Thread Safe Objects in CSharp - ConcurrentDictionary

I'm using ConcurrentDictionary to hold the records.. I'm reading files from the local system in parallel, to speed up the process.

Sample 1:

static ConcurrentDictionary<Int64, Byte[]> concurrentDictionary = new ConcurrentDictionary<Int64, Byte[]>();

private void ReadFilesFromDirectory()
{
  string[] docNames = Directory.GetFiles(CompleteLettersDirectory);
  Parallel.ForEach(docNames, new ParallelOptions { MaxDegreeOfParallelism = 8 }, file =>
    {
      try
      {
          int64 letterId = Convert.ToInt64(file.Split('_')[1]);
          Byte[] CompleteLetterContent;
          CompleteLetterContent = ConvertToPdfWithAspose(System.IO.File.ReadAllBytes(file), letterId);
          concurrentDictionary.GetOrAdd(letterId, CompleteLetterContent);
      }
      catch (Exception ex)
      {
          //Logger.WriteFile(letterId.ToString() + Environment.NewLine, ex);
      }
    }
}

Sample 2: In this sample I want to keep multiple values in dictionary as such I created a letter class which has multiple properties. As Letter class is not threadsafe, will it create any issues ?

static ConcurrentDictionary<Int64, Letter> concurrentDictionary = new ConcurrentDictionary<Int64, Letter>();

private void ReadFilesFromDirectory()
{
  string[] docNames = Directory.GetFiles(CompleteLettersDirectory);
  Parallel.ForEach(docNames, new ParallelOptions { MaxDegreeOfParallelism = 8 }, file =>
    {
      try
      {
          int64 letterId = Convert.ToInt64(file.Split('_')[1]);
          Byte[] LetterContent;
          Byte[] CompleteLetter;

          Letter letter = new Letter();
          letter.LetterId = letterId;

      if(file.Contains("CL"))
      {
    letter.CompleteLetter = ConvertToPdfWithAspose(System.IO.File.ReadAllBytes(file), letterId);
      }
      else
      {
    letter.LetterContent = ConvertToHTML(System.IO.File.ReadAllBytes(file), letterId);
      }
    concurrentDictionary.GetOrAdd(letterId, letter);
          }
      catch (Exception ex)
  {
          //Logger.WriteFile(letterId.ToString() + Environment.NewLine, ex);
      }
    }
}

public class Letter
  {
      public Int64 LetterId { get; set; }
      public Byte[] LetterContent { get; set; }
      public Byte[] CompleteLetter { get; set; }
  }

Can anyone advise if code samples are threadsafe. Also, should string docNames which holds documents from the directory and is passed to Parallel.ForEach be also threadsafe or not? Or any other recommendation.