louthy/language-ext · error · ArgumentException

Key doesn't exist in map

Error message

Key doesn't exist in map: {change.Key}

What it means

This throw fires in the TrieMap update traversal when the key is definitively absent (the node/section where it would live is empty or a terminal leaf) and the update type is SetItem. SetItem requires the key to already exist; TrySetItem returns without change instead. It is the strict-mode counterpart of the not-found-at-node case.

Solutions

1. Use AddOrUpdate for insert-or-replace behavior
2. Use TrySetItem to make the update a no-op for missing keys
3. Add the key first (Add) then SetItem, or restructure to a single AddOrUpdate
4. Log/inspect the key with FindOption to confirm presence before SetItem

Example fix

// before
map = map.SetItem(missingKey, value);
// after
map = map.TrySetItem(missingKey, value); // no-op if absent
// or
map = map.AddOrUpdate(missingKey, value);

if (map.ContainsKey(key)) map = map.SetItem(key, value); else map = map.Add(key, value);

try { map = map.SetItem(key, value); }
catch (ArgumentException) { /* key absent: add or skip */ }

When it happens

Trigger: Calling map.SetItem(key, value) for a key that is not in the TrieMap at all; also reached via bulk update APIs that set the update type to SetItem.

Common situations: Assuming SetItem upserts; calling SetItem after Remove on the same key; typos or casing mismatches between the add and set call sites.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• Key doesn't exist in map
• Key doesn't exist in map
• ArgumentOutOfRangeException
• Key not found in Map
• Key not found in set

AI-assisted analysis of louthy/language-ext@2f0e362824 (2026-09-15). Data as JSON: /api/errors/33a7540ee4c05b6d. Report an issue: GitHub.

Appendix: source

Thrown at LanguageExt.Core/Immutable Collections/TrieMap/TrieMap.cs:2375

                    return (1, new Entries(newEntryMap, newNodeMap, newItems, newNodes), default, true);
                }
            }
            else if (Bit.Get(NodeMap, mask))
            {
                // var nodeIndex = Index(NodeMap, mask);
                var nodeIndex = BitCount((int)NodeMap & ((int)mask - 1));

                var nodeToUpdate = Nodes[nodeIndex];
                var (cd, newNode, ov, ch) = nodeToUpdate.Update(env, change, hash, section.Next());
                var (newNodes, _) = SetItem(Nodes, nodeIndex, newNode, env.Mutate);
                return (cd, new Entries(EntryMap, NodeMap, Items, newNodes), ov, ch);
            }
            else
            {
                if (env.Type == UpdateType.SetItem)
                {
                    // Key must already exist to set it
                    throw new ArgumentException($"Key doesn't exist in map: {change.Key}");
                }
                else if (env.Type == UpdateType.TrySetItem)
                {
                    // Key doesn't exist, so there's nothing to set
                    return (0, this, default, false);
                }

                // var entryIndex = Index(EntryMap, mask);
                var entryIndex = BitCount((int)EntryMap & ((int)mask - 1));

                // var entries = Bit.Set(EntryMap, mask, true);
                var entries = EntryMap | mask;

                var newItems = Insert(Items, entryIndex, change);
                return (1, new Entries(entries, NodeMap, newItems, Nodes), default, true);
            }
        }

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