ln.bson/ln.bson/mapper/mappings/ClassStructMapping.cs

using System;
using System.Collections.Generic;
using System.Reflection;

namespace ln.bson.mapper.mappings
{
    public class ClassStructMapping<T> : BsonMapping
    {
        private FieldInfo[] mappedFields;
        //private PropertyInfo[] mappedProperties;
        
        public ClassStructMapping() : base(typeof(T))
        {
            mappedFields = typeof(T).GetFields(BindingFlags.Instance | BindingFlags.Public |
                                               BindingFlags.NonPublic);
        }

        public override bool TryMapValue(BsonMapper mapper, object o, out BsonValue bsonValue)
        {
            BsonDocument bsonDocument = new BsonDocument();
            foreach (FieldInfo fieldInfo in mappedFields)
            {
                object fieldValue = fieldInfo.GetValue(o);
                if (fieldValue is null)
                    bsonDocument.Add(fieldInfo.Name, BsonNull.Instance);
                else if (mapper.TryGetMapping(fieldInfo.FieldType, out BsonMapping fieldMapping) &&
                         fieldMapping.TryMapValue(mapper, fieldValue, out BsonValue bsonElementValue))
                    bsonDocument.Add(fieldInfo.Name, bsonElementValue);
            }

            bsonValue = bsonDocument;
            return true;
        }

        public override bool TryMapValue(BsonMapper mapper, BsonValue bsonValue, out object o)
        {
            o = Activator.CreateInstance(SourceType, true);
            
            if (bsonValue is BsonDocument bsonDocument)
            {
                foreach (FieldInfo fieldInfo in mappedFields)
                {
                    if (bsonDocument.Contains(fieldInfo.Name))
                    {
                        object fieldValue;
                        BsonValue bsonFieldValue = bsonDocument[fieldInfo.Name];
                        
                        if (bsonFieldValue is BsonNull)
                            fieldValue = null;
                        else if (mapper.TryGetMapping(fieldInfo.FieldType, out BsonMapping fieldMapping) && 
                                 fieldMapping.TryMapValue(mapper, bsonDocument[fieldInfo.Name], out fieldValue))
                        { }
                        else
                        {
                            o = null;
                            return false;
                        }
                        fieldInfo.SetValue(o, fieldValue);
                    }
                }
            }
            return true;
        }
    }
}