TheAlgorithms/C-Sharp · error · ArgumentException

Key already exists

Error message

Key already exists

What it means

HashTable.Add throws ArgumentException("Key already exists") when an entry with an equal key already occupies the computed bucket. Unlike Dictionary.Add, this table does not overwrite; duplicate insertion is treated as a programming error.

Solutions

1. Check table.ContainsKey(key) before calling Add, or use it to decide update vs insert.
2. Remove the existing entry first if overwrite semantics are wanted.
3. Catch ArgumentException and treat it as 'already present' if that is acceptable.

Example fix

// before
table.Add(key, value);
// after
if (!table.ContainsKey(key)) table.Add(key, value);

if (table.ContainsKey(key))
    throw new InvalidOperationException($"Key '{key}' already present");

try { table.Add(key, value); }
catch (ArgumentException ex) when (ex.Message == "Key already exists")
{
    // treat as duplicate insert; optionally update via Remove+Add
}

When it happens

Trigger: Calling Add twice with the same key without Remove in between, e.g. table.Add("a", 1); table.Add("a", 2); or re-running an idempotency-unsafe import that re-inserts existing keys.

Common situations: Data imports run twice; seeding a table in a loop over data containing duplicate keys; merging two datasets that share keys; retry logic re-executing an Add that already succeeded.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Key " " already in tree!
• Matrix must be square!
• Key " " already exists in AVL tree.
• Key " " already exists in B-Tree.
• Key " " already exists in tree!

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/6728062ae1b9e89d. Report an issue: GitHub.

Appendix: source

Thrown at DataStructures/Hashing/HashTable.cs:140

    /// If the number of elements in the hash table is greater than or equal to the threshold, the hash table is resized.
    /// </remarks>
    public void Add(TKey? key, TValue? value)
    {
        if (EqualityComparer<TKey>.Default.Equals(key, default))
        {
            throw new ArgumentNullException(nameof(key));
        }

        if (size >= threshold)
        {
            Resize();
        }

        var index = GetIndex(key);
        if (entries[index] != null &&
            EqualityComparer<TKey>.Default.Equals(entries[index]!.Key!, key))
        {
            throw new ArgumentException("Key already exists");
        }

        if (EqualityComparer<TValue>.Default.Equals(value, default))
        {
            throw new ArgumentNullException(nameof(value));
        }

        entries[index] = new Entry<TKey, TValue>(key!, value!);
        size++;
    }

    /// <summary>
    /// Removes the key-value pair associated with the specified key.
    /// </summary>
    /// <param name="key">Key to remove.</param>
    /// <returns>True if the key-value pair was removed, false otherwise.</returns>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="key"/> is null.</exception>
    /// <remarks>

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