.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

LibraryImport

DllImport

LibraryImport

LibraryImport

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

DllImport

LibraryImport

long

size_t

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

DllImport

LibraryImport

CharSet

StringMarshalling

SetLastError

SetLastPInvokeError

Step 2: Map Native Types to .NET Types

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

long

CLong

LibraryImport

[assembly: DisableRuntimeMarshalling]

size_t

nuint

UIntPtr

nuint

UIntPtr

ulong

BOOL

int

bool

BOOL

bool

[MarshalAs(UnmanagedType.U1)] bool

HANDLE

HWND

SafeHandle

IntPtr

LPWSTR

wchar_t*

string

in

wchar_t

LPSTR

char*

string

in

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

StdCall

Agent behavior: If you detect that Windows x86 is a target — through project properties (e.g.,

<PlatformTarget>x86</PlatformTarget>

win-x86

// 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

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

NativeMemory

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

Marshal.FreeCoTaskMem

malloc

Model 3 — Handle-based (callee allocates, callee frees): Use

SafeHandle

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

SafeHandle

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

[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

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

Marshal.GetFunctionPointerForDelegate

GC.KeepAlive

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

CLong

CULong

long

unsigned long

CLong

CULong

LibraryImport

[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

dotnet add package Microsoft.Windows.CsWin32

CsWinRT (WinRT APIs)

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

.winmd

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.