Active Directory get user List

I am trying with this code to change the password of a user in active directory.

Code:

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rivate Sub button1_Click(sender As Object, e As EventArgs)
        Try
                ' Use the user account you're changing the password on
                ' to validate the user really exists. Specifies the
                ' authentication type as secure as I had problems
                ' getting it to work otherwise.
                Dim entry As New DirectoryEntry("LDAP://CN=Users,DC=domaim,DC=com", "Domain\Administrator", "password", AuthenticationTypes.Secure Or AuthenticationTypes.Sealing Or AuthenticationTypes.ServerBind)

                Dim search As New DirectorySearcher(entry)
                search.Filter = "(SAMAccountName=testuser)"
                Dim result As SearchResult = search.FindOne()
                Dim user As DirectoryEntry = result.GetDirectoryEntry()
                Dim ret As [Object] = user.Invoke("ChangePassword", New Object() {"password", "newpassword"})
                user.CommitChanges()
        Catch exception As Exception
                Console.WriteLine(exception.Message)
        End Try

End Sub

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But this is returning the result as Logon failure. But I am sure the user name and password are correct

Any ideas ?