# On Ubuntu x64, using .NET 7 preview 5

$ dotnet new classlib -n lib1 && cd lib1
$ cat > Class1.cs <<EOF
using System;
using System.Runtime.InteropServices;

namespace lib1;
public class EntrypointWithCallback
{
    [UnmanagedCallersOnly(EntryPoint = "myadd")]
    public unsafe static int Add(int a, int b, delegate* unmanaged<int, int, int, int> callback)
    {
        Console.WriteLine("Hello from C# Add!");
        return callback (a, b, a + b);
    }
}
EOF
$ dotnet publish -c Release --use-current-runtime -o dist  -p:PublishAot=true -p:NativeLib=static

$ cat > test.c <<EOF
#include<stdio.h>

typedef int(*callback_iii)(int, int, int);
extern int myadd(int, int, callback_iii);

int foo(int x, int y, int sum)
{
  printf("Hello from C callback!\n");
  return sum;
}

int main()
{
  printf("sum is: %d\n", myadd(1, 2, &foo));
  return 0;
}
EOF

$ cc test.c dist/lib1.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/sdk/libbootstrapperdll.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/sdk/libRuntime.WorkstationGC.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/framework/libSystem.Native.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/framework/libSystem.Globalization.Native.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/framework/libSystem.IO.Compression.Native.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/framework/libSystem.Net.Security.Native.a \
  ~/.nuget/packages/runtime.linux-x64.microsoft.dotnet.ilcompiler/7.0.0-preview.5.22301.12/framework/libSystem.Security.Cryptography.Native.OpenSsl.a \
  -Wall -pthread -lstdc++ -ldl -lm -Wl,--require-defined,NativeAOT_StaticInitialization

$ ./a.out
Hello from C# Add!
Hello from C callback!
sum is: 3