This is VB.net but should be pretty easy for you to translate

rptTest is the report viewer

Dim parameters As ReportParameter() = New ReportParameter(2) {}

parameters(0) = New ReportParameter("param1", Trim("value1"), False)

parameters(1) = New ReportParameter("param2", Trim("value2"), False)

parameters(2) = New ReportParameter("param3", Trim("value3"), False)

rptTest.ServerReport.SetParameters(parameters)

rptTest.ServerReport.Refresh()