TheAlgorithms/C-Sharp · error

item

Error message

item

What it means

FibonacciSearcher.FindIndex throws ArgumentNullException with parameter name "item" when the search item itself is null. Since searching for null has no defined position in the ordering the searcher assumes, the library rejects it up front as an invalid argument.

Solutions

1. Guard the item before calling: if (item is null) return -1; or throw your own descriptive error.
2. Fix the source that produced the null key (binding, deserialization, optional field default).
3. Use a sentinel/default value of T for reference types when 'no key' should mean 'no search'.
4. Catch ArgumentNullException if null keys are expected and should map to not-found.

Example fix

// before
return searcher.FindIndex(array, userKey);
// after
if (userKey is null) return -1;
return searcher.FindIndex(array, userKey);

if (item is null) return -1; // or surface a validation error to the caller

bool HasKey<T>(T? item) => item is not null;

try { return searcher.FindIndex(array, item); }
catch (ArgumentNullException ex) when (ex.ParamName == "item") { return -1; }

When it happens

Trigger: Calling FindIndex(array, null) — typically a search key that came from an uninitialized variable, a failed map/deserialization, or an optional UI input left empty.

Common situations: Nullable reference types disabled so null keys flow silently; record fields defaulting to null; queries built dynamically where the user-supplied key was never assigned.

Related errors

• array
• searchItem
• sortedArray
• Input string cannot be null.
• Value cannot be null. (Parameter 'collection')

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/98853abd36b285d1. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Search/FibonacciSearcher.cs:29

    ///     Time complexity:
    ///     worst-case: O(log n),
    ///     average-case: O(log n),
    ///     best-case: O(1).
    /// </summary>
    /// <param name="array">Sorted array to be searched in. Cannot be null.</param>
    /// <param name="item">Item to be searched for. Cannot be null.</param>
    /// <returns>If an item is found, return index. If the array is empty or an item is not found, return -1.</returns>
    /// <exception cref="ArgumentNullException">Gets thrown when the given array or item is null.</exception>
    public int FindIndex(T[] array, T item)
    {
        if (array is null)
        {
            throw new ArgumentNullException("array");
        }

        if (item is null)
        {
            throw new ArgumentNullException("item");
        }

        var arrayLength = array.Length;

        if (arrayLength > 0)
        {
            // find the smallest Fibonacci number that equals or is greater than the array length
            var fibonacciNumberBeyondPrevious = 0;
            var fibonacciNumPrevious = 1;
            var fibonacciNum = fibonacciNumPrevious;

            while (fibonacciNum <= arrayLength)
            {
                fibonacciNumberBeyondPrevious = fibonacciNumPrevious;
                fibonacciNumPrevious = fibonacciNum;
                fibonacciNum = fibonacciNumberBeyondPrevious + fibonacciNumPrevious;
            }

View on GitHub (pinned to 96e2905cab)