Using external function from C++ DLL

I created a C++ DLL and a C# project. (The C# project calls the C++ DLL functions).

The DLL has the following header and cpp file:

// Header.h
#ifdef HEADER_EXPORTS
#define HEADER_API __declspec(dllexport)
#else
#define HEADER_API __declspec(dllimport)
#endif

#pragma once
#include <stdio.h>
#include <stdlib.h>

extern "C" HEADER_API bool function(...);

// cpp file - only function, no class!
bool function(...){
...
}

The c# project calls the function: (and throwing BadImageFormatException)

using System;
using System.Runtime.InteropServices;

namespace tempProject
{
    public partial class ViewResultWin : Form
    {
        [DllImport(DLL_PATH)]
        public static extern bool function(...);
        .
        .
        .
        public ViewResultWin()
        {
            // An error occurs here
            bool answer = function(...);
            .
            .
            .

and ... no matter what I try, I always get the same error: "System.BadImageFormatException: 'An attempt was made to load a program with an incorrect format. (0x8007000B)'"

I have read and implement all the possible solutions to this problem (64-bits calling 32-bits and vice versa) including changing the 'Platform target' to 'Any CPU' for both projects. I tried to add a reference to the DLL, but it is'nt possible (even, that apparently this is not possible for all DLL anyway).

*The DLL created step by step by the tutorial https://docs.microsoft.com/en-us/cpp/build/walkthrough-creating-and-using-a-dynamic-link-library-cpp?view=msvc-160.

*Using Visual Studio 2019.