Memory stream is not expandable

Question

I'm attempting to read an email attachment and I'm getting a "Memory Stream is not expandable" error. I researched this some and most of the solutions seemed related to determining the size of the buffer dynamically, but I'm already doing that. I'm not very experienced with memory streams, so I'd like to know WHY this is a problem. Thanks.

foreach (MailMessage m in messages)
{
   byte[] myBuffer = null;
   if (m.Attachments.Count > 0)
   {
      //myBuffer = new byte[25 * 1024];  old way 
      myBuffer = new byte[m.Attachments[0].ContentStream.Length];
      int read;
      while ((read = m.Attachments[0].ContentStream.Read(myBuffer, 0, myBuffer.Length)) > 0)
      {
          // error occurs on executing next statement
          m.Attachments[0].ContentStream.Write(myBuffer, 0, read);
      }

      ... more unrelated code ...

Answer 1

You need to replace the line

m.Attachments[0].ContentStream.Write(myBuffer, 0, read);

with a line that writes to a previously created MemoryStream, e.g.

foreach (MailMessage m in messages)
{
   byte[] myBuffer = null;
   if (m.Attachments.Count > 0)
   {
      //myBuffer = new byte[25 * 1024];  old way 
      myBuffer = new byte[m.Attachments[0].ContentStream.Length];
      int read;
      MemoryStream ms = new MemoryStream();
      while ((read = m.Attachments[0].ContentStream.Read(myBuffer, 0, myBuffer.Length)) > 0)
      {
          ms.Write(myBuffer, 0, read);
      }

Answer 2

If you create a MemoryStream over a pre-allocated byte array, it can't expand (ie. get longer than the size you specified when you started). Instead, why not just use:

using (var ms = new MemoryStream())
{
   // Do your thing, for example:
   m.Attachments[0].ContentStream.CopyTo(ms);

   return ms.ToArray(); // This gives you the byte array you want.
}