, MethodInfo method)
   at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable`1 arguments)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.ExpressionVisitor.VisitAndConvert[T](ReadOnlyCollection`1 nodes, String callerName)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitNewArray(NewArrayExpression node)
   at Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.NavigationRewritingQueryModelVisitor.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
   at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.Rewrite(QueryModel queryModel, QueryModel parentQueryModel)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel, Boolean asyncQuery)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel, Boolean asyncQuery)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, IQueryModelGenerator queryModelGenerator, IDatabase database, IDiagnosticsLogger`1 logger, Type contextType)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass13_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Remotion.Linq.QueryableBase`1.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)

Steps to reproduce

The relevant parts of the model, in pseudo-code:

class EventLog
{
    [ForeignKey<User>(x => x.ID)]
    public int? FromUserID { get; set; }
    
    public virtual User FromUser { get; set; }
}

class User
{
    [PrimaryKey]
    public int ID { get; set; }

    public int Guid { get; set; }
}

Code that triggers the exception:

var thisWorks = Unit.Set<EventLog>()
    .Where(x => x.FromUserID != null)
    .Select(x => x.FromUser)
    .Select(x => x.Guid.ToString())
    .ToList();

var thisThrows = Unit.Set<EventLog>()
    .Where(x => x.FromUserID != null)
    .Select(x => x.FromUser)
    .Select(x => x.ID.ToString())
    .ToList();

These are trimmed-down examples, not the full code, so the exception above could be a bit misleading..

Further Notes

• Adding .Where(x => x != null) before the final Select( ... ToString()) does not affect the outcome.
• Doing .Select(x => x != null ? x.ID.ToString() : null) throws the exception too.
• The way we solved it was by not using EF for the last bit, i.e:

var thisWorksNow = Unit.Set<EventLog>()
    .Where(x => x.FromUserID != null)
    .Select(x => x.FromUser)
    .Select(x => new { x.ID })
    .AsEnumerable()
    .Select(x => x.ID.ToString())
    .ToList();

Related Issues

• EF Core does not cast from Nullable to underlying type #10284 - seems to have been merged around the 2.1 milestone but looks quite similar.
• ProjectTo fails on mapping child entity guid property AutoMapper/AutoMapper#2836 - from a later date; mentions the one above.

Further technical details

EF Core version: 2.2.3, 2.2.4
Database Provider: Microsoft.EntityFrameworkCore.SqlServer, Microsoft.EntityFrameworkCore.Sqlite
Operating system: Windows 10
IDE: Visual Studio 2017 15.9

[smitpatel]

It's nav rewrite. Translation code is bug free wrt this kind of exception. Assigning to @maumar

[maumar]

this scenario works in the new query pipeline, running similar query on test model produced the following sql:

SELECT CONVERT(VARCHAR(11), CAST([t].[SquadId] AS int))
FROM [Tags] AS [t0]
LEFT JOIN (
    SELECT [g].[Nickname], [g].[SquadId], [g].[AssignedCityName], [g].[CityOrBirthName], [g].[Discriminator], [g].[FullName], [g].[HasSoulPatch], [g].[LeaderNickname], [g].[LeaderSquadId], [g].[Rank]
    FROM [Gears] AS [g]
    WHERE [g].[Discriminator] IN (N'Gear', N'Officer')
) AS [t] ON (([t0].[GearNickName] = [t].[Nickname]) AND [t0].[GearNickName] IS NOT NULL) AND (([t0].[GearSquadId] = [t].[SquadId]) AND [t0].[GearSquadId] IS NOT NULL)
WHERE [t0].[GearSquadId] IS NOT NULL