Remove Duplicates From a List in C#: fix the manual loops, add objects and in-place, .NET 10

[vladimir-pecanac-main]

Sample for https://code-maze.com/remove-duplicates-from-a-list-csharp/

Only collections-lists/RemoveDuplicatesFromLists changes.

What changes:

• All three projects move from net6.0 to net10.0.
• UsingIterationsAndShifting() and UsingIterationsAndSwapping() never wrote to the list: the inner loop assigned to a local variable, so Take(n) only truncated the input. On { 1, 1, 2, 3, 4, 5 } both returned { 1 }. Both now work on a copy of the list and write back into it (shift assigns list[k] = list[k + 1], swap exchanges list[j] and list[size]).
• The old fixture { 1, 2, 1, 2 } could not catch that. New theory tests run every method against { 1, 1, 2, 3, 4, 5 } and { 3, 1, 3, 2, 1, 4, 2, 5, 4 }. On the old helper four of these cases fail (the two loop methods, both fixtures); on this branch all 41 tests pass.
• WhenUsingDictionary_ThenRemovesDuplicates() called ConvertingToHashSet(); it now calls UsingDictionary().
• UsingRecursion() returned the input list from its stop branch; it now returns the list without duplicates (same result, the outer call already discarded that value).
• where T : notnull on RemoveDuplicatesHelper clears CS8714 and CS8602 on net10.0.
• New: RemoveDuplicatesInPlace() (List.RemoveAll with a HashSet), Person, PersonRecord and PeopleHelper (Distinct on a class keeps equal-valued objects, Distinct on a record removes them, DistinctBy by Email), each with tests. The in-place test asserts the same list instance changed.
• Benchmark: [Params(3, 100, 2_000)] distinct values in a 2,000 item list built in [GlobalSetup]; HashSetMethod and InitializingHashetMethod renamed to ConvertToHashSetMethod and InitializingHashSetMethod; SortMethod and the new RemoveAllInPlaceMethod start each call from a fresh copy because both change their list.

Packages (re-queried on NuGet 2026-10-04, newest stable):

• Microsoft.NET.Test.Sdk 18.10.1
• xunit 2.9.3
• xunit.runner.visualstudio 4.0.0
• coverlet.collector 10.1.0
• BenchmarkDotNet 0.15.8

Local: SDK 10.0.302, runtime 10.0.10. dotnet build -c Release: 0 warnings, 0 errors. dotnet test: 41 passed, 0 failed. dotnet list package --vulnerable --include-transitive: none.