Programmatically Expand/Collapse Groups

I use Visual Studio 2010 to develop SSRS reports.

I added 2 buttons in the UI that contains the Report View. One button is used to expand the group and the other to collapse the group.

The buttons work very well except one report that has multiple sub-groups.

Here's the code behind the buttones:

Microsoft.Reporting.WinForms.ReportParameter reportDataParameter = new ReportParameter("parameterExpand", "true"); //or "false" to collapse them.
reportUsage.LocalReport.SetParameters(reportDataParameter);
reportUsage.RefreshReport();

The group properties - visibility has been set with a expression correctly.

The problem for that particular report is:  All groups are expanded/collapsed correctly, but the "+" sign never changes.

I expect that when they are all expanded, the "+" sign should change to "-" sign, when they are all collapsed, the "-" sign should change to "+" sign.

Plesase help.

Thanks.