Incorrect loop bound in SizeOptIListSelectIterator.ToList

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Since Nov 7, 2026.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
64/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
csharp
Domain
backend

Research direction

Find SizeOptIListSelectIterator<TSource, TResult>.ToList() in the .NET runtime source and read the iterator implementation introduced in PR #118156. Reproduce the issue with <UseSizeOptimizedLinq>true</UseSizeOptimizedLinq> using the supplied reflection example. Done when the override returns the selected items and the result matches normal query enumeration; the issue suggests fixing or removing this non-public path.

Written by the indexing model from the issue text.

Description

area-System.Collections untriaged
Description

SizeOptIListSelectIterator<TSource, TResult>.ToList() checks the destination list’s Count immediately after constructing it with a capacity. Since Count is zero, the loop never executes and the override returns an empty list for a nonempty source.

Admittedly, this path is non-public and functionally dead, but it hides a ticking time bomb, so would be nice to fix for good hygiene (or remove altogether)

Reproduction Steps

With true

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

IEnumerable<int> query = new[] { 1, 2, 3 }.Select(x => x * 10);

MethodInfo method = query.GetType().GetMethod(
    "ToList",
    BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly)
    ?? throw new InvalidOperationException("ToList override not found");

List<int> result = (List<int>)(method.Invoke(query, null)
    ?? throw new InvalidOperationException("ToList returned null"));

Console.WriteLine(query.GetType().Name);
Console.WriteLine($"Reflection: [{string.Join(", ", result)}]");
Console.WriteLine($"Public API: [{string.Join(", ", query.ToList())}]");
Expected behavior

printing [10, 20, 30], matching enumeration of the query

Actual behavior

prints []

Regression?

The erroneous loop was included when this iterator was introduced in #118156

Known Workarounds

No workaround is needed

Configuration

Windows 10, x64
Reproduced on .NET 10, 11 rc3 and main

Other information
List<TResult> list = new List<TResult>(_source.Count);
for (int i = 0; i < list.Count; i++)
{
    list.Add(_selector(_source[i]));
}
return list;

The constructor initializes Capacity, not Count

Dominant language
C#
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