C# 15 Extension Indexers: Add [] Syntax to Types You Don't Own
C# 15 lets you declare indexers inside extension blocks, so types you don't own can support square-bracket access. Adding Count and Slice also unlocks index-from-end, ranges, and list patterns. The feature was tested on the .NET 11 RC 1 SDK, and the article lists its gotchas: real indexers win silently, arrays and strings are excluded, and expression trees reject it.