System.IO Exception The process cannot access the file because it is used by another

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C#3.0

C#

Hello All,
System.IO Exception The process cannot access the file because it is used by another process exception is thrown
but in my code i have used using block as well as close also but also exception is thrown what shall i do for this?
One process is reading the contents of file while writing the file at that time exception is occured how to avoid this?

private static void WritToXmlFile(string xmlfile, XmlDocument doc)
       {
         using (StreamWriter sw = new StreamWriter(xmlfile, false))
               {
                   sw.Write(doc.OuterXml);
                   sw.Close();
               }
      }