// Create a writer and open the file:
StreamWriter log;
string path = “c:\test\test.log”
if (!File.Exists(path))
{
    log = new StreamWriter(path);
}
else
{
    log = File.AppendText(path);
}

// Write to the file:
log.WriteLine(DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss") + " : " + ex.Message);
// Close the stream:
log.Close();