Yield return with try catch, how can I solve it?

Question

I've a piece of code:

using (StreamReader stream = new StreamReader(file.OpenRead(), Encoding))
{
    char[] buffer = new char[chunksize];
    while (stream.Peek() >= 0)
    {
       int readCount = stream.Read(buffer, 0, chunksize);

       yield return new string(buffer, 0, readCount);
    }
 }

Now I have to surround this with an try-catch block

try
{
   using (StreamReader stream = new StreamReader(file.OpenRead(), Encoding))
   {
       char[] buffer = new char[chunksize];
       while (stream.Peek() >= 0)
       {
          int readCount = stream.Read(buffer, 0, chunksize);

          yield return new string(buffer, 0, readCount);
       }
    } 
}
catch (Exception ex)
{
    throw ExceptionMapper.Map(ex, file.FullName)
}

I can't see any way to do what I want.

The method has the signature:

public IEnumerable<string> ReadPieces(int pieces)

I need a try/catch with a call to the ExceptionMapper in the catch case. The method is used deferred by all callers.

The exceptions I have to catch are coming from these calls:

File.OpenRead()
stream.Read()

Answer 1

Because you want to keep the Stream open for the duration of the enumeration AND deal with exceptions AND properly close the file handle either way, I don't think you can use a regular enumeration shortcut (the iterator block, yield-return/yield-break).

Instead, just do what the compiler would have done for you and add some:

By implementing IEnumerator yourself, you can also add IDisposable

public class LazyStream : IEnumerable<string>, IDisposable
{
  LazyEnumerator le;

  public LazyStream(FileInfo file, Encoding encoding)
  {
    le = new LazyEnumerator(file, encoding);
  }

  #region IEnumerable<string> Members
  public IEnumerator<string> GetEnumerator()
  {
    return le;
  }
  #endregion

  #region IEnumerable Members
  System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
  {
    return le;
  }
  #endregion

  #region IDisposable Members
  private bool disposed = false;

  public void Dispose()
  {
    Dispose(true);

    GC.SuppressFinalize(this);
  }

  protected virtual void Dispose(bool disposing)
  {
    if (!this.disposed)
    {
      if (disposing)
      {
        if (le != null) le.Dispose();
      }

      disposed = true;
    }
  }
  #endregion

  class LazyEnumerator : IEnumerator<string>, IDisposable
  {
    StreamReader streamReader;
    const int chunksize = 1024;
    char[] buffer = new char[chunksize];

    string current;

    public LazyEnumerator(FileInfo file, Encoding encoding)
    {
      try
      {
        streamReader = new StreamReader(file.OpenRead(), encoding);
      }
      catch
      {
        // Catch some generator related exception
      }
    }

    #region IEnumerator<string> Members
    public string Current
    {
      get { return current; }
    }
    #endregion

    #region IEnumerator Members
    object System.Collections.IEnumerator.Current
    {
      get { return current; }
    }

    public bool MoveNext()
    {
      try
      {
        if (streamReader.Peek() >= 0)
        {
          int readCount = streamReader.Read(buffer, 0, chunksize);

          current = new string(buffer, 0, readCount);

          return true;
        }
        else
        {
          return false;
        }
      }
      catch
      {
        // Trap some iteration error
      }
    }

    public void Reset()
    {
      throw new NotSupportedException();
    }
    #endregion

    #region IDisposable Members
    private bool disposed = false;

    public void Dispose()
    {
      Dispose(true);

      GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
      if (!this.disposed)
      {
        if (disposing)
        {
          if (streamReader != null) streamReader.Dispose();
        }

        disposed = true;
      }
    }
    #endregion
  }
}

I didn't test this, but I think it's close.

used like this:

using (var fe = new LazyStream(new FileInfo("c:\\data.log"), Encoding.ASCII))
{
  foreach (var chunk in fe)
  {
    Console.WriteLine(chunk);
  }
}

EDIT: I had totally forgotten to add the try-catch block placements. Oops.