HRESULT: 0x800401A8 when getting UsedRange of Worksheet

I'm trying to open an Excel Sheet, get the UsedRange and loop through it.

This already worked once, but then I had the exception

COM object that has been separated from its underlying RCW cannot be used

after using

System.Runtime.InteropServices.Marshal.ReleaseComObject(xlApp);
System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkBook);
System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkSheet);

Note that the exception kept coming up in the code below as well, not when actually releasing. I did this the first time and since then I couldn't get the code below to work. Sadly, I don't know at which line exactly he crashed anymore.

Because the exception kept comming up, I restarted my PC and since then when trying to access UsedRange, I get an Exception:

System.Runtime.InteropServices.COMException: "Ausnahme von HRESULT: 0x800401A8"

Here is the code:

using Excel = Microsoft.Office.Interop.Excel;
....
var xlApp = new Excel.Application();
var xlWorkBook = xlApp.Workbooks.Open(file);
Excel.Worksheet xlWorkSheet = xlWorkBook.Sheets["Tabelle1"];
for (int i = 1; i < xlWorkSheet.UsedRange.Rows.Count; i++)
{ ...

This worked once, so I'm thinking the COM object is still not working correctly. (Like in the first exception)

What can I do to make it work again?

What happened is that I accidentally closed the Workbook and quitted the App in the foor loop and not after it. I accidentally put it in front of the }.

for (int i = 1; i < xlWorkSheet.UsedRange.Rows.Count; i++)
{ 
  ...
  xlWorkBook.Close(false);
  xlApp.Quit();
}

Of course, that's supposed to go after the loop. You cannot access a variable in a workbook that isn't open anymore.

So if you've got the error, check if the workbook is actually really open.

Shame on me.