cswin32-interop

compatibility	Requires the .NET SDK and the Microsoft.Windows.CsWin32 source generator; projects Windows APIs across .NET 10 and .NET Framework. Owner/extender composition requires C# 14.
description	Guides CsWin32 P/Invoke interop in a multi-targeted .NET 10 and .NET Framework library. Consult when replacing [DllImport] with source-generated PInvoke.* calls, working with generated Windows.Win32 projections (HANDLE / HMODULE / HRESULT / BOOL and typed enum/constant types), configuring NativeMethods.txt / NativeMethods.json, composing a public PInvoke across foundation/owner/extender packages with extensionReceiver, deciding which support library owns a helper, auditing native allocation and byte/element lengths, selecting compile-time / runtime / analyzer guards for Windows-only code, or choosing between CsWin32 and [LibraryImport]. Not for managed-only changes with no native boundary. Paired with the cswin32-com skill for the struct-based COM layer.
license	MIT
metadata	{"applicability":"dotnet","binding":"optional-overlay","github-path":"skills/cswin32-interop","github-pinned":"v0.11.0","github-ref":"refs/tags/v0.11.0","github-repo":"https://github.com/JeremyKuhne/agent-skills","github-tree-sha":"14960bec081ff29eaa9bf5593b7a6ca9d5201ae8","maturity":"canary","portability":"portable","related":"cswin32-com, dotnet-polyfills, scratch-buffer-strategy, security-review","requires":"none","risk":"local-write"}
name	cswin32-interop

CsWin32 P/Invoke interop

If overlay.md exists beside this file, read it before acting; it contains repository-specific bindings. This core remains usable without it.

CsWin32 is a source generator that replaces hand-written [DllImport] declarations with generated PInvoke.* calls and strongly-typed projections of Win32 handles, structs, enums, and constants. You list the API, type, or constant names you need in NativeMethods.txt; the generator emits them and pulls in their dependencies.

This skill is the general P/Invoke layer. The struct-based COM layer builds on it - see the paired cswin32-com skill, whose vtable methods follow the same blittable-signatures rules.

Rules

1. Replace a hand-written [DllImport] with PInvoke.* when CsWin32 can project the API. After verifying the generated declaration and supporting types, delete their hand-written equivalents. Keep a source-generated [LibraryImport] on .NET 10 and [DllImport] on .NET Framework for an export absent from the available Win32 metadata. See types-and-constants.md.
2. Use the generated types directly (HANDLE, HMODULE, HRESULT.S_OK, FILE_FLAGS_AND_ATTRIBUTES, ...). Never keep a parallel local copy of a Win32 enum or struct CsWin32 already projects.
3. Call the generated method directly - no thin wrapper. Within one ownership layer, generate once and share internal types with friend assemblies. When a downstream package intentionally extends a public owner, use the owner/extender composition model instead.
4. Prefer CsWin32 for Windows APIs present in its metadata. Use [LibraryImport] (or [DllImport] on .NET Framework) for private, custom, or otherwise unprojectable Windows exports and for non-Windows native calls such as libc. Keep those signatures blittable under the same rules.
5. Preserve the original error-handling contract when migrating. Check the old declaration for PreserveSig / SetLastError / BOOL / HRESULT semantics and reproduce them: PreserveSig=false becomes .ThrowOnFailure(); SetLastError=true plus a failed BOOL becomes throw new Win32Exception(). Silently returning where the old code threw is a behavior change. The paired cswin32-com skill has the COM-side parity table.
6. Keep signatures blittable. CsWin32 is configured allowMarshaling: false, so every [DllImport] and every COM vtable method must be blittable. The full rule set is in blittable-signatures.md.
7. Audit ownership and size units. Generated wrappers do not decide which allocator frees an output, whether a COM reference remains caller-owned, or whether a length is bytes or elements. Record and test those contracts using ownership-and-units.md.

Platform and target-framework guards

CsWin32 projects Windows APIs, but compile inclusion, runtime reachability, and the public platform contract are separate decisions. Generated declarations can live in a cross-platform assembly; reachable calls still need a recognized Windows runtime guard or a Windows-only API context. Exclude source at compile time only when a target intentionally omits that interop surface or one of its dependencies. In the .NET 10 / .NET Framework target model, use NET for the .NET 10 leg and NETFRAMEWORK for the Framework leg. See gating.md for the decision table, CA1416, and guarded-build verification. An overlay records the concrete TFMs and any repository-specific compile symbol or source-item condition.

Configuration

• NativeMethods.txt - one API / type / constant name per line; the generator projects each and pulls in its dependencies.
• NativeMethods.json - generator options. allowMarshaling: false (raw blittable signatures, no marshaller) and useSafeHandles: false are the interop-friendly defaults these skills assume.
• Run the generator in a single project within an ownership layer and share via InternalsVisibleTo rather than adding CsWin32 to every implementation project. A package that extends another package's public projection is a separate layer: configure className / extensionReceiver as described in composition.md.

Source generator versus build task

The Roslyn source generator is the default. Setting CsWin32RunAsBuildTask=true replaces it with an MSBuild task that writes .cs files under the intermediate output directory and adds them to Compile; the source generator stands down. Treat this as an opt-in for a specific build ordering, generated-file, or tooling requirement, not as a general upgrade.

Before keeping build-task mode, clean and build all .NET 10 and .NET Framework TFMs, compare the emitted public API, and measure clean-build time. It can be materially slower. It also does not make an allowMarshaling: false project more AOT-safe by itself, so do not enable DisableRuntimeMarshalling merely because generation moved to the task.

Sibling pages

• blittable-signatures.md - the signature rules shared by [DllImport] and COM vtables (HRESULT, T** vs out T*, void*, nint, PCWSTR / PWSTR, typed enum parameters).
• types-and-constants.md - using the generated handle / enum / constant projections, "grep the metadata before redefining", type conversions, FILETIME, and native integers.
• composition.md - composing one public PInvoke across an owner package and downstream extender, including constants, XML docs, and package verification.
• library-layering.md - placing generic utilities, shared Win32 support, domain extensions, and applications in a directed package stack, with migration and release ordering.
• ownership-and-units.md - matching native allocators and deallocators, caller-owned COM references, failure cleanup, and byte-versus-element conversions.
• gating.md - source inclusion, runtime and TFM guards, CA1416, a stack-first scratch-buffer strategy, and guarded-build verification.