MemoryStream have one thread write to it and another read

You can't use a Stream with seeking capabilities from 2 threads simultaneous since a Stream is state full. e.g. A NetworkStream has 2 channels, one for reading and one for writing and therefore can't support seeking.

If you need seeking capabilities, you need to create 2 streams, one for reading and one for writing respectively. Else you can simply create a new Stream type which allows reading and writing from a underlying memory stream by taking exclusive access to the underlying stream and restore its write/read position. A primitive example of that would be:

class ProducerConsumerStream : Stream{    private readonly MemoryStream innerStream;    private long readPosition;    private long writePosition;    public ProducerConsumerStream()    {        innerStream = new MemoryStream();    }    public override bool CanRead { get { return true;  } }    public override bool CanSeek { get { return false; } }    public override bool CanWrite { get { return true; } }    public override void Flush()    {        lock (innerStream)        {            innerStream.Flush();        }    }    public override long Length    {        get         {            lock (innerStream)            {                return innerStream.Length;            }        }    }    public override long Position    {        get { throw new NotSupportedException(); }        set { throw new NotSupportedException(); }    }    public override int Read(byte[] buffer, int offset, int count)    {        lock (innerStream)        {            innerStream.Position = readPosition;            int red = innerStream.Read(buffer, offset, count);            readPosition = innerStream.Position;            return red;        }    }    public override long Seek(long offset, SeekOrigin origin)    {        throw new NotSupportedException();    }    public override void SetLength(long value)    {        throw new NotImplementedException();    }    public override void Write(byte[] buffer, int offset, int count)    {        lock (innerStream)        {            innerStream.Position = writePosition;            innerStream.Write(buffer, offset, count);            writePosition = innerStream.Position;        }    }}