Does *every* Excel interop object need to be released using Marshal.ReleaseComObject?

Question

I've seen several examples where Marshal.ReleaseComObject() is used with Excel Interop objects (i.e., objects from namespace Microsoft.Office.Interop.Excel), but I've seen it used to various degrees.

I'm wondering if I can get away with something like this:

var application = new ApplicationClass();
try
{
    // do work with application, workbooks, worksheets, cells, etc.
}
finally
{
    Marashal.ReleaseComObject(application)
}

Or if I need to release every single object created, as in this method:

public void CreateExcelWorkbookWithSingleSheet()
{
    var application = new ApplicationClass();
    var workbook = application.Workbooks.Add(_missing);
    var worksheets = workbook.Worksheets;
    for (var worksheetIndex = 1; worksheetIndex < worksheets.Count; worksheetIndex++)
    {
        var worksheet = (WorksheetClass)worksheets[worksheetIndex];
        worksheet.Delete();
        Marshal.ReleaseComObject(worksheet);
    }
    workbook.SaveAs(
        WorkbookPath, _missing, _missing, _missing, _missing, _missing,
        XlSaveAsAccessMode.xlExclusive, _missing, _missing, _missing, _missing, _missing);
    workbook.Close(true, _missing, _missing);
    application.Quit();
    Marshal.ReleaseComObject(worksheets);
    Marshal.ReleaseComObject(workbook);
    Marshal.ReleaseComObject(application);
}

What prompted me to ask this question is that, being the LINQ devotee I am, I really want to do something like this:

var worksheetNames = worksheets.Cast<Worksheet>().Select(ws => ws.Name);

...but I'm concerned I'll end up with memory leaks or ghost processes if I don't release each worksheet (ws) object.

Any insight on this would be appreciated.

Update

Based on the answers so far, it sounds like I really do need to release every single com object I create. I took the opportunity to build a ComObjectManager class to make it a little easier to deal with this headache. You have to remember to use the Get() method each time you instantiate a new com object, but if you do, it will take care of everything else for you. Please let me know if you see any problems with it (or edit and leave a comment if you are able). Here's the code:

public class ComObjectManager : IDisposable
{
    private Stack<object> _comObjects = new Stack<object>();

    public TComObject Get<TComObject>(Func<TComObject> getter)
    {
        var comObject = getter();
        _comObjects.Push(comObject);
        return comObject;
    }

    public void Dispose()
    {
        while (_comObjects.Count > 0)
            Marshal.ReleaseComObject(_comObjects.Pop());
    }
}

Here's a usage example:

public void CreateExcelWorkbookWithSingleSheet()
{
    using (var com = new ComObjectManager())
    {
        var application = com.Get<ApplicationClass>(() => new ApplicationClass());
        var workbook = com.Get<Workbook>(() => application.Workbooks.Add(_missing));
        var worksheets = com.Get<Sheets>(() => workbook.Worksheets);
        for (var worksheetIndex = 1; worksheetIndex < worksheets.Count; worksheetIndex++)
        {
            var worksheet = com.Get<WorksheetClass>(() => (WorksheetClass)worksheets[worksheetIndex]);
            worksheet.Delete();
        }
        workbook.SaveAs(
            WorkbookPath, _missing, _missing, _missing, _missing, _missing,
            XlSaveAsAccessMode.xlExclusive, _missing, _missing, _missing, _missing, _missing);
        workbook.Close(true, _missing, _missing);
        application.Quit();
    }
}

Answer 1

I believe you would have to call ReleaseComObject on each COM object. Since they're not garbage-collected, the parent-child hierarchy doesn't really come into the equation: even if you release the parent object it does not decrement the reference count on any child objects.

Answer 2

You should call Marshal.ReleaseComObject on every COM object you use in your code, not just the main application object.