using Excel = Microsoft.Office.Interop.Excel;
Most enterprises prefer embedding interop types to avoid PIA deployment headaches. microsoft.office.interop.excel version 15.0.0.0
Interop requires a functional desktop session. It is not thread-safe and can hang the server if a dialogue box pops up. using Excel = Microsoft
If your application does embed interop types, the target machine must have the exact PIA installed. For version 15.0.0.0, that means: using Excel = Microsoft.Office.Interop.Excel