Alice,

I had problems with the conversions, too.  Here is a code snippet of my
solution:  ( I am reading power data from an instrument)

double m_dTemp, m_dCh1LastPower; //

// This bypasses conversion error that shows up,
// Error is on return value of "" (Empty sting)
if ( Double.TryParse(strReading,
    System.Globalization.NumberStyles.Float,
    null, out m_dTemp) )
{  // True on successful conversion, so use it

        m_dCh1LastPower = m_dTemp;
               // To modify and return to string
               m_dCh1LastPower = m_dCh1LastPower * 1000;
        String sFormatter = "#0.00";
               string txtNew = m_dCh1LastPower.ToString(sFormatter)
}