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PathIo: generate System.IO forwarding APIs with a Roslyn source generator - #249
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ForNeVeR
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October 21, 2026 09:20
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…rator Add TruePath.PathIoGenerator, a Roslyn incremental source generator that fills the other partial part of PathIo (marked with GenerateSystemIoExtensionsAttribute) with forwarding extension methods for every public static File/Directory method of the current target framework, with the BCL XML documentation copied verbatim. - PathIo.cs keeps only the members with custom behavior or documentation; pure forwarders are generated (93 methods on net8.0, 66 on netstandard2.0), introducing previously missing APIs (symlinks, OpenHandle, Replace, EnumerateDirectories, GetParent, etc.). - Hand-written members win over generated ones, both by rewritten signature and by the forwarding call inside their body; File wins over Directory on duplicate signatures; path-io-exclusions.txt lists APIs deliberately not exposed. - BCL XML docs reach the generator via AdditionalFiles (the command-line compiler does not attach them to references), are extracted line-by-line to survive malformed netstandard.xml, and param/paramref names are normalized against the generated parameters; path parameters keep the historical sourceFile/destFile names. - Generated code uses short type names for imported namespaces. - Tests: generator unit tests plus a reflection completeness test asserting BCL API ⊆ generated ∪ hand-written ∪ exclusions.
…ndex - Drop dead test helpers, redundant nullability and qualifiers, an unused RenderFile parameter, and a redundant currentName check in BclDocIndex. - Store parsed XElement members in the doc index instead of raw strings: XElement.Parse is now called exactly once per member with its result actually used, instead of a validation-only call with a discarded return value followed by a second parse at emission time. The cached element is cloned before doc formatting, which renames param elements in place.
- Strip the UTF-8 BOM from TruePath.sln introduced by an editor. - Add a REUSE license companion for path-io-exclusions.txt. - Pin the generator test references to the Basic.Reference.Assemblies package: discovering the SDK's reference packs from the test host's runtime directory is not reliable on CI runners where the runtime and the SDK live in different roots. The test helper now also surfaces the driver diagnostics when the expected generated tree is missing. - The completeness test now mirrors the generator's third exclusion rule: BCL overloads that hand-written members forward to with path.Value as the first argument (previously flagged Directory.Delete(string,bool)).
The SPDX tags emitted into the generated file are string literals in the generator source; REUSE picked them up as this file's own tags and failed on the trailing C# syntax inside the expression.
- Reject nested marked classes with the new TPG004 diagnostic instead of silently generating a same-named top-level class, and derive the hint name from the full type name (sanitized) so same-named types cannot collide. - Hoist the exclusion file texts and the XML documentation index into their own cached pipeline nodes: the .xml filter now happens before BclDocIndex.Build, so editing path-io-exclusions.txt no longer re-scans the ~15 MB documentation files, and the exclusion file is matched by exact file name. - Move the hand-written member analysis into the syntax-anchored ForAttributeWithMetadataName transform (equatable MarkedType payload with sorted arrays), so it is cached across edits in unrelated files instead of re-running on every compilation change. - Clone the member element inside DocFormatting.ToDocLines rather than at the call site, and read the XML documentation files through SourceText.Lines instead of ToString/Replace/Split (three fewer full copies of the text). Generated output verified byte-identical on both target frameworks before and after the change.
…int names - Render every special type with its C# keyword inside generic arguments too: net10 span-based APIs like ReadOnlySpan<char> previously rendered as ReadOnlySpan<Char> because System_Char was missing from the table. - Emit the generated file under the simple type name again (PathIo.Generated.g.cs): the full-name sanitization was only needed for nested marked types, which are now rejected with TPG004. Both verified by temporarily building TruePath.SystemIo against net10.0, which generates 112 methods including the new span- and Memory-based overloads without any rule changes.
A handwritten member only covers the BCL overload it forwards to when its own parameter list is the leading part of the overload's generated parameter list, i.e. when it is that overload with its trailing arguments pinned. Overload identity is the parameter list alone, so a member that forwards to the same BCL method with a different parameter type is an addition next to the generated one rather than a replacement for it. GetFiles(AbsolutePath, LocalPathPattern) is exactly that case: it forwards to Directory.GetFiles(string, string) but replaces the second parameter type, so the old blanket "forwards to it" rule suppressed GetFiles(AbsolutePath, string) and silently dropped it from the public API. Signature identity also ignores nullability now, because C# overloads do not differ by it and because a reference assembly compiled before nullable reference types existed reports every reference type as oblivious. AppendAllText and WriteAllText keep handwritten `string? contents` overloads for the same reason: netstandard2.0's reference assembly has no NRT metadata, so the generator would render a non-nullable parameter and break the released contract. The BCL accepts null there by design (it writes an empty file or nothing), which the released baseline records as `string?`. The public API baselines gain the newly generated members: 25 for net8.0 and 16 for netstandard2.0.
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Directory.GetParent returns null for a filesystem root, and the net8.0 baseline already records GetParent as `DirectoryInfo?`. The netstandard2.0 reference assembly carries no NRT metadata, so the generator renders the same member as non-nullable and the baseline recorded a `DirectoryInfo!` contract that the implementation does not honour. Declare the member by hand for the same reason AppendAllText and WriteAllText are handwritten. Also compare generated overload keys against handwritten ones with the nullability-insensitive renderer on both sides. Previously the generated key kept its annotations while the handwritten key dropped them, so the two only matched for oblivious reference assemblies.
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Add a net10.0 target (the current LTS) to TruePath.SystemIo. With the source generator from ForNeVeR#249 in place this is nearly a one-line change: the generator projects the per-target-framework BCL automatically, exposing 123 extension methods on this target, including the new span- and Memory-based overloads: - AppendAllText/WriteAllText with ReadOnlySpan<char> contents, - AppendAllBytes/WriteAllBytes with ReadOnlySpan<byte>, - the ReadOnlyMemory-based async counterparts of all of the above. The existing netstandard2.0 and net8.0 targets are unchanged. The public API baselines gain the net10.0 surface. The four handwritten `string?` overloads and GetParent are kept as handwritten members here as well, so the nullable contracts match the other targets and only the ReadOnlySpan overloads are generated alongside them. TruePath.Tests also gains a net10.0 target so the reflection completeness test validates the generated net10.0 surface against the matching runtime.
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Summary
Introduce a Roslyn incremental source generator (TruePath.PathIoGenerator) that fills the other partial part of PathIo (marked with [GenerateSystemIoExtensions]) with forwarding extension methods for every public static File/Directory method of the current target framework, copying the BCL XML documentation verbatim. PathIo.cs shrinks to the members with custom behavior or documentation.
Changes
Testing