getting blank ParentId of forumreply

Does this work?

#set($reply =  $core_v2_forumReply.Get(934885))

$reply.ParentId

no in that i am getting nothing mean same $reply.ParentId is printing

That means its null, velocity scripts do not have a concept of null so it just prints the statement.  

or the reply is null, either one will have the same result

so in that case how we can get forumthreadid for a forumreply

$reply.ThreadId

But you need to ensure your $reply is not null and you're making the correct call to get it back

#set($reply =  $core_v2_forumReply.Get(934885))

$reply.ThreadId

when i am running this code i am getting 934885(same id)

in that case how it may be forumthread id because it is forumthreadreply id

While unlikely, Ids are not globally unique so it is possible.  Have you actually checked these Ids?

i am getting same $core_v2_forumThread.Get(934885) print

i am getting same $core_v2_forumThread.Get(934885) print

Please try this in the script console to figure out if your reply is null.

#set($reply = false)
#set($reply = $core_v2_forumReply.Get(934885))
#if ($reply) 
    Reply's Thread ID: $reply.ThreadId
#else
    Reply does not exist.
#end

Assuming your reply ID is correct and the reply exists, then your original syntax approach would have still worked, though with ThreadId instead of ParentId. That said, it's better to have those kinds of null checks for safety anyway.

$core_v2_forumReply.Get(934885).ThreadId