Change password using Active Directory

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ASP.NET

Hi, I'm Thinh. I saw your posts in some forums about AD, especially aboutt the way to communicate with AD using ASP.net. I would like to ask you to help me correct a problem like this:

This is a code I use to change password on AD

The problem here is :

- If the Admin creates a user and doesn't check "User must change password the next logon" then the user from client can change password successfully, and without error
- But if the Admin creates a user and check "User must change password the next logon"  then the user can't change password .
  I know the error caused by: When "User must change password the next logon" is checked, user have to change it immediately in a sign in window, and use new password to login instead of the old password.
How can I deal with this problem ?