.NET P/Invoke

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

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

Step 2: Map Native Types to .NET Types

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

For the complete type mapping table, struct layout, and blittable type rules, see 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:

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

DllImport:

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

LibraryImport:

[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.

// 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:

// 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.

// 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.

Model 1 — Caller allocates, caller frees (safest):

[LibraryImport("mylib")]
private static partial int GetName(
    Span<byte> buffer, nuint bufferSize, out nuint actualSize);

public static string GetName()
{
    Span<byte> buffer = stackalloc byte[256];
    int result = GetName(buffer, (nuint)buffer.Length, out nuint actualSize);
    if (result != 0) throw new InvalidOperationException($"Failed: {result}");
    return Encoding.UTF8.GetString(buffer[..(int)actualSize]);
}

Model 2 — Callee allocates, caller frees (common in Win32):

[LibraryImport("mylib")]
private static partial IntPtr GetVersion();
[LibraryImport("mylib")]
private static partial void FreeString(IntPtr s);

public static string GetVersion()
{
    IntPtr ptr = GetVersion();
    try { return Marshal.PtrToStringUTF8(ptr) ?? throw new InvalidOperationException(); }
    finally { FreeString(ptr); } // Must use the library's own free function
}

Critical rule: Always free with the matching allocator. Never use Marshal.FreeHGlobal or Marshal.FreeCoTaskMem on malloc'd memory.

Model 3 — Handle-based (callee allocates, callee frees): Use SafeHandle (see Step 6).

Pinning managed objects — when native code stores the pointer or runs asynchronously:

// Synchronous: use fixed
public static unsafe void ProcessSync(byte[] data)
{
    fixed (byte* ptr = data) { ProcessData(ptr, (nuint)data.Length); }
}

// Asynchronous: use GCHandle
var gcHandle = GCHandle.Alloc(data, GCHandleType.Pinned);
// Must keep pinned until native processing completes, then call gcHandle.Free()

Step 6: Use SafeHandle for Native Handles

Raw IntPtr leaks on exceptions and has no double-free protection. SafeHandle is non-negotiable.

internal sealed class MyLibHandle : SafeHandleZeroOrMinusOneIsInvalid
{
    // Required by the marshalling infrastructure to instantiate the handle.
    // Do not remove — there are no direct callers.
    private MyLibHandle() : base(ownsHandle: true) { }

    [LibraryImport("mylib", StringMarshalling = StringMarshalling.Utf8)]
    private static partial MyLibHandle CreateHandle(string config);

    [LibraryImport("mylib")]
    private static partial int UseHandle(MyLibHandle h, ReadOnlySpan<byte> data, nuint len);

    [LibraryImport("mylib")]
    private static partial void DestroyHandle(IntPtr h);

    protected override bool ReleaseHandle() { DestroyHandle(handle); return true; }

    public static MyLibHandle Create(string config)
    {
        var h = CreateHandle(config);
        if (h.IsInvalid) throw new InvalidOperationException("Failed to create handle");
        return h;
    }

    public int Use(ReadOnlySpan<byte> data) => UseHandle(this, data, (nuint)data.Length);
}

// Usage: SafeHandle is IDisposable
using var handle = MyLibHandle.Create("config=value");
int result = handle.Use(myData);

Step 7: Handle Errors

// Win32 APIs — check SetLastError
[LibraryImport("kernel32", SetLastPInvokeError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static partial bool CloseHandle(IntPtr hObject);

if (!CloseHandle(handle))
    throw new Win32Exception(Marshal.GetLastPInvokeError());

// HRESULT APIs
int hr = NativeDoWork(context);
Marshal.ThrowExceptionForHR(hr);

Step 8: Handle Callbacks (if needed)

Preferred (.NET 8+): UnmanagedCallersOnly — avoids delegates entirely, no GC lifetime risk:

[UnmanagedCallersOnly]
private static void LogCallback(int level, IntPtr message)
{
    string msg = Marshal.PtrToStringUTF8(message) ?? string.Empty;
    Console.WriteLine($"[{level}] {msg}");
}

[LibraryImport("mylib")]
private static unsafe partial void SetLogCallback(
    delegate* unmanaged<int, IntPtr, void> cb);

unsafe { SetLogCallback(&LogCallback); }

The method must be static, must not throw exceptions back to native code, and can only use blittable parameter types.

Fallback (older TFMs or when instance state is needed): delegate with rooting

[UnmanagedFunctionPointer(CallingConvention.Cdecl)] // Only needed on Windows x86
private delegate void LogCallbackDelegate(int level, IntPtr message);

// CRITICAL: prevent delegate from being garbage collected
private static LogCallbackDelegate? s_logCallback;

public static void EnableLogging(Action<int, string> handler)
{
    s_logCallback = (level, msgPtr) =>
    {
        string msg = Marshal.PtrToStringUTF8(msgPtr) ?? string.Empty;
        handler(level, msg);
    };
    SetLogCallback(s_logCallback);
}

If native code stores the function pointer, the delegate must stay rooted for its entire lifetime. A collected delegate means a crash.

