louthy/language-ext · error · IndexOutOfRangeException

IndexOutOfRangeException

Error message

IndexOutOfRangeException

What it means

SeqEmptyInternal is the empty sequence sentinel; its indexer always throws IndexOutOfRangeException because an empty sequence has no elements. Any code path that indexes an empty Seq without checking lands here.

Solutions

1. Check seq.IsEmpty / seq.Count > 0 before indexing
2. Use HeadOrNone / At which return Option instead of throwing
3. Pattern-match: seq.Match(x => x[0], () => fallback)

Example fix

// before
var first = seq[0];
// after
var first = seq.HeadOrNone().IfNone(default(A));

bool hasFirst = seq.Count > 0;

Option<A> TryFirst<A>(Seq<A> seq) => seq.HeadOrNone();

try { x = seq[index]; }
catch (IndexOutOfRangeException)
{ x = fallback; }

When it happens

Trigger: Indexing an empty Seq via seq[i] (the empty-seq representation is SeqEmptyInternal); accessing seq[0] after filtering/Where that removed all elements.

Common situations: Filtering a collection and assuming at least one result remains; indexing Head-less results of Empty, Filter, or Take(0); off-by-one on zero-length input.

Related errors

• IndexOutOfRangeException
• IndexOutOfRangeException
• IndexOutOfRangeException
• Nil iterator has no head
• Nil iterator has no head

AI-assisted analysis of louthy/language-ext@2f0e362824 (2026-09-15). Data as JSON: /api/errors/d72a29b97b293a11. Report an issue: GitHub.

Appendix: source

Thrown at LanguageExt.Core/Immutable Collections/Seq/DSL/SeqEmptyInternal.cs:18

using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using LanguageExt.Common;

namespace LanguageExt;

internal class SeqEmptyInternal<A> : ISeqInternal<A>
{
    public static ISeqInternal<A> Default = new SeqEmptyInternal<A>();

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ReadOnlySpan<A> AsSpan() =>
        ReadOnlySpan<A>.Empty;

    public A this[int index] => 
        throw new IndexOutOfRangeException();

    public Option<A> At(int index) => 
        default;

    public A Head =>
        throw Exceptions.SequenceEmpty;

    public ISeqInternal<A> Tail =>
        this;

    public bool IsEmpty => 
        true;

    public ISeqInternal<A> Init =>
        this;

    public A Last =>
        throw Exceptions.SequenceEmpty;

View on GitHub (pinned to 2f0e362824)