See the question and my original answer on StackOverflow

REGDB_E_CLASSNOTREG means the class that you ask for (whatever it is COM/WinRT etc.) is not registered/known to the activation system (hosted in combase.dll).

The problem probably comes from the fact you're trying to use a registration-free WinRT component.

Let's take this sample as a start for the C# component: Walkthrough: Create a C#/WinRT component and consume it from C++/WinRT. So, just create the C# component but don't create the C++/WinRT app. (I use Visual Studio 2019 and net5.0-windows10.0.19041.0).

Note: C#/WinRT does produce a .dll (here SampleComponent.dll), not only metadata.

If you don't build the C++/WinRT app, you still need to build a regular .h file to use the C# component. C++/WinRT does that for you, but since we don't use this tool, we must build it ourselves. For that, we need two other tools winmdidl.exe and midlrt.exe that you'll find from Developer Command Prompt for Visual Studio..See also How to: Use winmdidl.exe and midlrt.exe to create .h files from windows metadata

So from the SampleComponent.winmd that you have if you followed the tutorial, run:

winmdidl SampleComponent.winmd

this will create a SampleComponent.idl file. Now run:

midlrt SampleComponent.idl /metadata_dir "C:\Windows\System32\WinMetadata"

this will create multiple files (proxy, stub, etc.), but we only need SampleComponent.h. Now, create a standard C++ console app like this (I don't use C++/WinRT I still use Wrl to simplify my code, but this is not mandatory):

#include <windows.h>
#include <stdio.h>
#include <wrl.h>
#include <wrl/wrappers/corewrappers.h>
#include "path to SampleComponent.h"

#pragma comment(lib, "runtimeobject.lib")

using namespace Microsoft::WRL; // ComPtr
using namespace Microsoft::WRL::Wrappers; // RoInitializeWrapper, HStringReference, HString
using namespace Windows::Foundation; // GetActivationFactory, ActivateInstance

int main()
    RoInitializeWrapper init(RO_INIT_MULTITHREADED);
    HRESULT hr = init;

    // all error checks on hr omitted

    ComPtr<SampleComponent::IExampleClass> cls;
    hr = ActivateInstance(HStringReference(RuntimeClass_SampleComponent_Example).Get(), &cls);
    hr = cls->put_SampleProperty(42);

    INT32 i;
    hr = cls->get_SampleProperty(&i);
    wprintf(L"%u\n", i);

    ComPtr<SampleComponent::IExampleStatic> clsStatic;
    hr = GetActivationFactory(HStringReference(RuntimeClass_SampleComponent_Example).Get(), &clsStatic);

    HString str;
    hr = clsStatic->SayHello(str.GetAddressOf());
    wprintf(L"%s\n", str.GetRawBuffer(nullptr));

RuntimeClass_SampleComponent_Example is from SampleComponent.h and should be defined like this:

extern const __declspec(selectany) _Null_terminated_ WCHAR RuntimeClass_SampleComponent_Example[] = L"SampleComponent.Example";

If you compile that and run, hr will be REGDB_E_CLASSNOTREG because the system cannot find the 'SampleComponent.Example' component.

So what you must do is explained here: How Registration-free WinRT Works

You must add a file to the project with the .manifest extension (any name should work with recent versions of Visual Studio), for example like this:

<?xml version="1.0" encoding="utf-8"?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
  <assemblyIdentity version="" name="CppConsoleApp"/>
  <file name="WinRT.Host.dll">
        xmlns="urn:schemas-microsoft-com:winrt.v1" />

assemblyIdentity's name is not super important, what is super important is file and activatableClass's name: it must be the same as the host dll name (here it must be WinRT.Host.dll which is provided by C#/WinRT) and class name you're trying to activate (corresponding to RuntimeClass_SampleComponent_Example).

You must also copy all the C#/WinRT files mess needed aside your .exe file. That would be : SampleComponent.dll, Microsoft.Windows.SDK.NET.dll, WinRT.Host.dll, WinRT.Host.runtimeconfig.json, WinRT.Host.Shim.dll, WinRT.Runtime.dll.

Note you can use C++/WinRT to help building WinRT.Host.runtimeconfig.json.

And now, it should work.