dotnet-pinvoke

What this skill does

# .NET P/Invoke

Calling native code from .NET is powerful but unforgiving. Incorrect signatures, garbled strings, and leaked or freed memory are the most common sources of bugs — all can manifest as intermittent crashes, silent data corruption, or access violations far from the actual defect.

This skill covers both `DllImport` (available since .NET Framework 1.0) and `LibraryImport` (source-generated, .NET 7+). When targeting .NET Framework, always use `DllImport`. When targeting .NET 7+, prefer `LibraryImport` for new code. When native AOT is a requirement, `LibraryImport` is the only option.

## When to Use This Skill

- Writing a new `[DllImport]` or `[LibraryImport]` declaration from a C/C++ header
- Reviewing P/Invoke signatures for correctness (type sizes, calling conventions, string encoding)
- Wrapping an entire C library for use from .NET
- Debugging `AccessViolationException`, `DllNotFoundException`, or silent data corruption at the native boundary
- Migrating `DllImport` declarations to `LibraryImport` for AOT/trimming compatibility
- Diagnosing memory leaks or heap corruption involving native handles or buffers

## Stop Signals

- **Single function?** Map the signature (Steps 1-3), handle strings/memory only if relevant, skip tooling and migration sections.
- **Don't migrate** existing `DllImport` to `LibraryImport` unless the user asks or AOT/trimming is an explicit requirement.
- **Don't recommend CsWin32** unless the target is specifically Win32 APIs.
- **Don't generate callbacks** (Step 8) unless the native API requires function pointers.
- **Review request?** Use the validation checklist — don't rewrite working code.

## Inputs

| Input | Required | Description |
|-------|----------|-------------|
| Native header or documentation | Yes | C/C++ function signatures, struct definitions, calling conventions |
| Target framework | Yes | Determines whether to use `DllImport` or `LibraryImport` |
| Target platforms | Recommended | Affects type sizes (`long`, `size_t`) and library naming |
| Memory ownership contract | Yes | Who allocates and who frees each buffer or handle |

**Agent behavior:** When documentation and native headers diverge, always trust the header. Online documentation (including official Win32 API docs) frequently omits or simplifies details about types, calling conventions, and struct layout that are critical for correct P/Invoke signatures.

---

## Workflow

### Step 1: Choose DllImport or LibraryImport

| Aspect | `DllImport` | `LibraryImport` (.NET 7+) |
|--------|-------------|---------------------------|
| **Mechanism** | Runtime marshalling | Source generator (compile-time) |
| **AOT / Trim safe** | No | Yes |
| **String marshalling** | `CharSet` enum | `StringMarshalling` enum |
| **Error handling** | `SetLastError` | `SetLastPInvokeError` |
| **Availability** | .NET Framework 1.0+ | .NET 7+ only |

### Step 2: Map Native Types to .NET Types

The most dangerous mappings — these cause the majority of bugs:

| C / Win32 Type | .NET Type | Why |
|----------------|-----------|-----|
| `long` | **`CLong`** | 32-bit on Windows, 64-bit on 64-bit Unix. With `LibraryImport`, requires `[assembly: DisableRuntimeMarshalling]` |
| `size_t` | `nuint` / `UIntPtr` | Pointer-sized. Use `nuint` on .NET 8+ and `UIntPtr` on earlier .NET. Never use `ulong` |
| `BOOL` (Win32) | `int` | Not `bool` — Win32 `BOOL` is 4 bytes |
| `bool` (C99) | `[MarshalAs(UnmanagedType.U1)] bool` | Must specify 1-byte marshal |
| `HANDLE`, `HWND` | `SafeHandle` | Prefer over raw `IntPtr` |
| `LPWSTR` / `wchar_t*` | `string` | UTF-16 on Windows (lowest cost for `in` strings). Avoid in cross-platform code — `wchar_t` width is compiler-defined (typically UTF-32 on non-Windows) |
| `LPSTR` / `char*` | `string` | Must specify encoding (ANSI or UTF-8). Always requires marshalling cost for `in` parameters |

**For the complete type mapping table, struct layout, and blittable type rules**, see [references/type-mapping.md](references/type-mapping.md).

