try:
protected void btn_generatereport_onclick(object sender, EventArgs e)
{
DataObjectDataSource1.SelectParameters["xyz"].DefaultValue = "xxx";
DataObjectDataSource1.SelectParameters["abc"].DefaultValue ="xxx";
ReportParameter[] reportParameters = new ReportParameter[8];
reportParameters[0] = new ReportParameter("xyzName","xxx");
reportParameters[1] = new ReportParameter("abcName","xxx";
ReportViewer1.LocalReport.SetParameters(reportParameters);
ReportViewer1.DataBind();
ReportViewer1.LocalReport.Refresh();
}