louthy/language-ext · error · IndexOutOfRangeException

IndexOutOfRangeException

Error message

IndexOutOfRangeException

What it means

Set.CopyTo throws IndexOutOfRangeException when the index is negative, greater than the array length, or when index + Count exceeds the array capacity. It is the bounds check ensuring the set's elements fit into the target array starting at the given offset.

Solutions

1. Size the array as index + set.Count (or just set.Count when starting at 0) before copying
2. Copy from index 0 into a fresh arr = new A[set.Count]
3. Use ToArray()/ToArray extension to avoid manual bounds math
4. Clamp/validate caller-supplied index against array bounds before invoking

Example fix

// before
var buf = new A[10];
set.CopyTo(buf, 5); // needs Count <= 5
// after
var buf = new A[set.Count];
set.CopyTo(buf, 0);

if (array == null || index < 0 || index > array.Length || array.Length - index < set.Count) return false;
set.CopyTo(array, index);

static bool FitsInArray<A>(Set<A> set, A[] arr, int idx) => arr != null && idx >= 0 && idx <= arr.Length && arr.Length - idx >= set.Count;

try { set.CopyTo(arr, idx); }
catch (IndexOutOfRangeException) { arr = new A[set.Count]; set.CopyTo(arr, 0); }

When it happens

Trigger: Calling set.CopyTo(arr, idx) with idx < 0, idx > arr.Length, or arr.Length - idx < set.Count.

Common situations: Reusing a small scratch buffer for a larger set; offset arithmetic errors (page/segment-based copies); forgetting the set can grow between sizing the array and copying.

Related errors

• ArgumentNullException
• Ord attribute should have a struct type that derives from…
• Hashable attribute should have a struct type that derives…
• Don't use Equals - use either RecordType
• Don't use Equals - use either RecordType

AI-assisted analysis of louthy/language-ext@2f0e362824 (2026-09-15). Data as JSON: /api/errors/e96ddae544629bf1. Report an issue: GitHub.

Appendix: source

Thrown at LanguageExt.Core/Immutable Collections/Set/Internal/Set.Internal.cs:660

        foreach (A item in other)
        {
            if (Contains(item))
            {
                return true;
            }
        }
        return false;
    }

    /// <summary>
    /// Copy the items from the set into the specified array
    /// </summary>
    /// <param name="array">Array to copy to</param>
    /// <param name="index">Index into the array to start</param>
    public void CopyTo(A[] array, int index)
    {
        if (array == null) throw new ArgumentNullException(nameof(array));
        if (index < 0 || index > array.Length) throw new IndexOutOfRangeException();
        if (index + Count > array.Length) throw new IndexOutOfRangeException();

        foreach (var element in this)
        {
            array[index++] = element;
        }
    }

    /// <summary>
    /// Copy the items from the set into the specified array
    /// </summary>
    /// <param name="array">Array to copy to</param>
    /// <param name="index">Index into the array to start</param>
    public void CopyTo(Array array, int index)
    {
        if (array == null) throw new ArgumentNullException(nameof(array));
        if (index < 0 || index > array.Length) throw new IndexOutOfRangeException();
        if (index + Count > array.Length) throw new IndexOutOfRangeException();

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