Well, I answered my own question, which I will share here in case it is of use to anyone else.

My situation is that I had a very long list of StudentID's as a multi-valued parameter is SSRS (2008)--so long that it could sometimes exceed the 8000 char limit for using varchar as a datatype in my stored procedure. All I care about here are the StudentID's (parameter value), not their names (parameter label).

I added the following code to the custom code block in Report Properties in SSRS:

Function StrParmToXML(ByVal obj As parameter) As String

Dim strXML as String

Dim i As Integer

strXML = ""

If obj.IsMultiValue then

For i = 0 To obj.count - 1

strXML = strXML & "<M>" & Cstr(obj.Value(i)) & "</M>"

Next

Else

strXML = strXML & "<M>" & Cstr(obj.Value(i)) & "</M>"

End IF

Return strXML

End Function

In the dataset that calls my stored procedure, I invoke this function as an expression for the Student parameter that is passed to the sp:

=Code.StrParmToXML(Parameters!Student)

On the SQL side, I parse the xml parameter to table rows using the following function:

create function [dbo].[fn_xml_to_table] (@parmxml xml) RETURNS TABLE

AS

RETURN

with name_cte as

(

select @parmxml as Names

),

name_parse_rows as

(

select

ltrim(rtrim(Split.a.value('.', 'VARCHAR(100)'))) AS item

from name_cte

CROSS APPLY Names.nodes('/M') Split(a)

) select * from name_parse_rows

I then invoke the function to select all students using the following sql statement:

select

UserID

,StudentName

from v_Students

where UserID IN (select item from dbo.fn_xml_to_table(@xmlUserList_in));

Dan