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The server-side OfficeWriter renderers for Reporting Services use ExcelTemplate and WordTemplate to populate the report and stream it back to the user. However, since Reporting Services does not provide runtime control, it is not possible to post-process the output of SSRS with ExcelApplication inside of SSRS.
However, In version 9.1.0 new functionality was added to open ExcelApplication and call Workbook.CalculateFormulas() after running the template through on all templates after running them through ExcelTemplate by adding keys to the Reporting Services web.config. See below for more details.
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To set the key on the Server go to web.config located in \Reporting Services\ReportServer. Add the following code to post-process all of your output with Workbook.CalculateFormulas()
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