Use the following code to get the first running instance of Excel:
oExcelApp = (Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
Example
public Excel.Application StartExcel()
{
Excel.Application instance = null;
try
{
instance = (Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
}
catch (System.Runtime.InteropServices.COMException ex)
{
instance = new Excel.ApplicationClass();
}
return instance;
}