open specific part of table of contents of chm file c# or vb.net

open specific part of table of contents of chm file c# or vb.net

I have a .chm file called help, In that file I have a structure like:
Introduction
-item1
-item2
Topic1
-item1
-item2
Topic2
-item1
-item2
Topic3
Now I want to open Topic1 inside c# or vb.net I have tried:
Help.ShowHelp(ParentForm, "Help.chm", HelpNavigator.Index, "Topic1")
Help.ShowHelp(ParentForm, "Help.chm", HelpNavigator.TableOfContents,
"Topic1")
but is not working, then I tried to give inside chm file an index to
Topic1 (31) and tried:
Help.ShowHelp(ParentForm, "Help.chm", HelpNavigator.Index, "31")
Help.ShowHelp(ParentForm, "Help.chm", HelpNavigator.Index, "Item1")
It gives execption, only working code is:
Help.ShowHelp(ParentForm, "Help.chm", HelpNavigator.TableOfContents,
Nothing)
How to open Topic1 ?