> ❌ **NEVER** use `int` or `long` for C `long` — it's 32-bit on Windows, 64-bit on Unix. Always use `CLong`.
> ❌ **NEVER** use `ulong` for `size_t` — causes stack corruption on 32-bit. Use `nuint` or `UIntPtr`.
> ❌ **NEVER** use `bool` without `MarshalAs` — the default marshal size is wrong.

### Step 3: Write the Declaration

Given a C header:

```c
int32_t process_records(const Record* records, size_t count, uint32_t* out_processed);
```

**DllImport:**

```csharp
[DllImport("mylib")]
private static extern int ProcessRecords(
    [In] Record[] records, UIntPtr count, out uint outProcessed);
```

**LibraryImport:**

```csharp
[LibraryImport("mylib")]
internal static partial int ProcessRecords(
    [In] Record[] records, nuint count, out uint outProcessed);
```

Calling conventions only need to be specified when targeting Windows x86 (32-bit), where `Cdecl` and `StdCall` differ. On x64, ARM, and ARM64, there is a single calling convention and the attribute is unnecessary.

**Agent behavior:** If you detect that Windows x86 is a target — through project properties (e.g., `<PlatformTarget>x86</PlatformTarget>`), runtime identifiers (e.g., `win-x86`), build scripts, comments, or developer instructions — flag this to the developer and recommend explicit calling conventions on all P/Invoke declarations.

```csharp
// DllImport (x86 targets)
[DllImport("mylib", CallingConvention = CallingConvention.Cdecl)]

// LibraryImport (x86 targets)
[LibraryImport("mylib")]
[UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
```

If the managed method name differs from the native export name, specify `EntryPoint` to avoid `EntryPointNotFoundException`:

```csharp
// DllImport
[DllImport("mylib", EntryPoint = "process_records")]
private static extern int ProcessRecords(
    [In] Record[] records, UIntPtr count, out uint outProcessed);

// LibraryImport
[LibraryImport("mylib", EntryPoint = "process_records")]
internal static partial int ProcessRecords(
    [In] Record[] records, nuint count, out uint outProcessed);
```

### Step 4: Handle Strings Correctly

1. **Know what encoding the native function expects.** There is no safe default.
2. **Windows APIs:** Always call the `W` (UTF-16) variant. The `A` variant needs a specific reason and explicit ANSI encoding.
3. **Cross-platform C libraries:** Usually expect UTF-8.
4. **Specify encoding explicitly.** Never rely on `CharSet.Auto`.
5. **Never introduce `StringBuilder` for output buffers.**

> ❌ **NEVER** rely on `CharSet.Auto` or omit string encoding — there is no safe default.

```csharp
// DllImport — Windows API (UTF-16)
[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
private static extern int GetModuleFileNameW(
    IntPtr hModule, [Out] char[] filename, int size);

// DllImport — Cross-platform C library (UTF-8)
[DllImport("mylib")]
private static extern int SetName(
    [MarshalAs(UnmanagedType.LPUTF8Str)] string name);

// LibraryImport — UTF-16
[LibraryImport("kernel32", StringMarshalling = StringMarshalling.Utf16,
    SetLastPInvokeError = true)]
internal static partial int GetModuleFileNameW(
    IntPtr hModule, [Out] char[] filename, int size);

// LibraryImport — UTF-8
[LibraryImport("mylib", StringMarshalling = StringMarshalling.Utf8)]
internal static partial int SetName(string name);
```

**String lifetime warning:** Marshalled strings are freed after the call returns. If native code stores the pointer (instead of copying), the lifetime must be manually managed. On Windows or .NET Framework, `CoTaskMemAlloc`/`CoTaskMemFree` is the first choice for cross-boundary ownership; on non-Windows targets, use `NativeMemory` APIs. The library may have its own allocator that must be used instead.

### Step 5: Establish Memory Ownership

When memory crosses the boundary, exactly one side must own it — and both sides must agree.

> ❌ **NEVER** free with a mismatched allocator — `Marshal.FreeHGlobal` on `malloc`'d memory is heap corruption.

**Mod