Advent-of-Code-2021 VS MoreLINQ

You can grab MoreLinq off of Nuget for pre-.NET6 and it has a DistinctBy implementation that you might be able to use: https://github.com/morelinq/MoreLINQ

Install "morelinq" 3.3.2 from Nuget: https://www.nuget.org/packages/morelinq You know you got the right one if it has about 37.2 million downloads.

If you don't already know about it, I highly recommend checking out the MoreLINQ open source project which adds a bunch of these less common but still occasionally useful extensions to LINQ. They also include an Index method which does exactly what your WithIndex method does, yielding out KeyValuePair entries for each iteration.

yield return is a significant core piece of the framework. It's the basis of which most deferred execution and LINQ is built upon and any extension one would want to add to LINQ. (for example)

For a time, I'd say the Query Syntax was comparatively as popular as the Method Syntax. But over the years as LINQ became ubiquitous for working with non-database queries (LINQ-to-Objects) and more users extended them (for example, MoreLinq) the Method Syntax became more popular. I'd say nowadays the Query Syntax is relatively rare to see and perhaps mostly limited to direct database queries or these scenarios like "join" that are easier.

I suggest morelinq https://github.com/morelinq/MoreLINQ

If you don't already know about it, take a gander at the MoreLINQ extensions. I don't think it solves this particular problem, but it's a great example of extending LINQ.

It's part of the MoreLinq library. The Window() function code is here.