Retrieve related entity records along with the Primary entity using Retrieve Method

namespace D365HelperApplication.CommonClasses
{
    class FetchRecords
    {
        public static List<Guid> FetchRelatedRecords(string relatedEntityName, string relatedEntityPrimaryAttrName, string primaryEntityAttrName, string primaryEntityGuid, CrmServiceClient client)
        {
            List<Guid> relatedEntityIds = new List<Guid>();


            // Define Condition Values
            var query_primaryEntityid = primaryEntityGuid;

            // Instantiate QueryExpression query
            var query = new QueryExpression(relatedEntityName);

            // Add columns to query.ColumnSet
            query.ColumnSet.AddColumns(relatedEntityPrimaryAttrName);

            // Define filter query.Criteria
            query.Criteria.AddCondition(primaryEntityAttrName, ConditionOperator.Equal, query_primaryEntityid);

            EntityCollection ec = client.RetrieveMultiple(query);

            if(ec.Entities.Count() > 0)
            {
                foreach(Entity e in ec.Entities)
                {
                    relatedEntityIds.Add(e.Id);
                }
            }

            return relatedEntityIds;
        }
    }
}