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Enumerable.DistinctBy keeps the first element per key. To keep the last one and still return results in the order they appear in the source, buffer the input, scan it backwards with a HashSet, and yield the survivors forwards. Eight implementations checked for order, null keys, double enumeration and argument validation on .NET 10.0.10 and .NET 11 RC3, and benchmarked on .NET 10.

Put [MemberNotNullWhen(true, nameof(X), nameof(Y))] on the HasValue property itself, naming the members that become non-null. It works for nullable value types and reference types, but the compiler barely checks the getter, ignores base-class members, and never notices when the state changes afterwards. Verified on .NET 10 SDK 10.0.302 and .NET 11 RC3.

ModelContextProtocol 2.4.0 ships ModelContextProtocol.Extensions.Skills, an experimental package that implements the SEP-2640 Skills extension: one call to publish a folder of SKILL.md directories, and client helpers that verify every file against a SHA-256 manifest.

OrderBy(...).ThenBy(...) runs every key selector for every element before it compares anything, even when the OrderBy keys are already unique. Measured on .NET 10 and .NET 11 RC3: what the runtime does, why the popular lazy-key workaround still computes keys it never needs, and three patterns that call an expensive secondary key only for tied elements.

On November 5, 2026 Microsoft added a Windows Arm64 migration scenario to the GitHub Copilot upgrade agent. Here is what the blockers look like in a real .NET project, what the agent can and cannot verify, and how to check the result yourself.

You changed a List or Dictionary while a foreach was walking it. Iterate a copy, use RemoveAll, loop backwards with for, or lock both sides. Which mutations throw on .NET 10 and 11, and why Dictionary.Remove does not.

foreach over (ReadOnlySpan<T>)[a, b] allocates nothing on .NET 8 and later and works for reference types, which stackalloc cannot do. On .NET 10 the compiler backs it with the BCL's InlineArray2<T>, and the JIT can even stack-allocate a plain new[] { a, b } once the method reaches Tier 1. Measured on .NET 10.0.10 and .NET 11 RC 3.

1.10m prints "1.10" because System.Decimal stores a scale alongside its digits, and arithmetic, parsing, and JSON all preserve it. How the scale is chosen, why 1.1m == 1.10m but their strings differ, three ways to strip trailing zeros (G29, a 0.## pattern, dividing by 1.000...m) and the 1E-05 trap in G29, how to force a fixed scale, and why .NET 11's double to decimal change produces digits no trick removes. Verified on .NET 10.0.14 and .NET 11 RC3.

AutoMapper 16.3.0 and 15.1.4 fix two high-severity advisories where deeply nested input bypassed MaxDepth and crashed the process with an uncatchable StackOverflowException. Here is which mapping paths were exposed and what changes when you upgrade.

List<T>.Sort, Array.Sort, Span<T>.Sort and ImmutableArray.Sort are unstable introsorts, so items with equal keys can swap places. Enumerable.OrderBy is stable. Measured on .NET 10: why lists of 16 or fewer items look stable, why three items are not, how to get a stable in-place sort, and what it costs.