Subset of messages for which there is more than one MRN per account number

Question:
How can we get the subset of messages for which there is more than one MRN associated with an account number?

Answer:
The best way to do this is to use a dictionary and track each message received based on the account number. At the end of processing all messages, check to see if any account has more than 1 MRN in its list. See the code below.

 // Called once or many times based on RunTypeRequested
 public class Tracker
 {
   private readonly List<string> m_medicalRecordNumbers = new List<string>(1);
   public string AccountNumber {get;set;}
   public List<string> MedicalRecordNumbers { get{return m_medicalRecordNumbers;} }
 }
 
 private readonly Dictionary<string,Tracker> m_items = new Dictionary<string,Tracker>(StringComparer.OrdinalIgnoreCase);  
 
 // Called once before the first message is processed.
 // It is a good place to initialize objects.
 // Always called from the UI thread.
 public override void OnStart()
 {
   m_items.Clear();
 }
   
 public override void Run()
 {
   // Get an HL7 Message in parsed format
   HL7Message message = GetParsedMessage();
   PID pid = message.GetFirstSegment<PID>();
   if(pid==null)
     return; // no pid segment

   var accountNumber = pid.PatientAccountNumber_18.Value;
   if(string.IsNullOrEmpty(accountNumber))
     return; // no account number
   
   var mrn = pid.PatientIdentifierList_03.First.Value;
   
   Tracker tracker;
   if(!m_items.TryGetValue(accountNumber, out tracker))
   {
     tracker = new Tracker{AccountNumber = accountNumber};
     m_items.Add(accountNumber, tracker);
   }
   
   if(!tracker.MedicalRecordNumbers.Any(m=>string.Compare(m, mrn, true)==0))
     tracker.MedicalRecordNumbers.Add(mrn);
 }
 
 // Called once after the last message has been processed.
 // It is a good place to perform cleanup and to report information.
 // Always called from the UI thread.
public override void OnFinish()
{
 string path = Path.GetTempFileName();
 using(var stream = new System.IO.StreamWriter(path))
 {
   List<Tracker> itemsWithMultipleMrns = m_items.Values.Where(i=>i.MedicalRecordNumbers.Count > 1).ToList();
   if(itemsWithMultipleMrns.Count == 0)
   {
     stream.WriteLine("No Account numbers were found with duplicate MRNs");
   } else
   {      
     foreach(Tracker item in itemsWithMultipleMrns)
     {
       stream.Write(item.AccountNumber);
       foreach(var mrn in item.MedicalRecordNumbers)
       {
         stream.Write(" ,");
         stream.Write(mrn);
       }
       stream.WriteLine();      
     }
   }
   // show the results in notepad
   try
   {
     System.Diagnostics.Process.Start("Notepad", path);
   } catch(Exception ex)
   {
     this.Log(Severity.Error, string.Format("Could not open file '{0}' with notepad",path));
   }
 }
}