How do I marshal complex data types?

I'm currently working on a .NET Framework 4.7.2 application which needs to expose some methods from C++ to be able to use them later on in a .NET Standard/Core environment.

I know, I cannot expose Classes from C++ to C# using pInvoke. Furthermore I figured it's not possible to expose abstract .NET data types such as List because the implementation in C++ and C# is different.

Thus I can only use structs with fundamental data types to pass data from C# to C++.

I'm trying to find the unmanaged equivalent for this managed C++ signature:

List<KeyValuePair<int, Dictionary<System::String^, float>^>>^ tomatoes

My current C++ code looks like this:

typedef struct  
{
    //int arr[10];
    // here I need to define a structure similiar to List<KeyValuePair<int, Dictionary<System::String^, float>^>>^ tomatoes

} TOMATO_DATA;


extern "C"
{
    __declspec(dllexport) int Veggy_Create(TOMATO_DATA & data)
    {
        return 42;
    }
}

Unfortunately it's really difficult, I don't know how to picture

List<KeyValuePair<int, Dictionary<System::String^, float>^>>^ tomatoes

as a struct data type to use it for marshaling.

Do you know how to solve this issue?

Thanks!