UploadReports.cs

string directoryDatasetFile = "D:\upload\Report_Dataset.rsd";
string datasetFileName = Path.GetFileName(datasetFile);
string datasetFullUrl = string.Concat("http://sharepoint/dashboard/Data Connections/Datasets/", datasetFileName)
SPFolder folder = ....; //get folder "Datasets" from library "Data Connections"

using(FileStream fileStream = File.OpenRead(directoryDatasetFile))
{
    folder.Files.Add(datasetFileName, fileStream, true);
}

//Add DataSource
DataSourceReference reference = new DataSourceReference();
reference.Reference = dataSourcesFileFullUrl; 

//if you have different datasource files for some reports, so you can get 
//information directly from dataset file
/**
XmlDocument xDocument = new XmlDocment();
using(FileStream stream = File.OpenRead(directorySourceFile))
{
    xDocument.Load(stream)
}
XmlNamespaceManager namespaces = new XmlNamespaceManager(xDocument.NameTable);
namespaces.AddNamespace("sdsd", "http://schemas.microsoft.com/sqlserver/reporting/2010/01/shareddatasetdefinition");
XmlElement root = xDocument.DocumentElement;
XmlNode dataSourceReferenceNode = root.SelectSingleNode("sdsd:DataSet/sdsd:Query/sdsd:DataSourceReference", namespaces);
string dataSourceFileName = dataSourceReferenceNode.InnerText;
**/

DataSource[] itemDataSources = reportingService.GetItemDataSources(datasetFullUrl);
DataSource dataSource = new DataSource();
dataSource = itemDataSource[0];
dataSource.Item = reference;
reportingService.SetItemDataSources(datasetFullUrl, itemDataSources);