Does matter COM object not assigned to var is not released?

Question

Will be all unmanaged COM objects released in case if I use code like this

var worksheet = new Application().Workbooks.Add().Worksheets.Add();
Marshal.ReleaseComObject(worksheet);

instead of code like this

var excel = new Application();
var workbook = excel.Workbooks.Add();
var worksheet = workbook.Worksheets.Add();
Marshal.ReleaseComObject(excel);
Marshal.ReleaseComObject(workbook);
Marshal.ReleaseComObject(worksheet);

?

If there is some documentation please send a link in answer.

Answer 1

Actually, both code samples will leave an Excel process running in the background. You need to call Application.Quit() on the application object, for example. The following works:

private static void DoExcel()
    {
        var application = new Application();
        var workbook = application.Workbooks.Add();
        var worksheet = workbook.Worksheets.Add();

        // Name that this will be saved as
        string name = workbook.FullName + ".xlsx";

        string fullPath = Path.Combine(Directory.GetCurrentDirectory(), name);
        // If a file of the same name exists, delete it so that we won't be prompted if
        // we want to overwrite it when we save
        if (File.Exists(fullPath))
            File.Delete(fullPath);

        // Save the workbook - otherwise we may be prompted as to whether we want to save when we go to quit
        workbook.Save();

        // Quit the application
        application.Quit();

        // Release the references
        Marshal.ReleaseComObject(worksheet);
        Marshal.ReleaseComObject(workbook);
        Marshal.ReleaseComObject(application);

        // Release the .NET reference and run the garbage collector now to make sure the application is closed immediately
        worksheet = null;
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }

A few other good things to remember: I didn't use it here, but there's a Marshal.FinalReleaseComObject method that's very useful in these cases. Also, again I didn't use this in my code sample, but the Marshal.ReleaseComObject method returns the current count, so you could always do the release in a loop if you wanted to make sure the count reached zero:

while (Marshal.ReleaseComObject(comObject) > 0) { }

You can also use this for debugging purposes - eg

int count = Marshal.ReleaseComObject(comObject);
Trace.TraceInformation("Current COM object reference count: " + count.ToString());