Hi! you can use conditions to assign values to variables depending on the comboboxes in following way...
string pid="";
if(ComboBox1.SelectedValue != "")
pid="P001";
add parameter field in crystal report
then..
ReportDocument cryRpt = new ReportDocument();
cryRpt.SetParameterValue("pid", pid);
CrystalRepotViewer1.ReportSource = cryRpt;
Hope this helps you!:cool: