In order to genrate an XLSX file without deep diving into Open XML SDK 2 and/or SpreadsheetML details we have decided to do the following:

1. Suggest an easy way to create a template of desired Excel document.
2. Provide a way for streaming of exported data.
3. Develop helpers that simplify template handling .

As an example, we think of a "download" web service, which on a client's request with parameters returns an XLSX file as file attachment.

After some research and Internet crawling, we've come to the conclusion that the best tool for creation of XLSX template is... Microsoft Office Excel. Thus, we suggest the following procedure for creating a template:

• Create a simplified prototype for desired output in Excel (including charts, sheets etc.);
• Save the created file as "Excel Workbook (*.xlsx)";
• Rename the saved file to *.zip and then extract it to the corresponding folder. Files in this folder will be used as a template.
• Replace all XML files, which depend on exported data, like xl/worksheets/sheet*.xml, xl/drwaings/drawing*.xml and xl/charts/chart*.xml with the corresponding XSLT files.

P.S. We know about SQL Server Reporting Services, which is a powerfull tool that allows to solve most of our reporting tasks. We, however, talking about a lightweight service that:

• uses REST services as inputs;
• uses XSLT as primary rendering engine;
• efficiently deals with big inputs;
• supports: html, text, xslx, docx, other formats;
• provides simple report definition registry;
• does not depend on SQL Server.