Extension members ^{C# 14.0} added extension properties, methods and operators but left out indexers.

C# 15 completes the set by allowing indexers to be declared in an extension block so you can index into a type as though the indexer were declared on it.

Code

Now

IEnumerable<int> numbers = Enumerable.Range(1, 10);
int third = numbers[2];

public static class SequenceIndexer
{
    extension(IEnumerable<int> sequence)
    {
        public int this[int index] => sequence.ElementAt(index);
    }
}

Before

IEnumerable<int> numbers = Enumerable.Range(1, 10);
int third = numbers.GetAt(2);

public static class SequenceExtensions
{
    public static int GetAt(this IEnumerable<int> sequence, int index) => sequence.ElementAt(index);
}

Notes

• Indexers are always instance members so the extension block must declare a named receiver parameter
• Indexers declared on the type itself always win; extension indexers are only considered when none of the type's own indexers apply
• Indexer syntax looks cheap but this one calls ElementAt, which can walk the sequence and be O(n) for sources that aren't lists

More information

• What's new in C# 15.0
• C# Reference
• Feature specification