LibraryImportとソース生成P/Invoke

LibraryImportを使う理由

LibraryImportメソッドの宣言

using System.Runtime.InteropServices;

internal static partial class NativeMethods
{
    // Source generator creates the P/Invoke body at compile time
    [LibraryImport("mylib", EntryPoint = "add_integers")]
    internal static partial int AddIntegers(int a, int b);

    // String marshaling must be explicit in LibraryImport
    [LibraryImport("mylib", StringMarshalling = StringMarshalling.Utf8)]
    internal static partial int ProcessString(string text);
}

LibraryImportでの文字列マーシャリング

// Option 1: StringMarshalling enum (all strings in method)
[LibraryImport("kernel32", StringMarshalling = StringMarshalling.Utf16)]
static partial bool CreateDirectory(string lpPathName, nint lpSecurityAttributes);

// Option 2: Per-parameter MarshalAs
[LibraryImport("libc")]
static partial int Open(
    [MarshalAs(UnmanagedType.LPUTF8Str)] string path,
    int flags);

// Option 3: MarshalUsing for custom marshalers
[LibraryImport("mylib")]
static partial void Process(
    [MarshalUsing(typeof(Utf8StringMarshaller))] string name);

構造体のマーシャリング

[NativeMarshalling(typeof(PointMarshaller))]
public struct Point
{
    public int X;
    public int Y;
}

[CustomMarshaller(typeof(Point), MarshalMode.Default, typeof(PointMarshaller))]
public static class PointMarshaller
{
    public static Point ConvertToManaged(NativePoint native)
        => new Point { X = native.X, Y = native.Y };
    public static NativePoint ConvertToUnmanaged(Point managed)
        => new NativePoint { X = managed.X, Y = managed.Y };

    [StructLayout(LayoutKind.Sequential)]
    public struct NativePoint { public int X, Y; }
}

DllImportとLibraryImportの比較

// Old — DllImport (still works, but avoid for new AOT code)
[DllImport("kernel32", CharSet = CharSet.Unicode, SetLastError = true)]
static extern bool MoveFile(string src, string dst);

// New — LibraryImport (AOT-safe, source-generated)
[LibraryImport("kernel32", StringMarshalling = StringMarshalling.Utf16,
                SetLastError = true)]
static partial bool MoveFile(string src, string dst);
// The compiler generates the actual P/Invoke wrapper body

SetLastErrorとエラー処理

[LibraryImport("kernel32", StringMarshalling = StringMarshalling.Utf16,
                SetLastError = true)]
static partial bool CreateDirectory(string path, nint secAttr);

bool ok = CreateDirectory(@"C:\Temp\NewDir", 0);
if (!ok)
{
    int err = Marshal.GetLastPInvokeError();
    // err is ERROR_ALREADY_EXISTS (183) if folder exists
    throw new Win32Exception(err);
}

Span<T>とメモリのマーシャリング

[LibraryImport("mylib")]
static partial int ProcessBuffer(
    ReadOnlySpan<byte> data,
    int length);

// Usage — no fixed or GCHandle needed
byte[] buffer = Encoding.UTF8.GetBytes("hello");
int result = ProcessBuffer(buffer, buffer.Length);

// For output buffers use Span<byte>
[LibraryImport("mylib")]
static partial int FillBuffer(Span<byte> output, int maxLen);

ソース生成の有効化

<!-- In your .csproj — no extra package needed on .NET 7+ -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <!-- Enable Roslyn analyzers and source generators -->
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <!-- AllowUnsafeBlocks may be needed for some marshalers -->
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>
</Project>

生成されたコードの確認

// The generator emits something like this for a LibraryImport method:
// (simplified view of generated stub)

internal static partial int AddIntegers(int a, int b)
{
    // Direct call — no reflection, no boxing, fully inlineable
    return __PInvoke(a, b);

    [System.Runtime.InteropServices.DllImportAttribute(
        "mylib",
        EntryPoint = "add_integers",
        ExactSpelling = true)]
    static extern int __PInvoke(int a, int b);
}

実践：Cライブラリのラッパー作成

internal static partial class LibSodiumNative
{
    private const string LibName = "libsodium";

    [LibraryImport(LibName, EntryPoint = "crypto_secretbox_keybytes")]
    internal static partial nuint KeyBytes();

    [LibraryImport(LibName, EntryPoint = "crypto_secretbox_easy")]
    internal static partial int SecretBoxEasy(
        Span<byte> ciphertext,
        ReadOnlySpan<byte> message,
        ulong mlen,
        ReadOnlySpan<byte> nonce,
        ReadOnlySpan<byte> key);
}

// Public safe wrapper hides the native signature
public static byte[] Encrypt(byte[] message, byte[] key, byte[] nonce)
{
    var cipher = new byte[message.Length + 16];
    LibSodiumNative.SecretBoxEasy(cipher, message, (ulong)message.Length, nonce, key);
    return cipher;
}

理解度チェック

振り返り：LibraryImportとソース生成P/Invoke