cswin32-com

name	cswin32-com
description	Guides struct-based COM interop using CsWin32 - AOT-compatible raw pointer calls without [ComImport] or built-in CLR marshalling; managed [ComImport] mirrors are limited to CCW bridges. Consult when working with ComScope, IID.Get, delegate* unmanaged vtables, CoCreateInstance or a class factory, IComIID across .NET 10 and .NET Framework, connection-point Advise/Unadvise ownership, caller-owned CCW references, manually defining COM interfaces absent from Win32 metadata, COM pointer lifetime in fields, cross-assembly CCWs, or mocking struct-based COM in tests. Not for ordinary RCW-based COM consumption with no raw struct pointers or CCW bridge. Paired with the cswin32-interop skill for the general P/Invoke layer and blittable-signature rules.
license	MIT
compatibility	Requires the .NET SDK and the Microsoft.Windows.CsWin32 source generator; Windows COM APIs across .NET 10 and .NET Framework. Cross-assembly extension-block examples require C# 14.
metadata	{"portability":"portable","applicability":"dotnet","binding":"optional-overlay","risk":"local-write","maturity":"canary","requires":"cswin32-interop","related":"security-review, il-copy-inspection"}

CsWin32 struct-based COM interop

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

Struct-based COM interop on top of CsWin32: raw delegate* vtable calls and no built-in CLR marshalling on the native pointer surface, so it is AOT- and trimming-friendly. A managed [ComImport] mirror is allowed only as a narrow CCW bridge; runtime calls still use the generated struct. This skill covers the COM-specific layer; the general P/Invoke layer and the blittable-signature rules that vtable methods also follow live in the paired cswin32-interop skill.

Helper conventions

Examples use ComScope<T> for a disposable scope that owns one AddRef'd COM reference and IID.Get<T>() for a support helper that returns a Guid* to T's IComIID.Guid. CsWin32 generates IComIID, but these two helpers come from a support library. The overlay names their concrete implementations. If a repository does not provide them, use an equivalent owned-pointer scope and a stable IID pointer; keep the same ownership and ABI contracts.

AcquireOwnedCcwPointer<T>() in examples is pseudocode for a repository helper that returns one AddRef'd, caller-owned interface pointer for a managed object. Its implementation determines compatibility: classic Marshal.GetComInterfaceForObject uses built-in COM interop and is not a NativeAOT path, while a ComWrappers implementation can be AOT-compatible.

Workflow

1. In Win32 metadata? If yes, add the interface name to NativeMethods.txt and CsWin32 generates it. If not (for example, Setup Configuration or a private / third-party interface), define a manual struct in its own file - see manual-structs.md.
2. Lifetime: using ComScope<T> scope = new(); for every transient owned COM reference, and free every owned COM BSTR out-param (SysFreeString / FreeBSTR, or a repo-provided scoped BSTR wrapper). A correct try / finally also releases on early return, but an owned-pointer scope centralizes initialization, release, and ownership transfer. Never release a borrowed pointer unless the caller first acquires a reference. See lifetime.md.
3. Activate via a class-factory helper or CoCreateInstance with the exact interface IID; prefer IID.Get<T>() when the support helper is available. See Activation.
4. Call via scope.Pointer->Method(...). Pass ComScope<T> directly where the API expects T** / void** when the concrete scope provides those conversions.
5. Match the caller's error contract. If the top-level consumer treats COM failure as a documented "absent" result, helpers may return default / false; otherwise call .ThrowOnFailure(). Do not change a shared helper's contract merely because one caller catches COMException. See the parity table in migration-and-testing.md.
6. Gate .NET 10-only COM features with #if NET. ComWrappers-based CCW support is unavailable on .NET Framework. Generated IComIID works on both supported runtime families with CsWin32 0.3.287 or later; see comiid-and-cls.md.
7. Compose across packages at the owner boundary. The owner implements generated partial hooks and publishes shared COM structs; extenders add behavior with extension blocks or uniquely named CCW providers. See ccw-composition.md.

Activation

// Via CoCreateInstance and the repository's IID support helper.
Guid clsid = SomeCoClass.CLSID;
using ComScope<ISomething> instance = new();
PInvoke.CoCreateInstance(
    &clsid, null, CLSCTX.CLSCTX_INPROC_SERVER, IID.Get<ISomething>(), instance)
    .ThrowOnFailure();
instance.Pointer->DoThing(...);

• Prefer IID.Get<T>() when available; it returns a Guid* to the type's IComIID.Guid without a per-call copy. Otherwise pass a pointer to a stable Guid containing the exact IID. A local Guid is valid for the duration of the native call; the problem is an incorrect or ad hoc IID, not local storage.
• Initialize ComScope<T> with new(); it implicitly converts to the T** / void** output parameter when the concrete support type supplies those conversions.
• A repo may also ship a class-factory helper (activate via CoGetClassObject then CreateInstance), the AOT-friendly path when a CLSID is registered - see the overlay.

Sibling pages

• lifetime.md - ComScope<T>, BSTR, owned and borrowed references, field-lifetime strategies, and ownership transfer out of a helper.
• manual-structs.md - defining an interface not in Win32 metadata: delegate* unmanaged[Stdcall] vtables, exact slot indices, the dual-target IComIID member, and strongly-typed handle/token wrappers.
• comiid-and-cls.md - IComIID across .NET 10 and .NET Framework (both-family auto-emit versus the older Framework per-struct polyfill) and CLS compliance.
• migration-and-testing.md - the error-handling parity table when migrating off [ComImport], and mocking struct-based COM in tests via a CCW bridge.
• ccw-composition.md - owner-side IUnknown vtable initialization, extending imported COM structs across assemblies, and local CCW provider types.