[Bug]: UpdateValueAndRemoveDeletedReferenceProperties throws on every removal — the Assembler deletion path is dead #349

What type of issue is this?

• Bug report
• Feature request

Description

UpdateValueAndRemoveDeletedReferenceProperties throws
InvalidOperationException: Collection was modified on every removal, so the deletion path of
Assembler.Synchronize is dead — any Synchronize whose DTOs drop an element from an enumerable
reference property aborts mid-assembly.

Source: SysML2.NET.CodeGenerator/Templates/Uml/core-dal-poco-uml-extensions.hbs:78-90, replicated
into 231 call sites across 167 files under SysML2.NET.Dal/Core/AutoGenPocoExtension/. Affects
OwnedRelationship, OwnedRelatedElement, and Dependency.Client / Supplier.

The emitted shape, e.g. SysML2.NET.Dal/Core/AutoGenPocoExtension/ForkNodeExtensions.cs:105-112:

var ownedRelationshipToDelete = poco.OwnedRelationship.Select(x => x.Id).Except(dto.OwnedRelationship);

foreach (var identifier in ownedRelationshipToDelete)
{
    ((IContainedElement)poco).OwnedRelationship.Remove(poco.OwnedRelationship.Single(x => x.Id == identifier));
}

identifiersOfObjectsToDelete.AddRange(ownedRelationshipToDelete);

• Throws. Except is deferred over the live ContainerList; the loop body removes from the
  collection it is streaming, invalidating the enumerator on the first removal.
• Silently loses the deletions. Line 90 re-runs the deferred query after the removals, so
  identifiersOfObjectsToDelete returns empty and the removed POCOs leak in the cache. Fixing only
  the throw leaves this in place.
• Quadratic. Single(x => x.Id == identifier) is the scan-per-element [Tech Debt]: Assembler.Synchronize is O(n²) — 89 s to assemble a 50 000-element model #347 removed from
  Assembler.Synchronize.

Steps to Reproduce

var assembler = new Assembler();
assembler.Synchronize([owner, membership, package]);   // owner.OwnedRelationship = [membership.Id]

owner.OwnedRelationship = [];

assembler.Synchronize([owner, membership, package]);   // InvalidOperationException

Expected: membership and its owned package are removed from the cache.

Proposed fix

In the template — do not hand-edit generated files:

var ownedRelationshipRetained = new HashSet<Guid>(dto.OwnedRelationship);
var ownedRelationshipToDelete = poco.OwnedRelationship.Where(x => !ownedRelationshipRetained.Contains(x.Id)).ToList();

foreach (var relationship in ownedRelationshipToDelete)
{
    ((IContainedElement)poco).OwnedRelationship.Remove(relationship);
}

identifiersOfObjectsToDelete.AddRange(ownedRelationshipToDelete.Select(x => x.Id));

Tasks

• Fix the template and regenerate SysML2.NET.Dal/Core/AutoGenPocoExtension/.
• Add removal coverage to AssemblerTestFixture — none exists today, which is why this was missed.
• Verify cascading deletion now that identifiersOfObjectsToDelete is populated for the first
  time; the Assembler deletion loop has never executed.

System Configuration

• SysML2.NET version: 0.19.0
• Found while verifying [Tech Debt]: Assembler.Synchronize is O(n²) — 89 s to assemble a 50 000-element model #347; runtime-independent (BCL enumerator semantics)