static void Main(string[] args)
{
    using (var ms = new MemoryStream())
    {
        var sw = new StreamWriter(ms);
        sw.WriteLine("Hello World");
        // The string is currently stored in the 
        // StreamWriters buffer. Flushing the stream will 
        // force the string into the MemoryStream.
        sw.Flush();

        // If we dispose the StreamWriter now,it will close 
        // the BaseStream (which is our MemoryStream) which 
        // will prevent us from reading from our MemoryStream
        //DON'T DO THIS - sw.Dispose();

        // The StreamReader will read from the current 
        // position of the MemoryStream which is currently 
        // set at the end of the string we just wrote to it. 
        // We need to set the position to 0 in order to read 
        // from the beginning.
        ms.Position = 0;
        var sr = new StreamReader(ms);
        var myStr = sr.ReadToEnd();
        Console.WriteLine(myStr);
    }

    Console.WriteLine("Press any key to continue.");
    Console.ReadKey();
}