UploadReports.cs

string directoryReportFile = "D:\upload\Reports\DailyReport.rdl";
string reportFileName = Path.GetFileName(datasetFile);
string reportFullUrl = string.Concat("http://sharepoint/dashboard/Reports/", reportFileName)
SPList reportLibrary = <Reports>; 

//First upload all report files
using(FileStream fileStream = File.OpenRead(directoryDatasetFile))
{
    reportLibrary.Files.Add(datasetFileName, fileStream, true);
}

//Use this step, if all report files are uploaded. Some reports use subreports
ItemReferenceData[] itemDataList = reportingService.GetItemReferences(reportFullUrl, "DataSet");
ItemReference[] items = new ItemReference[itemDataList.Length];

if (itemDataList.Length > 0)
{
    //get all dataset files from one report
    for (int i = 0; i < itemDataList.Length; i++)
    {
        ItemReferenceData itemData = itemDataList[i];
        ItemReference item = new ItemReference();
        item.Name = itemData.Name; //Name of dataset
        reportFile.DatasetFullUrl = <GetDatasetFullUrl(itemData.Name, datasetFiles)>;
        item.Reference = reportFile.DatasetFullUrl;
        items[i] = item;
    }

    reportingService.SetItemReferences(reportFile.FullUrl, items);
}