Using Microsoft Visual Studio Professional 2019:

Created a simple addition function in C++ to generate a DLL file.
In a new C# project, referenced the above DLL file.

Win32: For both the C++ DLL and the C# program, when the active platform is set to Win32, using ‘Add reference’ in the reference manager works. The project builds and runs without any issues.

x64: For both the C++ DLL and the C# program, when the active platform is set to x64, attempting to use ‘Add reference’ in the reference manager results in a pop-up error

and the reference is not added to the project.

For my work, I need to use x64. What settings should I adjust to make this possible?

CPP file:

#include "pch.h"
#include "Divide.h"

Divide::Divide(int x) {
    this->x = x;
}

int Divide::adding(int y) {
    return this->x + y;
}

int Divide::get() {
    return this->x;
}


extern "C" __declspec(dllexport) void* Create(int x) {
    return (void*) new Divide(x);
}
extern "C" __declspec(dllexport) int DivideAdding(Divide * a, int y) {
    return a->adding(y);
}
extern "C" __declspec(dllexport) int DivideGet(Divide * a) {
    return a->get();
}

header file:

class Divide {
    int x;
public:
    Divide(int x);
    int adding(int y);
    int get();
};

In C#:

[DllImport("DivideDLL.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr Create(int x);

        [DllImport("DivideDLL.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern int DivideAdding(IntPtr a, int y);

        [DllImport("DivideDLL.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern int DivideGet(IntPtr a);

        static void Main(string[] args)
        {
            try
            {
                IntPtr a = Create(5);
                int temp = DivideGet(a);
                Console.WriteLine("Initial:" + temp);
                DivideAdding(a, 10);

For my work, I need to use x64. What settings should I adjust to make this possible?
If you need more information, please let me know.