namespace ConsoleApplicationLibrary
{
    class Program
    {
        static void Main(string[] args)
        {
            using (SPSite siteColl = new SPSite("http://localhost/Deps/SUP"))
            {
                using (SPWeb web = siteColl.OpenWeb())
                {
                    try
                    {
                        SPDocumentLibrary library = (SPDocumentLibrary)web.Lists["Общие документы"];

                        //write text-based content into memory stream
                        MemoryStream DocumentSream = new MemoryStream();
                        StreamWriter writer = new StreamWriter(DocumentSream);
                        writer.Write("Hello");
                        writer.Close();

                        //create new document using text content in memory sream
                        library.RootFolder.Files.Add("test.txt", DocumentSream, true);

                        // dispose memory stream object to release memory 
                        DocumentSream.Dispose();
                    }
                    catch (Exception ex)
                    {
                        SPSecurity.RunWithElevatedPrivileges(delegate()
                        {
                            Log.Add("Exception", ex.Message + ":::" + ex.StackTrace,"wer","kjh");
                        });
                    }
                }
            }
        }
    }
}

 library.RootFolder.Files.Add("test.txt", DocumentSream, true);

StreamWriter sw = File.AppendText(@"D:\Variables.txt", true, Encoding.ASCII)
{
     sw.WriteLine("Новая строка bla bla bla");
}