List of union type inside object type causes an exception when using fluent API #5528

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1. dotarj

   added

   🐛 bug

   on Nov 3, 2022
2. michaelstaib

   added

   🌶️ hot chocolate

   on Nov 10, 2022
3. Bery0za commented on Aug 4, 2023

   

   Bery0za

   on Aug 4, 2023

   Last edited by Bery0za

   More actions

   I'm experiencing the same issue, any usage of the lists of the union types with UseProjection() causes the errors or incorrect behavior.

   I've tried to change the generic type of the collection property to object as a workaround mentioned by @dotarj:

   class ContainingEntity {
       /*List<EntityWithSubtypes> ListOfUnionEntities { get; set; }*/
       List<object> ListOfUnionEntities { get; set; }
   }

   It helps to solve the Argument types do not match error, but if I query entities in the collection by a concrete type then I get null instead of any array:

   {
       containingEntities {
           listOfUnionEntities {
               ... on SubtypeA {
                   someProperty
               }
           }
       }
   }

   "containingEntities": {
       "listOfUnionEntities": null
   }

   If I query union entities by some common interface or when there are only one instance of EntityWithSubtypes in the database then the solution works.

   {
       containingEntities {
           listOfUnionEntities {
               ... on EntityWithSubtypesInterface {
                   someProperty
               }
           }
       }
   }

   "containingEntities": {
       "listOfUnionEntities": [
           ...actual entities...
       ]
   }

   ---

   I also tried to check if I can query those entities from a separate repository.

   public class Query
   {
      public IQueryable<EntityWithSubtypes> UnionEntities(IRepository repository) => repository.EntitiesWithSubtypes;
   }
   
   public class QueryType : ObjectType<Query>
   {
       protected override void Configure(IObjectTypeDescriptor<Query> descriptor)
       {
           descriptor
               .Field(q => q.UnionEntities)
               .Type<ListType<NonNullType<EntityWithSubtypesUnion>>>
               .UseProjection();
       }
   }

   {
       unionEntities {
           ... on EntityWithSubtypesInterface {
              someProperty
           }
       }
   }

   But I'm getting the error Unable to cast object of type…, no matter if I query entities by a concrete type or by the common interface.

   {
       "errors": [
           {
               "message": "Unexpected Execution Error",
               "locations": [
                   {
                       "line": 2,
                       "column": 5
                   }
               ],
               "path": [
                   "unionEntities"
               ],
               "extensions": {
                   "message": "Unable to cast object of type 'System.Linq.Expressions.Expression1`1[System.Func`2[System.Object,System.Object]]' to type 'System.Linq.Expressions.Expression`1[System.Func`2[ListOfUnionTest.EntityWithSubtypes,ListOfUnionTest.EntityWithSubtypes]]'.",
                   "stackTrace": "   at HotChocolate.Data.Projections.Expressions.QueryableProjectionScopeExtensions.Project[T](QueryableProjectionScope scope)
      at HotChocolate.Data.Projections.Expressions.QueryableProjectionContextExtensions.Project[T](QueryableProjectionContext context)
      at HotChocolate.Data.Projections.Expressions.QueryableProjectionProvider.<CreateApplicator>b__7_0[TEntityType](IResolverContext context, Object input)
      at HotChocolate.Data.Projections.Expressions.QueryableProjectionProvider.<>c__DisplayClass4_0`1.<<CreateExecutor>g__ExecuteAsync|1>d.MoveNext()
   --- End of stack trace from previous location ---
      at HotChocolate.Execution.Processing.Tasks.ResolverTask.ExecuteResolverPipelineAsync(CancellationToken cancellationToken)
      at HotChocolate.Execution.Processing.Tasks.ResolverTask.TryExecuteAsync(CancellationToken cancellationToken)"
               }
           }
       ],
       "data": {
           "unionEntities ": null
       }
   }

   If I change the return type of the query to the collection of objects, then I'm getting the same behavior as in the first part of this comment, i. e. queries only work when I try to get the union entities by the common interface.

   public class Query
   {
      public /*IQueryable<EntityWithSubtypes>*/ IQueryable<object> UnionEntities(IRepository repository) => repository.EntitiesWithSubtypes;
   }
4. Bery0za commented on Aug 5, 2023

   

   Bery0za

   on Aug 5, 2023

   Last edited by Bery0za

   More actions

   Well, I found a solution to the described problem and it is quite simple and straigtforward. UnionType class has a generic extension UnionType<T> which is supposed to be used with some marker interface, but it's also possible to use the base class of the union types as a generic parameter. It helps to get rid of Argument types do not match and Unable to cast object of type… errors with the collections which include entities of the various subtypes (and there is no need to change the generic type of the collections to object).
5. michaelstaib

   removed

   🐛 bug

   on Oct 28, 2024
6. michaelstaib

   added

   Area: Type SystemIssue is related to the Type SystemIssue is related to the Type System

   on Feb 19, 2026
7. michaelstaib

   closed this as completedin #9222on Feb 25, 2026