Use backing field when determining collection navigation type to create

[ajcvickers]

Looks like right now we are only looking at the property CLR type. See #10800.

Repro:

public class Blog
{
    private List<Post> _posts;
    
    public int Id { get; set; }

    public IEnumerable<Post> Posts
    {
        get => _posts;
        set => _posts = (List<Post>) value;
    }
}

public class Post
{
    public int Id { get; set; }
    public Blog Blog { get; set; }
}

public class BloggingContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>();
    }
    
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Test;ConnectRetryCount=0");
}

public class Program
{
    public static void Main()
    {
        using (var context = new BloggingContext())
        {
            context.Database.EnsureDeleted();
            context.Database.EnsureCreated();

            context.Add(new Blog
            {
                Posts = new List<Post>
                {
                    new Post(),
                    new Post(),
                    new Post()
                }
            });
            
            context.SaveChanges();
        }
        
        using (var context = new BloggingContext())
        {
            var blogs = context.Set<Blog>().Include(e => e.Posts).ToList();
        }
    }
}

Throws:

Unhandled Exception: System.InvalidCastException: Unable to cast object of type 'System.Collections.Generic.HashSet`1[Post]' to type 'System.Collections.Generic.List`1[Post]'.
   at Blog.set_Posts(IEnumerable`1 value) in C:\Stuff\TwoTwo\TwoTwo\Program.cs:line 20
   at Microsoft.EntityFrameworkCore.Metadata.Internal.ClrCollectionAccessorFactory.CreateAndSetHashSet[TEntity,TCollection,TElement](TEntity entity, Action`2 setterDelegate)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.ClrICollectionAccessor`3.GetOrCreateCollection(Object instance)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.IncludeCollection[TEntity,TRelated,TElement](Int32 includeId, INavigation navigation, INavigation inverseNavigation, IEntityType targetEntityType, IClrCollectionAccessor clrCollectionAccessor, IClrPropertySetter inverseClrPropertySetter, Boolean tracking, TEntity entity, Func`1 relate
dEntitiesFactory, Func`3 joinPredicate)
   at lambda_method(Closure , QueryContext , Blog , Object[] )
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler._Include[TEntity](QueryContext queryContext, TEntity entity, Object[] included, Action`3 fixup)
   at lambda_method(Closure , Blog )
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider._TrackEntities[TOut,TIn](IEnumerable`1 results, QueryContext queryContext, IList`1 entityTrackingInfos, IList`1 entityAccessors)+MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Program.Main() in C:\Stuff\TwoTwo\TwoTwo\Program.cs:line 70

[mhDuke]

i am not sure whether am missing something, or this issue isn't really fixed! but i have a private HashSet backing field for public IEnumerable. if i didnt initialize the hashset myself (on the field declaration). ef core would initialize it with some ICollection type.

private HashSet<VehicleFeature> _features;
public IEnumerable<VehicleFeature> Features 
        { 
            get => _features; 
            private set => _features = new HashSet<VehicleFeature>(value); 
        }

with above code, when ef initializes the entity. The HashSet _features would gladly add duplicate values. _features is no longer a hashSet! But if _features initialization be:
private HashSet<VehicleFeature> _features = new HashSet<VehicleFeature>();
it works just fine.

More over, i created a test entity to make sure it's not something that i am missing.

private HashSet<int> _ints;
public IEnumerable<int> Ints 
        { 
            get => _ints;
            private set => _ints = new HashSet<int>(value); 
        }

But this time i initialize the _ints or not, i receive

System.InvalidCastException
HResult=0x80004002
Message=Unable to cast object of type 'System.Collections.Generic.List1[System.Int32]' to type 'System.Collections.Generic.HashSet1[System.Int32]'.
Source=Microsoft.EntityFrameworkCore.Relational
StackTrace:
at Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitor.QueryingEnumerable1.Enumerator.MoveNext() at System.Linq.Enumerable.Single[TSource](IEnumerable1 source)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
at System.Linq.Queryable.First[TSource](IQueryable`1 source)

@ajcvickers your input is much appreciated.
thats ef 3 with Sqlite 3

[ajcvickers]

@mhDuke It's not clear to me what is happening here. Please open a new issue and post a small, runnable project/solution or complete code listing so that we can investigate.