duplicatestreamasync()

I was trying to organize some older projects that dealt heavily with streams and I came across a nice little extension method I wrote a while back. It duplicates a stream and lets you specify the position of the source and destination streams. I can’t count the number of times I’ve copied a stream or breathed wrong on a stream and ended up with some weird bug because I didn’t reset the position of a stream. This little extension method takes care of that.

public static async Task<MemoryStream> DuplicateStreamAsync(
    this Stream source,
    StreamPosition sourcePosition = StreamPosition.Start,
    StreamPosition destinationPosition = StreamPosition.Start,
    CancellationToken cancellationToken = default)
{
    _ = source ?? throw new ArgumentNullException(nameof(source));
    if (!source.CanSeek && !source.CanRead)
        throw new ObjectDisposedException(
            nameof(source), "Cannot access a closed Stream.");

    var position = source.Position;
    var destination = new MemoryStream();
    try
    {
        source.Position = 0;
        await source.CopyToAsync(
            destination: destination,
            cancellationToken: cancellationToken).ConfigureAwait(false);

        if (destinationPosition == StreamPosition.Start)
            destination.Position = 0;
        else if (destinationPosition == StreamPosition.Current)
            destination.Position = position;

        return destination;
    }
    catch
    {
        destination.Dispose();
        throw;
    }
    finally
    {
        if (sourcePosition == StreamPosition.Start)
            source.Position = 0;
        else if (sourcePosition == StreamPosition.Current)
            source.Position = position;
        else source.Position = source.Length;
    }
}

public enum StreamPosition
{
    Start,
    Current,
    End
}

Granted, copying a stream isn’t rocket science, but I had one project where I was doing a lot of stream manipulation… and this was the result. By default, it resets the source and destination streams to the start position. You can also specify the current position or the end of the stream for either stream.

Actually, add this to it:

public static async Task<MemoryStream> DuplicateStreamAsync(
    this Stream source,
    CancellationToken cancellationToken) =>
    await source.DuplicateStreamAsync(
        sourcePosition: StreamPosition.Start,
        destinationPosition: StreamPosition.Start,
        cancellationToken: cancellationToken).ConfigureAwait(false);

This overload is a little more convenient if you don’t need to specify the positions, but do want to use a cancellation token without naming it. Anyway, it was a nice little helper for me at the time and I thought I’d share it. May all your Positions be 0.