Cannot close Excel.exe after Interop process

Simple rule: avoid using double-dot-calling expressions, such as this:

var workbook = excel.Workbooks.Open(/*params*/)

…because in this way you create RCW objects not only for workbook, but for Workbooks, and you should release it too (which is not possible if a reference to the object is not maintained).

So, the right way will be:

var workbooks = excel.Workbooks;
var workbook = workbooks.Open(/*params*/)

//business logic here

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