Avoid allocations in GetHashCode implementations

[DustinCampbell]

Two simple tweaks to stop calling ToLowerInvariant() and ToUpperInvariant() in GetHashCode implementations in order to produce a case-insensitive hash. Instead, use StringComparer.OrdinalIgnoreCase.GetHashCode(...) to avoid allocating strings in a potentially hot path.

Context

Calling ToLowerInvariant() or ToUpperInvariant() inside GetHashCode allocates a temporary string just to compute a hash — an unnecessary allocation in a potentially hot path. Using StringComparer.OrdinalIgnoreCase.GetHashCode(...) produces the same case-insensitive result without any allocation.

Changes Made

• Replace ToLowerInvariant()/ToUpperInvariant()-based hashing with StringComparer.OrdinalIgnoreCase.GetHashCode(...) in TargetPlatformSDK.GetHashCode and SdkReference.GetHashCode.
• Fix SdkReference.GetHashCode to always apply the (hashCode * 397) position multiplier for every field (treating null as 0), preserving the original combining behavior and preventing hash collisions between different null patterns (e.g. {Name, null, "bar"} vs {Name, "bar", null}).

Testing

• Existing SdkReference unit tests in Microsoft.Build.Framework.UnitTests pass.

Notes

The SdkReference.GetHashCode fix ensures the hash-combining pattern is positionally stable: skipping the * 397 multiply when a field is null would cause different null arrangements to produce identical hashes.

[baronfel]

Is there some analyzer for this that we could use? "Prefer GetHashCode from Comparer instead of string instance"?

[Copilot]

Pull request overview

Updates GetHashCode implementations to avoid allocating temporary strings for case-insensitive hashing, which is beneficial in potentially hot paths and better aligns hashing with the existing OrdinalIgnoreCase equality semantics.

Changes:

• Replace ToLowerInvariant()/ToUpperInvariant()-based hashing with StringComparer.OrdinalIgnoreCase.GetHashCode(...).
• Minor refactor of SdkReference.GetHashCode control flow while preserving case-insensitive behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Utilities/TargetPlatformSDK.cs	Switches to StringComparer.OrdinalIgnoreCase in GetHashCode to avoid allocations.
src/Framework/Sdk/SdkReference.cs	Uses StringComparer.OrdinalIgnoreCase for hashing of Name/Version/MinimumVersion to avoid allocations.

[DustinCampbell]

Is there some analyzer for this that we could use? "Prefer GetHashCode from Comparer instead of string instance"?

I don't know if there's an existing analyzer. However, it's fine to call GetHashCode on a string instance. The issue is with allocating a new throwaway string in order to call GetHashCode.

[rainersigwald]

Had me scared for a second here, but fortunately these are both low-frequency uses, not our hottest hash path. Still worth fixing!