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Native library loading

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This article explains which paths the runtime searches when loading native libraries via P/Invoke. It also shows how to use SetDllImportResolver.

Library name variations

To facilitate simpler cross platform P/Invoke code, the runtime adds the canonical shared library extension (.dll, .so or .dylib) to native library names. On Unix-based platforms, the runtime will also try prepending lib. These library names variations are automatically searched when you use APIs that load native libraries, such as DllImportAttribute.

Note

Absolute paths in library names (for example, /usr/lib/libc.so) are treated as-is and no variations will be searched.

Consider the following example of using P/Invoke:

[DllImport("nativedep")]
static extern int ExportedFunction();

When running on Windows, the DLL is searched for in the following order:

  1. nativedep
  2. nativedep.dll (if the library name does not already end with .dll or .exe)

When running on Linux or macOS, the runtime will try prepending lib and appending the canonical shared library extension. On these OSes, library name variations are tried in the following order:

  1. nativedep.so / nativedep.dylib
  2. libnativedep.so / libnativedep.dylib 1
  3. nativedep
  4. libnativedep 1

On Linux, the search order is different if the library name ends with .so or contains .so. (note the trailing .). Consider the following example:

[DllImport("nativedep.so.6")]
static extern int ExportedFunction();

In this case, the library name variations are tried in the following order:

  1. nativedep.so.6
  2. libnativedep.so.6 1
  3. nativedep.so.6.so
  4. libnativedep.so.6.so 1

1 Path is checked only if the library name does not contain a directory separator character (/).

Custom import resolver

In more complex scenarios, you can use SetDllImportResolver to resolve DLL imports at run time. In the following example, nativedep is resolved to nativedep_avx2 if the CPU supports it.

Tip

This functionality is only available in .NET Core 3.1 and .NET 5+.

using System;
using System.Reflection;
using System.Runtime.InteropServices;

namespace PInvokeSamples
{
    public static partial class Program
    {
        [LibraryImport("nativedep")]
        private static partial int ExportedFunction();

        public static void Main(string[] args)
        {
            // Register the import resolver before calling the imported function.
            // Only one import resolver can be set for a given assembly.
            NativeLibrary.SetDllImportResolver(Assembly.GetExecutingAssembly(), DllImportResolver);

            int value = ExportedFunction();
            Console.WriteLine(value);
        }

        private static IntPtr DllImportResolver(string libraryName, Assembly assembly, DllImportSearchPath? searchPath)
        {
            if (libraryName == "nativedep")
            {
                // On systems with AVX2 support, load a different library.
                if (System.Runtime.Intrinsics.X86.Avx2.IsSupported)
                {
                    return NativeLibrary.Load("nativedep_avx2", assembly, searchPath);
                }
            }

            // Otherwise, fallback to default import resolver.
            return IntPtr.Zero;
        }
    }
}

See also