C# 15 Extension Indexers Round Out Extension Members in .NET 11 Preview 6

.NET 11 Preview 6 shipped on July 14, 2026, and buried in the C# section of the release notes is the piece that finally makes extension members feel complete: extension indexers. Extension methods have been around since C# 3.0, extension properties arrived with the extension block in C# 14, and now you can bolt this[...] access onto a type you do not own.

What was missing until now

The obvious workaround has always been a plain extension method. If you wanted index-from-the-end semantics on an IReadOnlyList<T>, which does not carry an indexer that understands Index, you wrote something like this:

public static class ReadOnlyListExtensions
{
    public static T At<T>(this IReadOnlyList<T> list, Index index)
        => list[index.GetOffset(list.Count)];
}

// call site
IReadOnlyList<string> log = ["start", "work", "done"];
Console.WriteLine(log.At(^1));   // done
Console.WriteLine(log.At(^2));   // work

It works, but the call site reads log.At(^1) instead of the log[^1] syntax that every other list-like type gives you. You lose the square-bracket notation, and you lose it in list patterns too.

The extension indexer version

C# 15 lets you declare the indexer inside an extension block, exactly the way you would declare an instance indexer:

public static class ReadOnlyListExtensions
{
    extension<T>(IReadOnlyList<T> list)
    {
        public T this[Index index] => list[index.GetOffset(list.Count)];
    }
}

// call site
IReadOnlyList<string> log = ["start", "work", "done"];
Console.WriteLine(log[^1]);   // done
Console.WriteLine(log[^2]);   // work

The receiver captured by the extension<T>(IReadOnlyList<T> list) header is in scope inside the body, so the indexer forwards to list directly. The call site now uses real indexer syntax, and because the compiler treats it as an indexer, it participates in list patterns and range expressions the same way a built-in one would.

Extension indexers are not limited to the one-parameter, get-only shape. They support multiple parameters and both get and set accessors, so you can project a two-dimensional lookup onto a flat backing store or expose a writable view over a type that only offers method-based access.

Turning it on

This is still a preview language feature, so it does not light up on the default language version. Add the following to your .csproj:

<LangVersion>preview</LangVersion>

Extension indexers are the last of the three classic member kinds to reach extension blocks, which means the mental model is now uniform: methods, properties, and indexers all declare the same way inside extension(...). If you have been reaching for wrapper types or awkward .At() helpers to paper over a missing indexer, Preview 6 is the release that lets you delete them. See the C# section of the Preview 6 release notes for the full write-up.