internal string GetMemberType(CrmNumber crmNumber)
{
    switch (crmNumber.Value)
    {
        case ListType.Account:
            return EntityName.account.ToString();
        case ListType.Contact:
            return EntityName.contact.ToString();
        case ListType.Lead:
            return EntityName.lead.ToString();
        default:
            break;
    }
}

BusinessEntityCollection RetrieveMembers(list marketinglist)
{
 
 string targetEntity = GetMemberType(marketinglist.membertype);
 
 QueryExpression query = new QueryExpression
 {
   EntityName = targetEntity;
   ColumnSet = new AllColumns();
 };
 
 // link from target entity to listmember
 LinkEntity linkToListMemberEntity = new LinkEntity
 {
   LinkFromEntityName = EntityName.account.ToString();
   LinkFromAttributeName = targetEntity + "id";
   LinkToEntityName = EntityName.listmember.ToString();
   LinkToAttributeName = "entityid";
 };
 
 //link from listmember to list
 LinkEntity linkToMarketingList = new LinkEntity
 {
   LinkFromEntityName = EntityName.listmember.ToString();
   LinkFromAttributeName = "listid";
   LinkToEntityName = EntityName.list.ToString();
   LinkToAttributeName = "listid";
 };
 
 // filter result with the id of the marketing list
 ConditionExpression marketingListIdCondition = new ConditionExpression
 {
   AttributeName = "listid";
   Operator = ConditionOperator.Equal;
   Values = new [] { marketinglist.listid.Value };
 };
 
 linkToMarketingList.LinkCriteria = new FilterExpression();
 linkToMarketingList.LinkCriteria.Conditions.Add(marketingListIdCondition);
 
 linkToListMemberEntity.LinkEntities.Add(linkToMarketingList);
 
 query.LinkEntities.Add(linkToListMemberEntity);
 
 BusinessEntityCollection members = service.RetrieveMultiple(query);
 
 return members;
}