public class Order
{
     public object codeName {set;get;}
     public object comment {set;get;}
     public List<Material> materials {set;get;}
}

public class Material
{
      public object Name {set;get;}
      public object Plan {set;get;}
      public object Fakt {set;get;}
      public object Diff {set;get;}
}

public FormReportPeriod()
        {
            .....
            reportViewer1.LocalReport.SubreportProcessing += new SubreportProcessingEventHandler(SetSubDataSource);
            
        }

public void SetSubDataSource(object sender, SubreportProcessingEventArgs e)
        {
            var mainSource = ((LocalReport)sender).DataSources["DataSetPeriodItem"];
            var subSource = ((List<ReportPeriodRecord>)mainSource.Value).First().materials;
            e.DataSources.Add(new ReportDataSource("DataSetPeriodMaterials", subSource));
        }

public void SetSubDataSource(object sender, SubreportProcessingEventArgs e)
        {
            var mainSource = ((LocalReport)sender).DataSources["DataSetPeriodItem"];
            var subSource = ((List<ReportPeriodRecord>)mainSource.Value).ElementAt(index++).materials;
            e.DataSources.Add(new ReportDataSource("DataSetPeriodMaterials", subSource));
        }