How to check Validity of a COM Object

How to check Validity of a COM Object

Hi,
I was trying to Automate MS Excel, I want to check the validity of a COM Object in C#. For Instance, after releasing a sheet Object using "Marshal.ReleaseComObject(m_oSheets)" Sheet object is not equal to null. How to check a COM Object is valid or not.
Thanks in Advance.


//sample code

Excel._Application m_oApplication;
Excel.Workbooks m_oWorkBooks;
Excel._Workbook m_oWorkBook;
Excel.Sheets m_oSheets;
Excel._Worksheet m_oWorkSheet;

Marshal.ReleaseComObject(m_oSheets);
if (m_oSheets == null)
{
......
}