Writing to then reading from a MemoryStream

Three things:

• Don’t close the StreamWriter. That will close the MemoryStream. You do need to flush the writer though.
• Reset the position of the stream before reading.
• If you’re going to write directly to the stream, you need to flush the writer first.

So:

using (var stream = new MemoryStream())
{
    var sw = new StreamWriter(stream);
    sw.Write("{");

    foreach (var kvp in keysAndValues)
    {
        sw.Write("'{0}':", kvp.Key);
        sw.Flush();
        ser.WriteObject(stream, kvp.Value);
    }    
    sw.Write("}");            
    sw.Flush();
    stream.Position = 0;

    using (var streamReader = new StreamReader(stream))
    {
        return streamReader.ReadToEnd();
    }
}

There’s another simpler alternative though. All you’re doing with the stream when reading is converting it into a string. You can do that more simply:

return Encoding.UTF8.GetString(stream.GetBuffer(), 0, (int) stream.Length);

Unfortunately MemoryStream.Length will throw if the stream has been closed, so you’d probably want to call the StreamWriter constructor that doesn’t close the underlying stream, or just don’t close the StreamWriter.

I’m concerned by you writing directly to the the stream – what is ser? Is it an XML serializer, or a binary one? If it’s binary, your model is somewhat flawed – you shouldn’t mix binary and text data without being very careful about it. If it’s XML, you may find that you end up with byte-order marks in the middle of your string, which could be problematic.