How do I properly clean up Excel interop objects?

Asked 2008-10-01T17:18:43.513

812

I'm using the Excel interop in C# (ApplicationClass) and have placed the following code in my finally clause:

while (System.Runtime.InteropServices.Marshal.ReleaseComObject(excelSheet) != 0) { }
excelSheet = null;
GC.Collect();
GC.WaitForPendingFinalizers();

Although this kind of works, the Excel.exe process is still in the background even after I close Excel. It is only released once my application is manually closed.

What am I doing wrong, or is there an alternative to ensure interop objects are properly disposed of?

c# excel interop com-interop

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2 Answers

715

Excel does not quit because your application is still holding references to COM objects.

I guess you're invoking at least one member of a COM object without assigning it to a variable.

For me it was the excelApp.Worksheets object which I directly used without assigning it to a variable:

Worksheet sheet = excelApp.Worksheets.Open(...);
...
Marshal.ReleaseComObject(sheet);

I didn't know that internally C# created a wrapper for the Worksheets COM object which didn't get released by my code (because I wasn't aware of it) and was the cause why Excel was not unloaded.

I found the solution to my problem on this page, which also has a nice rule for the usage of COM objects in C#:

Never use two dots with COM objects.

So with this knowledge the right way of doing the above is:

Worksheets sheets = excelApp.Worksheets; // <-- The important part
Worksheet sheet = sheets.Open(...);
...
Marshal.ReleaseComObject(sheets);
Marshal.ReleaseComObject(sheet);

POST MORTEM UPDATE:

I want every reader to read this answer by Hans Passant very carefully as it explains the trap I and lots of other developers stumbled into. When I wrote this answer years ago I didn't know about the effect the debugger has to the garbage collector and drew the wrong conclusions. I keep my answer unaltered for the sake of history but please read this link and don't go the way of "the two dots": Understanding garbage collection in .NET and answered 2008-10-01T17:30:41.020

234

Preface: my answer contains two solutions, so be careful when reading and don't miss anything.

There are different ways and advice of how to make Excel instance unload, such as:

Releasing EVERY com object explicitly with Marshal.FinalReleaseComObject() (not forgetting about implicitly created com-objects). To release every created com object, you may use the rule of 2 dots mentioned here:
How do I properly clean up Excel interop objects?
Calling GC.Collect() and GC.WaitForPendingFinalizers() to make CLR release unused com-objects * (Actually, it works, see my second solution for details)
Checking if com-server-application maybe shows a message box waiting for the user to answer (though I am not sure it can prevent Excel from closing, but I heard about it a few times)
Sending WM_CLOSE message to the main Excel window
Executing the function that works with Excel in a separate AppDomain. Some people believe Excel instance will be shut, when AppDomain is unloaded.
Killing all excel instances which were instantiated after our excel-interoping code started.

BUT! Sometimes all these options just don't help or can't be appropriate!

For example, yesterday I found out that in one of my functions (which works with excel) Excel keeps running after the function ends. I tried everything! I thoroughly checked the whole function 10 times and added Marshal.FinalReleaseComObject() for everything! I also had GC.Collect() and GC.WaitForPendingFinalizers(). I checked for hidden message boxes. I tried to send WM_CLOSE message to the main Excel window. I executed my function in a separate AppDomain and unloaded that domain. Nothing helped! The option with closing all excel instances is inappropri

answered 2009-12-12T14:50:15.970

How do I properly clean up Excel interop objects?

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