how to set the autofresh parameter through expression?

if the user chooses type=a
then the autofresh is true,eg.every 10secs.It should be something like this:
=iif(Parameters!type = "a", 10, 0)
This same pattern is used to write lots of expressions for conditional
formatting for instance in the FontWeight property for a textbox cell
you can place:
=iif(Value > 0, Bold,Normal)
This will bold all values greater than 0. Notice that you don't in
this case you can reference just value, but if you you wanted to base
it off another cell you would need to do something like this:
=iif(ReportItems!Textbox1.Value > 0, Bold,Normal)
Occasionally values such as colors have to be in quotes e.g. "Black".
Others such as FontWeight have constants associated with them.
Collections that can be accessed in this way are:
ReportItems
Globals
Parameters
Fields|||thanks a lot,
how to get all the collections that can be accessed?
i mean the specific parameter? e.g the auto refesh of the report?
can you show me some sites that are helpful for beginner?|||Except for ReportItems all of the collections are enumerated when you
go into the expression editor for a cell or property. For ReportItems
just click on the item to find out its name. If it has a nonspecific
name I recommend renaming it before you reference it.
I haven't been able to find any good sites. Books on reporting services
will give you a basic overview, but I haven't found any that go beyond
that. Most of what I have learned has been hacking it out trial and
error, patient googling for references to specific topics, and digging
around Microsoft's documentation, which is fairly spartan. This news
group is really one of the best sources of information, and
unfortunately I only found it a few days ago. I access it through
google groups(there are other ways, I recommend google). Google lets
you search just this news group for information. The group has been
around a while so queries for 'expression' or 'collection' turn up lots
of hits. Search the group first before posting a question.|||i also found the google group a few days ago, my mother language is not
english,so it is a little difficult for me to express my question
clearly,i really know that what i have asked is old,but patient
googling for referernces means costing much time,unfortunately i have
little time on that project.
i really apprecaite those who help me.thanks.