I need a recommendation on how to do the validation in this case without postback

[ssystems]

Fair enough. Either way the implementation should be just similar to what I posted earlier.

private Boolean _MyLockedToken;
protected Boolean MyLockedToken
{
    get
    {
        if(_MyLockedToken == null)
        {
            try
            {
                _MyLockedToken = (Boolean) Session[SESSION_KEY];
            }
            catch(Exception exc)
            {
                 // Your exception handling
            }
        }
        return _MyLockedToken;
    }
    set
    {
         Session[SESSION_KEY] = value;
         _MyLockedToken = value;
    }
}