I need to wrap a c dll so that it can be called from c#, the issue is that a property of the struct passed in is complex. Frustratingly the child property is only accessed internally in the c code, but it needs to be defined in the c# wrapper.

struct outer {
    .
    .
    .
    unsigned int rsi;

    unsigned int flags;

    struct internal_state *state;
};


struct internal_state {
    int (*mode)(struct outer *);

    int (**id_table)(struct outer *);

    void (*flush_output)(struct outer *);

    int id;

    int id_len;
    .
    .
    .
}

In the c code there are three methods, essentially init, act, end each taking an instance of the outer struct. The offending properties are mode, id_table and flush_output if I leave them out of the struct I can call init and it returns successfully. Init sets the state property on the outer but on inspection state does not have any of the values set.

If I try to add in mode, id_table and flush_output as Func/Action types the call blows up with the following exception (unsurprisingly)

Cannot marshal field ‘mode’ of type ‘internal_state’: Non-blittable generic types cannot be marshaled.

Given the c# code does not touch the state property is there any way to “con” the c#/c code for that property?

Example c# code

        public unsafe byte[] GetData(byte[] data)
        {

            var myOuter = new outer
            {
        .
        .
        .
                rsi = 128,
                flags = 8
            };

            var intitResponse = Init(ref myOuter);
            var actResponse = Act(ref myOuter, 0);
            var endResponse = End(ref myOuter);

            return dataOut;
        }

        [DllImport("lib/aec.dll", EntryPoint = "init")]
        public static extern int Init(ref outer outr);

        [DllImport("lib/aec.dll", EntryPoint = "act")]
        public static extern int Act(ref outer outr, nuint flush);

        [DllImport("lib/aec.dll", EntryPoint = "end")]
        public static extern int End(ref outer outr);

        public struct outer
        {
            .
            .
            .
            public nuint rsi;

            public nuint flags;

            public internal_state state;
        }

I have tried leaving off the complex properties but that causes the state property to not be initialised properly in the init method, even though it returns a 0 from the call.