How to make select as variable for Export-csv?

Question

Hello, 
I'm trying to get the script to work which can iterate over topics and make predefined selection based on current active topic on iteration. It seems that the problem is simple, but I can't find the solution.
 The source file is the following:
topic=interface_bridge ruleaction=add name=vlan10
topic=interface_bridge ruleaction=add name=vlan13
topic=interface_bridge ruleaction=add name=vlan14
topic=interface_bridge ruleaction=add name=vlan20
topic=interface_ethernet set=0 comment=DMZ
topic=interface_ethernet set=1 comment=DMZ
topic=interface_ethernet set=2 comment=Servers
topic=interface_ethernet set=4 comment=WKS
topic=interface_ethernet set=5 comment=VMWARE
topic=interface_ethernet set=6 comment=WKS
topic=interface_ethernet set=10 comment=External
The current script is the following:
$file="c:\scripts\mk.txt"
#selections
$interface_ethernet='comment','name','set','switch','master-port'
$interface_bridge='comment','name','protocol-mode'
#topics
$topics1=@($interface_ethernet, $interface_bridge)
$topic_int_bridge='interface_bridge'
$topic_int_ethernet='interface_ethernet'
$topics=@($topic_Int_bridge, $topic_int_ethernet)
foreach ($topic in $topics)
$match='^topic='+$topic+'\s+'
$result_File = 'config_'+$topic+'.txt'
$select = '$'+$topic
$interface_ethernet='comment','name','set','switch','master-port'
$interface_bridge='comment','name','protocol-mode'
$topics1=@($interface_ethernet,$interface_bridge)
$select1=invoke-expression ('$'+$topic)
$results = Get-Content mk.txt | Where-Object { $_ -Match $match }| foreach { new-object PSObject -Property $($_ -replace '^' -replace ' ',"`n" | convertfrom-stringdata) }
$results | select $select1
I'm hang on the following: when I run the script it only takes first selection ($interface_bridge) gives the result about first topic (bridge), and for second (ethernet) - only comments.
comment name protocol-mode
vlan10
vlan13
vlan14
vlan20
DMZ
DMZ
Servers
WKS
VMWARE
WKS
External
There can be many topics, so write If expression for each it's not the case.
Question:
How to modify the script that the result is printed for each topic with adequate section:
1) when looping over topics, when the topic interface_ethernet is processed select and print results for variable $interface_ethernet
(selected items 'comment','name','set','switch','master-port')
2) when looping over topics, when the topic interface_bridge is processed select and print results for variable $interface_bridge
(selected items 'comment','name','protocol-mode')
3) etc...
Each topic should be on separated iteration block because for each will be generated csv file with each own header. 
Thanks.

Answer 1

Hello!
I'm trying to understand why the script does not work. The question is:
Is it correct way of using the "invoke-epression" on code?
$select1=invoke-expression ('$'+$topic)