Show distinct parameter list in reporting services

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reporting-services

In my report, I get the DataSet from the OLAP Analysis Server. Then add a calculated filed to the DataSet based on one existed field. Now I want to add a parameter to the report by using the calculated field. After a preview, I find there is duplicate items in the parameter list. How can I make a distinct list?
I have tried to add code to remove the duplicates, but no success.
reference to the code snippet here. http://blogs.msdn.com/b/sqlforum/archive/2011/04/28/walkthrough-how-to-get-distinct-values-of-a-column-of-a-sharepoint-list-using-sql-server-reporting-services.aspx[^]

Some Update for the question.
I have managed to make the sample code work by adding a parameter based on the calculated filed mentioned above. It is stupid as I need to first make a select for this dummy parameter and then the distinct parameter list will show. After a dig, it should be caused by the dummy parameter cannot be set some default value.
Any suggestion on this?