linq sum example / csharp

 var activefees = from af in invoice.AccountFees
                         where af.StatusID == EnumStatusType.Open
                         select af;
 return activefees.Sum(af => af.Amount);

 // or more complicated with condition 
 decimal am = 0;
 foreach (XAttribute v in rs.RuleAccountFeesToBill(customer_id)){
            long f = long.Parse(v.Value);
            am += activefees.Sum(acf => acf.AccountFeeSubType.AccountFeeType.FeeTypeID == f ? acf.Amount : 0 );
 }

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