Can I return a string if I use "using (StreamReader sw = new StreamReader(new FileStream("out1.xml", FileMode.Open)))"

[C# Programming]

string message = Timetable(doc);

public string Timetable(XmlDocument xmldocument)
{
string timetableResult = "";
if (xmldocument != null)
{
Output("Processing...");

using (StreamReader sw = new StreamReader(new FileStream("out1.xml", FileMode.Open)))
{
string str = sw.ReadToEnd();
//extraction and suppose there's a string here which I want to return
}
}
}

I can't seem to return a string, the error says :Could not load file or assembly 'RS232, Version=1.24.0.0, Culture=neutral, PublicKeyToken=515d87df384dcc81' or one of its dependencies. The system cannot find the file specified.

Please help.