The goal of this project is to start a .NET Core 3.1 WinForms application, from a native (ie unmanaged) process. It can be easily modified to start any other type of .NET Core project.

There are three methods that can be used to host .NET Core runtime :

• The "Mscoree" method : the oldest one, also used for the older .NET Framework, this project is a good example.
• The "CoreClrHost" method : introduced in .NET Core 2.1
• The "HostFxr" method : introduced with .Net Core 3.1, it is now the recommended API.

This project uses the "HostFxr" method, and is based on the official sample.

Code overview

1. The get_hostfxr_path method retrieves the path of hostfxr.dll

char_t buffer[MAX_PATH];
size_t bufferSize = std::size(buffer);
int result = get_hostfxr_path(buffer, &bufferSize, nullptr);

2. hostfxr.dll exposes methods to load and initialize the runtime

HMODULE handleHostFxr = LoadLibraryW(buffer);
auto initRuntime = reinterpret_cast<hostfxr_initialize_for_runtime_config_fn>( GetProcAddress(handleHostFxr, "hostfxr_initialize_for_runtime_config") );
auto getRuntimeDelegate = reinterpret_cast<hostfxr_get_runtime_delegate_fn>( GetProcAddress(handleHostFxr, "hostfxr_get_runtime_delegate") );

3. The runtime is initilized with a runtime.json configuration file. It's automatically generated for every .NET Core project after compilation.

hostfxr_handle runtimeContext{ nullptr };
result = initRuntime(managed->runtimeConfigPath, nullptr, &runtimeContext); //runtimeConfigPath points to WinFormsSample.runtimeconfig.json

4. Load the runtime

load_assembly_and_get_function_pointer_fn loadAssemblyAndGetFunctionPointer = nullptr;
result = getRuntimeDelegate( runtimeContext, hdt_load_assembly_and_get_function_pointer, (void**)&loadAssemblyAndGetFunctionPointer);

5. Execute a specified method from the managed application. It this sample we start the HostEntryPoint method, in the Program class, in the WinFormsApp namespace, in the WinFormsApp assembly. We do not pass any arguments, but it can be done, as shown in the official sample

component_entry_point_fn ManagedEntryPoint = nullptr;
result = loadAssemblyAndGetFunctionPointer(
    assemblyPath.c_str(),   // path to WinFormsSample.dll
    typeName.c_str(),       // "WinFormsSample.Program, WinFormsSample"
    methodName.c_str(),     // "HostEntryPoint"
    nullptr,
    nullptr,
    (void**)&ManagedEntryPoint);

ManagedEntryPoint(nullptr, 0);

6. The entry point needs to match this signature

namespace WinFormsApp
{
    public static class Program
    {
        [STAThread]
        public static int HostEntryPoint(IntPtr arg, int argLength)
        {
            ...
        }
    }
}

Remarks

The following files were copied from the dotnet runtime repo :

• coreclr_delegates.h
• hostfxr.h
• error_codes.h

nethost.dll

In order to locate hostfxr.dll, you will need a method from nethost.dll. nethost.dll is shipped with the SDK, after installation it can be found in these directories :

• 32bit C:\Program Files (x86)\dotnet\packs\Microsoft.NETCore.App.Host.win-x86\3.1.2\runtimes\win-x86\native
• 64bit C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Host.win-x64\3.1.2\runtimes\win-x64\native

You might need to adjust the paths according to your version.

These path are defined with user macros, then made accessible programatically with the "Preprocessor definitions" option.

Debugging

If you want to debug the Winforms application, you can make a call to Debugger.Launch(), like I did on the "Debug" button, or attach to a running processs.

You cannot debug both the code loading the runtime (unmanaged) and the managed application at the same time. Even if you select both the native and CoreClr debuggers. This is a documented limitation.