EF Core does not cast from Nullable to underlying type

[AndrewBoklashko]

Having the following mapping:

        public static IQueryable<UserInListDTO> MapToUserInListToDTO(this IQueryable<Parent> parents)
            => parents.Select(p => new UserInListDTO
                {
                    Id = p.Id,
                    FirstName = p.FirstName,
                    LastName = p.LastName,
                    Email = p.ApplicationUser != null
                        ? p.ApplicationUser.Email
                        : null,
                    PhoneNumber = p.ApplicationUser != null
                        ? p.ApplicationUser.PhoneNumber
                        : null,
                    Address = p.Address != null
                        ? p.Address.MappedAddress
                        : null,
                    Schools = p.Childs.Select(c => c.School.SchoolName).Distinct(),
                    Status = p.ApplicationUser != null
                        ? p.ApplicationUser.Status.ToFriendlyString()
                        : null,
                    Verified = p.IsVerified,
                         ....
                });

Query fails with exception on this part:

Status = p.ApplicationUser != null ? p.ApplicationUser.Status.ToFriendlyString() : null

Status property type is enum. Relation beetwen Parent and ApplicationUser:

            builder.Entity<ApplicationUser>()
                .HasOne(u => u.Parent)
                .WithOne(p => p.ApplicationUser)
                .IsRequired()
                .HasForeignKey<ApplicationUser>(u => u.ParentId)
                .OnDelete(DeleteBehavior.Cascade);

System.ArgumentException: Expression of type 'System.Nullable`1[KidsCareApp.Model.Enums.UserStatuses]' cannot be used for parameter of type 'KidsCareApp.Model.Enums.UserStatuses' of method 'System.String ToFriendlyString[UserStatuses](KidsCareApp.Model.Enums.UserStatuses)'
Parameter name: arg0
   at System.Dynamic.Utils.ExpressionUtils.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arguments, ParameterInfo pi, String methodParamName, String argumentParamName, Int32 index)
   at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, Expression arg0)
   at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable`1 arguments)
   at System.Linq.Expressions.MethodCallExpression.Update(Expression object, IEnumerable`1 arguments)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.Expressions.Internal.NullConditionalExpression.VisitChildren(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.VisitExtension(Expression node)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
   at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
   at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.ExpressionTransformingQueryModelVisitor`1.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.NavigationRewritingExpressionVisitor.NavigationRewritingQueryModelVisitor.VisitSelectClause(SelectClause selectClause, QueryModel queryModel)
   at Remotion.Linq.Clauses.SelectClause.Accept(IQueryModelVisitor visitor, 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.CreateAsyncQueryExecutor[TResult](QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileAsyncQuery[TResult](QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileAsyncQueryCore[TResult](Expression query, INodeTypeProvider nodeTypeProvider, IDatabase database)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass24_0`1.<CompileAsyncQuery>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddAsyncQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileAsyncQuery[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.System.Collections.Generic.IAsyncEnumerable<TResult>.GetEnumerator()
   at System.Linq.AsyncEnumerable.<Aggregate_>d__6`3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at KidsCareApp.Repository.Services.Admin.AdminUserService.<GetUsersAsync>d__1.MoveNext()

Further technical details

EF Core version: 2.0.0
Database Provider: Microsoft.EntityFrameworkCore.SQLServer
Operating system: Windows 10 Pro 1703 15063.540
IDE: Visual Studio 2017

[anpete]

@maumar Nav expansion?

[maumar]

looks like it

[AndrewBoklashko]

By the way with explicit casting it works:
Status = ((UserStatuses)p.ApplicationUser.Status).ToFriendlyString()

[maumar]

The problem here is that during nav rewrite we deduce that since Person->ApplicationUser is principal to dependent, ApplicationUser can be null, so we apply null protection logic on the call accessing Status property. This changes the type to Nullable<UserStatuses> which then no longer matches the signature of the method ToFriendlyString that is being called.

[anpete]

@maumar So we should unwrap before calling into client method here? cc @smitpatel

[maumar]

fixed in f7e93de