Header named "interface.h" with func, defined outside class/struct, generates "public unsafe partial class interface"

[AleXr64]

Subj.
OS: Windows

Used headers

void Func();
in file named "interface.h"

incompilable generated code

    public unsafe partial class interface
    {
        public partial struct __Internal
        {
            [SuppressUnmanagedCodeSecurity]
            [DllImport("__Internal", CallingConvention = global::System.Runtime.InteropServices.CallingConvention.Cdecl,
                EntryPoint="?Func@@YAXXZ")]
            internal static extern void Func();
        }

        public static void Func()
        {
            __Internal.Func();
        }
    }