Simple example of loading/unloading DLLs in D. This could be extended to provide a hot-swapping mechanism.

a.d

import core.stdc.stdio;

extern (C) int dll() {
    printf("hello from dll A!\n");
    return 0;
}

shared static this() {
    printf("liba.so shared static this\n");
}

shared static ~this() {
    printf("liba.so shared static ~this\n");
}

b.d

import core.stdc.stdio;

extern (C) int dll() {
    printf("hello from dll B!\n");
    return 0;
}

shared static this() {
    printf("libb.so shared static this\n");
}

shared static ~this() {
    printf("libb.so shared static ~this\n");
}

hotswap.d

import core.stdc.stdio;
import core.stdc.stdlib;
import core.sys.posix.dlfcn;
import std.string;

extern (C) int dll();

void* loadAndRunDll(string dllPath) {
  void* lh = dlopen(toStringz(dllPath), RTLD_LAZY);
  if (!lh) {
    fprintf(stderr, "dlopen error: %s\n", dlerror());
    exit(1);
  }
  printf("libmyfn.so is loaded\n");

  int function() fn = cast(int function())dlsym(lh, "dll");
  char* error = dlerror();
  if (error) {
    fprintf(stderr, "dlsym error: %s\n", error);
    exit(1);
  }
  printf("dll() function is found\n");

  fn();

  return lh;
}


int main() {
    printf("+main()\n");

    void* ah = loadAndRunDll("liba.so");
    printf("unloading DLL A\n");
    dlclose(ah);

    printf("getting user input...\n");
    void* bh = loadAndRunDll("libb.so");
    
    printf("unloading DLL B\n");
    dlclose(bh);

    printf("-main()\n");
    return 0;
}

shared static this() { printf("main shared static this\n"); }

shared static ~this() { printf("main shared static ~this\n"); }

meson.build

project('hotproj', 'd',)
libA = shared_library('a', 'dll.d')
libB = shared_library('b', 'dll2.d')
executable('hotswap', 'hotswap.d', 
  link_with : [libA, libB])