SSRS Drilling through on dependent grouped parameters - reporting-services

Question

I have been asked to add a group to a drill-through report depending on the parent report's multi-value parameter called Projects.
This group works perfectly on the parent chart however when drilling through into the child other parameters dependent on Projects are not being selected despite appearing in the list.
For instance the project parameter controls the available options for another multi-select Team parameter. When multiple projects are selected you can run the report across multiple teams, when selecting a single project from the grouped report this limits the team filter in the child, despite some of the original parameters from the main report being available in the child none are selected.
When passing the parameters to the drill-through report can I produce a sublist from the parent Team parameter filtered on the grouped Project parameter?
Pseudo-LINQ example:
=Parameters!TeamIds.Where(t => t.ProjectId == Fields!ProjectId.Value)
Alternatively is there any way to pass through the full list of teams to the child report and for the report to select the teams that match the selected project?
Edit
Available values for the Project parameter in both parent and child reports:
select id, header from projects
Available values for the Team parameter in both parent and child reports:
select t.id, t.header from teams t where t.ProjectId = #ProjectId
When running the report we display a lot of data in grouped grids with actions to run another report.
This Drillthrough passes the full list of teams. This stops the child report because some of the teams passed in are not valid values. Then the child report does not automatically run and forces the user to reselect the team.
Despite the Drillthrough implicitly setting the team from the main parameter.
I believe this is breaking due to the child report not being able to handle the multiple team projects originally selected in the main report.
My question is how do I:
a) Filter the team parameter to only include the project I have selected to drillthrough
or
b) Make the child report accept the full list of teams passed into it and only select the appropriate ones from the new filtered team list based on the single Project
Edit 2
Public Function GetTeamsForProject(ByVal ItemWithProjectInHeader as Parameter, ByVal ProjectHeader as String) as Collection
Dim s as new Collection
For i as integer = 0 to ItemWithProjectInHeader.Count-1
if ItemWithProjectInHeader.Label(i).Contains(ProjectHeader)
s.Add(ItemWithProjectInHeader.Value(i))
End If
Next
Return s
End Function
Started trying to put together some code to cherry pick the teams but I can't seem to get the collection return to work, for now i am calling it like
=Code.GetTeamsForProject(Parameters!TeamIds,CSTR(Parameters!ProjectId.Label(0)))
This just returns #Error in the text box i am using.
Hopefully someone could at least spot the 'deliberate' mistake on this.
Thanks
Edit 3
I am also trying something like this
Public Function GetItemsForProject(ByVal ItemWithProjectInHeader as Parameter, ByVal ProjectHeader as String) as Collection
Dim s as new Collection
For i as integer = 0 to ItemWithProjectInHeader.Count-1
if INSTR(1,ItemWithProjectInHeader.Label(i), ProjectHeader,1) > 0
s.Add(ItemWithProjectInHeader.Value(i))
End If
Next
Return s
End Function
and calling it like...
=Code.GetItemsForProject(Parameters!TeamIds,Fields!ProjectId.Value)

Answer 1

As you suggest, It looks like the core problem is that a parameter is being passed into the child report that the child report considers invalid.
Seems like two directions to solve this: one, Filter down what you send pass as the parameter value from the parent, or two, allow all the values that are coming in. Not knowing the structure of the report it seems like the second of these is easier. If you accepted extra teams into the child report will it cause the report to display incorrect values?
That is, if you change the possible values for the Team parameter in the child report to:
select t.id, t.header from teams t
That would certainly limit the usefulness of the child report when used directly, but do users go straight to the child report?
Another option would be to create two parameters in the child report. PassthroughTeams would be a hidden parameter that is passed in by the parent.
Teams would stay as it is, but have some default value (maybe all possible teams)
Then change the core data query of the child report to something like:
SELECT mydata.*
FROM mydata
WHERE mydata.teamId IN (#TeamIds)
AND mydata.teamId IN (#PassthroughTeams)

Answer 2

Finally got a working solution, I added the Project header to the team parameter so that I can use the team's label in the pass through.
Some problems with the original code:
Missing the 'Then' from the ifs
Potential that the instr or contains were missing references so moved to using the startsWith function
Public Function startsWith(str As String, prefix As String) As Boolean
startsWith = Left(str, Len(prefix)) = prefix
End Function
Public Function GetItemsForProject(ByVal ItemWithProjectInHeader as Parameter, ByVal ProjectHeader as String) as Collection
Dim s as new Collection
If ItemWithProjectInHeader.IsMultiValue Then
For i as integer = 0 to ItemWithProjectInHeader.Count-1
If startsWith(ItemWithProjectInHeader.Label(i), ProjectHeader) Or startsWith(ItemWithProjectInHeader.Label(i), "(Not") Then
s.Add(ItemWithProjectInHeader.Value(i))
End If
Next
Else
If startsWith(ItemWithProjectInHeader.Label(0), ProjectHeader) Then
s.Add(ItemWithProjectInHeader.Value(0))
End If
End If
Return s
End Function