FundingStatusServer/Models/ContractData.cs

using System.Collections.Generic;
using PaymentServer.Models;
using Newtonsoft.Json;

namespace PaymentServer.Models
{
    public class ContractData
    {
        public Dictionary<string, List<VendorPayment>> contracts = new Dictionary<string, List<VendorPayment>>();

        public void AddPayments(List<VendorPayment> vendorPayments)
        {
            List<string> existingContracts = new List<string>(this.contracts.Keys);

            foreach (VendorPayment payment in vendorPayments)
            {
                if (existingContracts.Contains(payment.contractNumber)) 
                {
                    this.contracts[payment.contractNumber].Add(payment);
                }
                else
                {
                    this.contracts.Add(payment.contractNumber, new List<VendorPayment> { payment });
                    existingContracts.Add(payment.contractNumber);
                }
            }
            Console.WriteLine(this.contracts.Count);
        }
        public string ToJson()
        {
            return JsonConvert.SerializeObject(this.contracts.Values);
        }
    }
}