Find the last ADT^A08 message for each patient in the message collection?

How can I find the last ADT^A08 message for every patient in a collection of messages?

The best way to do this is to create a dictionary based on the patient Medical Record Number (PID-3). For each message receive. Look up the patient in the dictionary, if there is no entry, add one. Otherwise, replace the existing entry’s message with the new version. See the code below.

string _fileName = @"c:\_test.txt";
Dictionary<string,Patient> _data = new Dictionary<string,Patient>(10000);

public class Patient
   public Patient(string patientId)
    PatientId = patientId;

   public IMessageData Message;
   public string PatientId;

public override void Run() {
  // Get an HL7 Message in parsed format
  HL7Message message = GetParsedMessage();
  MSH msh = message.GetFirstSegment<MSH>();
  if(msh==null || msh.MessageType_09.TriggerEvent_02.Value!="A08")
    return; // not an ADT^A08 message

  PID pid = message.GetFirstSegment<PID>();
    return; // no pid segment

  string patientId = pid.PatientIdentifierList_03.First.Value; 
  Patient pat;
  if(!_data.TryGetValue(patientId,out pat))
     pat = new Patient(patientId);

  pat.Message = Message;

// 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()
   SaveMessage(_data.Values.Select(p=>p.Message),"Last A08");