stride3d/stride · error · InvalidOperationException

Unable to set this value at the given index because this…

Error message

Unable to set this value at the given index because this value is already contained in this ObservableSet.

What it means

ObservableSet enforces set semantics: it keeps a parallel HashSet to guarantee uniqueness. SetItem (indexer) removes the old item from the hash set and tries to add the new one; if the new item is already present elsewhere in the set, it would create a duplicate, so the old item is restored and InvalidOperationException is thrown.

Solutions

Check Contains(newItem) before index assignment and remove the duplicate first
Use Add/Remove semantics instead of indexer assignment when values may collide
If the intent is replacement, Remove(existing equal item) then assign
Verify your equality/comparer semantics — the 'duplicate' may be unintended

Example fix

// before
set[0] = existingItem; // throws if already contained
// after
if (!set.Contains(existingItem))
    set[0] = existingItem;
else
    set.Remove(set[0]); // or handle duplicate explicitly

Defensive patterns

Strategy: validation

Validate before calling

if (!observableSet.Contains(newItem)) observableSet[index] = newItem;

Try / catch

try { set[i] = value; } catch (InvalidOperationException) { /* value already in set; remove duplicate first */ }

Prevention

Treat ObservableSet as a set, not a list — prefer Add/Remove over indexer assignment
Check Contains before assigning
Review your equality comparer for unintended collisions

When it happens

Trigger: Assigning via collection[index] = item where item already exists in the set (per equality comparer); replacing an item with one equal to a different existing element.

Common situations: Treating the set like a List and assigning values; equality comparer changes making items collide; updating an item to a value equal to another tracked item.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/ce46e86c8f52c7d6. Report an issue: GitHub.

Appendix: source

Thrown at sources/presentation/Stride.Core.Presentation/Collections/ObservableSet.cs:98

        }

        protected override void InsertItem(int index, T item)
        {
            if (hashSet.Add(item))
            {
                base.InsertItem(index, item);
            }
        }

        protected override void SetItem(int index, T item)
        {
            var oldItem = base[index];
            hashSet.Remove(oldItem);
            if (!hashSet.Add(item))
            {
                // restore removed item
                hashSet.Add(oldItem);
                throw new InvalidOperationException("Unable to set this value at the given index because this value is already contained in this ObservableSet.");
            }
            base.SetItem(index, item);
        }

        protected override void RemoveItem(int index)
        {
            var item = base[index];
            if (!hashSet.Remove(item))
            {
                // safety check: shouldn't happen. If it does, we have a failed logic somewhere.
                throw new InvalidOperationException("Unable to remove this value at the given index because it wasn't found in the ObservableSet.");
            }
            base.RemoveItem(index);
        }

        /// <inheritdoc/>
        [CollectionAccess(CollectionAccessType.None)]
        public override string ToString()

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