host_interop.c

#include <stdint.h>
#include <mono-wasi/driver.h>
#include <mono/metadata/appdomain.h>
// #include <mono/metadata/mono.h>
#include <mono/metadata/object.h>
#include <mono/metadata/exception.h>
#include <mono/metadata/image.h>
#include <string.h>
#include <assert.h>
#include <stdio.h>

// from https://github.com/dotnet/dotnet-wasi-sdk/blob/2dbb00c779180873d3ed985e59e431f56404d8da/src/Wasi.AspNetCore.Server.Native/native/dotnet_method.h
#define DEFINE_DOTNET_METHOD(c_name, assembly_name, namespc, class_name, method_name) \
MonoMethod* method_##c_name;\
MonoObject* c_name(MonoObject* target_instance, void* method_params[]) {\
    if (!method_##c_name) {\
        method_##c_name = lookup_dotnet_method(assembly_name, namespc, class_name, method_name, -1);\
        assert(method_##c_name);\
    }\
\
    MonoObject* exception;\
    MonoObject* res = mono_wasm_invoke_method(method_##c_name, target_instance, method_params, &exception);\
    assert(!exception);\
    return res;\
}

DEFINE_DOTNET_METHOD(_helloworld, "MyFirstWasiApp.dll", "Atmo", "Interop", "HelloWorld");

__attribute__((export_name("helloworld")))
void helloworld() {
    void *params[] = {  };
	_helloworld(NULL, params);
}

Lib.cs

namespace Atmo;

public class Interop
{
    public static void HelloWorld()
    {
		Console.WriteLine(DateTimeOffset.UnixEpoch);
	}

	static void Main() {
		// Console.WriteLine(DateTimeOffset.UnixEpoch);
	}
}

MyFirstWasiApp.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net7.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <!-- <PublishAot>true</PublishAot> -->
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Wasi.Sdk" Version="0.1.4-preview.10020" />
    <WasiNativeFileReference Include="$(MSBuildThisFileDirectory)\host_interop.c" />
  </ItemGroup>

</Project>