Declare EmbeddedAttribute as partial so it can coexist with Reflectify

[dennisdoomen]

The problem

Pathy ships as source and compiles into the consuming assembly. It declares Microsoft.CodeAnalysis.EmbeddedAttribute so its own types can be marked [Embedded]. That attribute tells Roslyn to hide those types from every other assembly, including InternalsVisibleTo friends.

Reflectify is another source-only package that declares the same attribute for the same reason. A project that uses both packages ended up with two declarations of one type in a single compilation:

CS0101: The namespace 'Microsoft.CodeAnalysis' already contains a definition for 'EmbeddedAttribute'

This is not theoretical. FluentAssertions already consumes Reflectify, so any project combining it with Pathy would hit this.

The fix

Declare the attribute as partial:

internal sealed partial class EmbeddedAttribute : Attribute

Two partial declarations of the same type merge into one type instead of colliding. Roslyn matches [Embedded] by name, so it does not care how many parts the type came from. Embedding keeps working exactly as before.

Reflectify made the matching change in dennisdoomen/reflectify#170. Both sides are needed. Neither one works alone.

Why the type-level attributes had to be removed

This part is not optional, and it is easy to undo by accident.

C# does not allow a non-AllowMultiple attribute to be repeated across the parts of a partial type. If both Pathy's part and Reflectify's part carry [AttributeUsage], the build fails:

CS0579: Duplicate 'System.AttributeUsage' attribute

So re-adding [AttributeUsage(AttributeTargets.All)] would silently bring the collision straight back, just with a different error code. Please do not add it back. It is also redundant: AttributeTargets.All is already the default when an attribute carries no [AttributeUsage], so removing it changes nothing at runtime.

[ExcludeFromCodeCoverage] was removed for the same reason.

Both parts must have an identical shape for the merge to work: same accessibility (internal), same sealed, same base type, and no type-level attributes. Reflectify's copy is now internal sealed partial class EmbeddedAttribute : System.Attribute { }. An XML <remarks> block on the declaration records this constraint for the next reader.

Analyzer suppressions

None were needed. Removing [AttributeUsage] normally triggers CA1018, MA0010 and RCS1203, and this repo escalates warnings to errors. I checked by forcing the analyzer package set on (it is currently gated on a net6.0 target framework that no project in this repo builds) and confirmed those three analyzers do fire on a plain test attribute type in this project, but none of them fire on this declaration. No #pragma and no .editorconfig change was required.

Verification

• dotnet build -c Release on the full solution: 0 warnings, 0 errors.
• Tests: Pathy.Specs net8.0 109 passed, Pathy.ApiVerificationTests net8.0 8 passed, 0 failures. The net472 run reports "No test is available ... make sure that test discoverer & executors are registered". I confirmed this happens on unmodified main as well, so it is a pre-existing local environment issue and not a regression from this change.
• A scratch consumer project outside the repo compiled Pathy's sources the way a real consumer does (without PATHY_PUBLIC), next to a second file holding Reflectify's declaration, with InternalsVisibleTo pointing at a friend assembly. Results:
  • the library compiles with both declarations present,
  • the friend assembly can still use the library's own internal types,
  • the friend assembly cannot see Pathy.ChainablePath (CS0234), so embedding still works.
• Two negative controls confirmed the reasoning rather than assumed it: reverting partial reproduces the collision (CS0260), and putting [AttributeUsage] back on both parts reproduces CS0579.

The scratch project was deleted afterwards.

[github-actions]

Test Results

117 tests  ±0   117 ✅ ±0   1s ⏱️ ±0s
  3 suites ±0     0 💤 ±0 
  3 files   ±0     0 ❌ ±0 

Results for commit be3dad8. ± Comparison against base commit 6b9ac41.