Question

I use some objects like workbook, worksheet, app for Excel tasks. After I am done with Excel, I try releasing them by the code below. After all, I still see EXCEL.EXE at Task Manager. Why doesn’t it release completely?

My class for Excel tasks:

Excel._Application app = new Excel.Application();
Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
Excel._Worksheet worksheet = null;
app.Visible = true;

worksheet.Cells[1, 1] = "test string";

workbook.SaveAs("C:\testfile.xlsx");

object misValue = System.Reflection.Missing.Value;
workbook.Close(true, misValue, misValue);
app.Quit();

releaseObject(worksheet2);
releaseObject(workbook);
releaseObject(app);

releaseObject class:

    private void releaseObject(object obj)
    {
        try
        {
            System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
            obj = null;
        }
        catch (Exception ex)
        {
            obj = null;
            MessageBox.Show("Unable to release the Object " + ex.ToString());
        }
        finally
        {
            GC.Collect();
        }
    } 

EXCEL.EXE is still at Task Manager:

Answer 1

Already answered see:

How do I properly clean up Excel interop objects?

Basically COM objects are unmanaged objects (duh) and are not released when they go out of scope.
System.Runtime.InteropServices.Marshal.ReleaseComObject() can be used to release them before they get finalized (in the garbage collector), but you have to make sure to keep a reference of all the objects:
excelApp.Worksheets.Open() creates two objects: the collection ‘Worksheets’ and the ‘Worksheet’ opened with the Open() method.