GC.KeepAlive for short-lived callbacks: When converting a delegate to a function pointer with Marshal.GetFunctionPointerForDelegate, the GC does not track the relationship between the pointer and the delegate. Use GC.KeepAlive to prevent collection before the native call completes:

var callback = new LogCallbackDelegate((level, msgPtr) =>
{
    string msg = Marshal.PtrToStringUTF8(msgPtr) ?? string.Empty;
    Console.WriteLine($"[{level}] {msg}");
});

IntPtr fnPtr = Marshal.GetFunctionPointerForDelegate(callback);
NativeUsesCallback(fnPtr);
GC.KeepAlive(callback); // prevent collection — fnPtr does not root the delegate

Cross-Platform Library Loading

Use NativeLibrary.SetDllImportResolver for complex scenarios, or conditional compilation for simple cases. Use CLong/CULong for C long/unsigned long. Note: CLong/CULong with LibraryImport requires [assembly: DisableRuntimeMarshalling].

// Simple: conditional compilation
// WINDOWS, LINUX, MACOS are predefined only when targeting an OS-specific TFM
// (e.g., net8.0-windows). For portable TFMs (e.g., net8.0), these symbols are
// not defined — use the runtime resolver approach below instead.
#if WINDOWS
    private const string LibName = "mylib.dll";
#elif LINUX
    private const string LibName = "libmylib.so";
#elif MACOS
    private const string LibName = "libmylib.dylib";
#endif

// Complex: runtime resolver
NativeLibrary.SetDllImportResolver(typeof(MyLib).Assembly,
    (name, assembly, searchPath) =>
    {
        if (name != "mylib") return IntPtr.Zero;
        string libName = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
            ? "mylib.dll"
            : RuntimeInformation.IsOSPlatform(OSPlatform.OSX)
                ? "libmylib.dylib" : "libmylib.so";
        NativeLibrary.TryLoad(libName, assembly, searchPath, out var handle);
        return handle;
    });

Migrating DllImport to LibraryImport

For codebases targeting .NET 7+, migrating provides AOT compatibility and trimming safety.

1. Add partial to the containing class and make the method static partial
2. Replace [DllImport] with [LibraryImport]
3. Replace CharSet with StringMarshalling
4. Replace SetLastError = true with SetLastPInvokeError = true
5. Remove CallingConvention unless targeting Windows x86
6. Build and fix SYSLIB1054–SYSLIB1057 analyzer warnings

Enable the interop analyzers:

<PropertyGroup>
    <EnableTrimAnalyzer>true</EnableTrimAnalyzer>
    <EnableAotAnalyzer>true</EnableAotAnalyzer>
</PropertyGroup>

Tooling

CsWin32 (Win32 APIs)

For Win32 P/Invoke, prefer Microsoft.Windows.CsWin32 over hand-written signatures. It source-generates correct declarations from metadata. Add a NativeMethods.txt listing the APIs you need:

dotnet add package Microsoft.Windows.CsWin32

CsWinRT (WinRT APIs)

For WinRT interop, use Microsoft.Windows.CsWinRT to generate .NET projections from .winmd files.

Objective Sharpie (Objective-C APIs)

For binding Objective-C libraries (macOS/iOS), use Objective Sharpie to generate initial P/Invoke and binding definitions from Objective-C headers.

Validation

Review checklist

• Every signature matches the native header exactly (types, sizes)
• Calling convention specified if targeting Windows x86; omitted otherwise
• String encoding is explicit — no reliance on defaults or CharSet.Auto
• Memory ownership is documented and matched (who allocates, who frees, with what)
• SafeHandle used for all native handles (no raw IntPtr escaping the interop layer)
• Delegates passed as callbacks are rooted to prevent GC collection
• SetLastError/SetLastPInvokeError set for APIs that use OS error codes
• Struct layout matches native (packing, alignment, field order)
• CLong/CULong used for C long/unsigned long in cross-platform code
• If using CLong/CULong with LibraryImport, [assembly: DisableRuntimeMarshalling] is applied
• No bool without explicit MarshalAs — always specify UnmanagedType.Bool (4-byte) or UnmanagedType.U1 (1-byte) to ensure normalization across the language boundary.

Runnable validation steps

1. Build with interop analyzers enabled — confirm zero SYSLIB1054–SYSLIB1057 warnings:

   <EnableTrimAnalyzer>true</EnableTrimAnalyzer>
   <EnableAotAnalyzer>true</EnableAotAnalyzer>
   

2. Verify struct sizes match — for every struct crossing the boundary, assert Marshal.SizeOf<T>() equals the native sizeof
3. Round-trip test — call the native function with known inputs and verify expected outputs
4. Test with non-ASCII strings — pass strings containing characters outside the ASCII range to confirm encoding is correct

Reference Files

• references/type-mapping.md — Complete native-to-.NET type mapping table, struct layout patterns, blittable type rules. Load when encountering types not covered in Step 2 above, or when working with struct layout or blittable type questions.
• references/diagnostics.md — Common pitfalls, failure modes and recovery, debugging approach, external resources. Load when debugging an existing P/Invoke failure or reviewing interop code for correctness issues.