Why does Microsoft.Office.Interop.Excel.Application.Quit() leave the background process running?

Got it!

application.Workbooks != application.Workbooks

This property doesn’t expose a variable, it generates a value. So every time I access the Workbooks property I create a new COM object.

I fixed the code and all is well. Thanks, everybody.

var excelApplication = new Application();
var workbooks = excelApplication.Workbooks;
var workbook = workbooks.Open(pathToExcelWorkbook); // Fixed

workbook.Close();
workbooks.Close();
excelApplication.Quit();

Marshal.ReleaseComObject(workbook);
Marshal.ReleaseComObject(workbooks);
Marshal.ReleaseComObject(excelApplication);