How to change System.DirectoryEntry "uSNChanged" attribute value to an Int64

I'm trying to get the Int64 value of a Directory Services object's "uSNChanged" value. Unfortunately, it is always coming back as a COM object of some kind. I've tried using casting to Int64, calling Int64.Parse(), and calling Convert.ToInt64(). None of these work.

For a given DirectoryEntry object, this code will display the properties:

    private static void DisplaySelectedProperties(DirectoryEntry objADObject)
    {
        try
        {
            string[] properties = new string[] {
                "displayName",
                "whenCreated",
                "whenChanged",
                "uSNCreated",
                "uSNChanged",
            };

            Console.WriteLine(String.Format("Displaying selected properties of {0}", objADObject.Path));
            foreach (string strAttrName in properties)
            {
                foreach (var objAttrValue in objADObject.Properties[strAttrName])
                {
                    string strAttrValue = objAttrValue.ToString();
                    Console.WriteLine(String.Format("   {0, -22} : {1}", strAttrName, strAttrValue));
                }
            }
            Console.WriteLine();
        }
        catch (Exception ex)
        {
            throw new ApplicationException(string.Format("Fatal error accessing: {0} - {1}", objADObject.Path, ex.Message), ex);
        }
    }

This is the output:

Displaying selected properties of LDAP://server/o=org/cn=obj
   displayName            : Display Name
   whenCreated            : 7/8/2009 7:29:02 PM
   whenChanged            : 7/8/2009 10:42:23 PM
   uSNCreated             : System.__ComObject
   uSNChanged             : System.__ComObject

How do I convert that System.__ComObject into a Int64?

---

Solution Used:

This is the solution I used based on marc_s's solution below:

    public static Int64 ConvertADSLargeIntegerToInt64(object adsLargeInteger)
    {
         var highPart = (Int32)adsLargeInteger.GetType().InvokeMember("HighPart", System.Reflection.BindingFlags.GetProperty, null, adsLargeInteger, null);
         var lowPart  = (Int32)adsLargeInteger.GetType().InvokeMember("LowPart",  System.Reflection.BindingFlags.GetProperty, null, adsLargeInteger, null);
         return highPart * ((Int64)UInt32.MaxValue + 1) + lowPart;
    }

I'm using this snippet of code in my ADSI Browser BeaverTail which is written in C#:

Int64 iLargeInt = 0;

IADsLargeInteger int64Val = (IADsLargeInteger)oPropValue.LargeInteger;
iLargeInt = int64Val.HighPart * 4294967296 + int64Val.LowPart;

As far as I can tell, this should work just fine.

Marc