Just some feedback - I have no clue what's going on here but here is my code that does indeed seem to set the default value for multi-value parameters to all available options in a custom resolver:
Thanks Katia.
ReportConnectionStringManager csm = newReportConnectionStringManager(providerConnStr);var appPath = HttpContext.Current.Server.MapPath("~/");var reportsPath = Path.Combine(appPath, "Reports");var uri = Path.Combine(reportsPath, report);var sourceReportSource = newUriReportSource() { Uri = uri };reportInstance = csm.UpdateReportSource(sourceReportSource);//Set parametersforeach(var reportParm in(reportInstance asInstanceReportSource).ReportDocument.ReportParameters){ if(reportParm.Name.ToLower() == "Department".ToLower()) { reportParm.Value = "= AllValues(Fields.Department)"; }}