Correctly call native (C/C++) libraries from .NET using P/Invoke and LibraryImport. Covers function signatures, string marshalling, memory lifetime, SafeHandle, and cross-platform patterns. USE FOR: writing new P/Invoke or LibraryImport declarations, reviewing or debugging existing native interop code, wrapping a C or C++ library for use in .NET, diagnosing crashes, memory leaks, or corruption at the managed/native boundary. DO NOT USE FOR: COM interop, C++/CLI mixed-mode assemblies, or pure managed code with no native dependencies.
Correctly call native (C/C++) libraries from .NET using P/Invoke and LibraryImport. Covers function signatures, string marshalling, memory lifetime, SafeHandle, and cross-platform patterns. USE FOR: writing new P/Invoke or LibraryImport declarations, reviewing or debugging existing native interop code, wrapping a C or C++ library for use in .NET, diagnosing crashes, memory leaks, or corruption at the managed/native boundary. DO NOT USE FOR: COM interop, C++/CLI mixed-mode assemblies, or pure managed code with no native dependencies.
license
MIT
.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 (available since .NET Framework 1.0) and (source-generated, .NET 7+). When targeting .NET Framework, always use . When targeting .NET 7+, prefer for new code. When native AOT is a requirement, is the only option.
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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.
❌ 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.
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.
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):
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 x86privatedelegatevoidLogCallbackDelegate(int level, IntPtr message);
// CRITICAL: prevent delegate from being garbage collectedprivatestatic LogCallbackDelegate? s_logCallback;
publicstaticvoidEnableLogging(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].
// Trivial: use platform naming convention// The default naming convention adds corresponding prefix and extension when// searching the nativelibrary. The resultant file name will be mylib.dll on// Windows, libmylib.so on Linux and libmylib.dylib on macOS.privateconststring LibName = "mylib";
// 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 WINDOWSprivateconststring LibName = "mylib.dll";
#elif LINUXprivateconststring LibName = "libmylib.so";
#elif MACOSprivateconststring LibName = "libmylib.dylib";
#endif// Complex: runtime resolver// When targeting netstandard2.0 or other frameworks when OperatingSystem.IsXXX// is not available, use RuntimeInformation.IsOSPlatform(OSPlatform.XXX) api.
NativeLibrary.SetDllImportResolver(typeof(MyLib).Assembly,
(name, assembly, searchPath) =>
{
if (name != "mylib") return IntPtr.Zero;
string libName = OperatingSystem.IsWindows()
? "mylib.dll"
: OperatingSystem.IsMacOS()
? "libmylib.dylib" : "libmylib.so";
NativeLibrary.TryLoad(libName, assembly, searchPath, outvar handle);
return handle;
});
Migrating DllImport to LibraryImport
For codebases targeting .NET 7+, migrating provides AOT compatibility and trimming safety.
Add partial to the containing class and make the method static partial
Replace [DllImport] with [LibraryImport]
Replace CharSet with StringMarshalling
Replace SetLastError = true with SetLastPInvokeError = true
Remove CallingConvention unless targeting Windows x86
Build and fix SYSLIB1054–SYSLIB1057 analyzer warnings
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:
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
Build with interop analyzers enabled — confirm zero SYSLIB1054–SYSLIB1057 warnings:
Verify struct sizes match — for every struct crossing the boundary, assert Marshal.SizeOf<T>() equals the native sizeof
Round-trip test — call the native function with known inputs and verify expected outputs
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.
