This is an old question, but I had the same issue, found the answer, so thought I'd share it here for others.

I'm using https://github.com/ieski/OdooXmlRpc as the core, but re-engineered to use .Net 4.8. Behind that is Cook.Computing XmlRPCV2 (from nuget).

Basically, the Many2one is an object array, so you need to cast it and assign the value to a new field, at least that was my solution.

The state name in the many2one returned something like Victoria(AU), which doesn't look very good, so I used the GetStateCode method to return the code from Odoo model.

               Object[] state = (Object[])oa.GetField("state_id").Value;
                address.State = OdooClient.GetStateCode(Int32.Parse(state[0].ToString()));

        public string GetStateCode(int StateId)
        {
            List results = new List();
            try
            {
                var rpcContext = new RpcContext(_OdooConn, "res.country.state");
                rpcContext
                    .RpcFilter
                    .Equal("id", StateId);                    
                results = rpcContext.Execute(true).ToList();
                return results.FirstOrDefault().GetField("code").Value.ToString();
            }
            catch 
            {
                return string.Empty;
            }
        }