How do I properly clean up Excel interop objects

Question

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

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Here is a really easy way to do it    DllImport  User32 dll    static extern uint GetWindowThreadProcessId IntPtr hWnd  out int lpdwProcessId        int objExcelProcessId   0   Excel Application objExcel   new Excel Application     GetWindowThreadProcessId new IntPtr objExcel Hwnd   out objExcelProcessId    Process GetProcessById objExcelProcessId  Kill

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You need to be aware that Excel is very sensitive to the culture you are running under as well   You may find that you need to set the culture to EN-US before calling Excel functions  This does not apply to all functions - but some of them       CultureInfo en US   new System Globalization CultureInfo  en-US         System Threading Thread CurrentThread CurrentCulture   en US      string filePathLocal    applicationObject ActiveWorkbook Path      System Threading Thread CurrentThread CurrentCulture   orgCulture    This applies even if you are using VSTO   For details  http   support microsoft com default aspx scid kb en-us Q320369

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A great article on releasing COM objects is 2 5 Releasing COM Objects  MSDN    The method that I would advocate is to null your Excel Interop references if they are non-local variables  and then call GC Collect   and GC WaitForPendingFinalizers   twice  Locally scoped Interop variables will be taken care of automatically   This removes the need to keep a named reference for every COM object   Here s an example taken from the article   public class Test           These instance variables must be nulled or Excel will not quit     private Excel Application xl      private Excel Workbook book       public void DoSomething                 xl   new Excel Application            xl Visible   true          book   xl Workbooks Add Type Missing               These variables are locally scoped  so we need not worry about them             Notice I don t care about using two dots          Excel Range rng   book Worksheets 1  UsedRange             public void CleanUp                 book   null          xl Quit            xl   null           GC Collect            GC WaitForPendingFinalizers            GC Collect            GC WaitForPendingFinalizers              These words are straight from the article      In almost all situations  nulling the RCW reference and forcing a garbage collection will clean up properly  If you also call GC WaitForPendingFinalizers  garbage collection will be as deterministic as you can make it  That is  you ll be pretty sure exactly when the object has been cleaned up   on the return from the second call to WaitForPendingFinalizers  As an alternative  you can use Marshal ReleaseComObject  However  note that you are very unlikely to ever need to use this method

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You can actually release your Excel Application object cleanly  but you do have to take care    The advice to maintain a named reference for absolutely every COM object you access and then explicitly release it via Marshal FinalReleaseComObject   is correct in theory  but  unfortunately  very difficult to manage in practice  If one ever slips anywhere and uses  two dots   or iterates cells via a for each loop  or any other similar kind of command  then you ll have unreferenced COM objects and risk a hang  In this case  there would be no way to find the cause in the code  you would have to review all your code by eye and hopefully find the cause  a task that could be nearly impossible for a large project   The good news is that you do not actually have to maintain a named variable reference to every COM object you use  Instead  call GC Collect   and then GC WaitForPendingFinalizers   to release all the  usually minor  objects to which you do not hold a reference  and then explicitly release the objects to which you do hold a named variable reference    You should also release your named references in reverse order of importance  range objects first  then worksheets  workbooks  and then finally your Excel Application object   For example  assuming that you had a Range object variable named xlRng  a Worksheet variable named xlSheet  a Workbook variable named xlBook and an Excel Application variable named xlApp  then your cleanup code could look something like the following      Cleanup GC Collect    GC WaitForPendingFinalizers     Marshal FinalReleaseComObject xlRng   Marshal FinalReleaseComObject xlSheet    xlBook Close Type Missing  Type Missing  Type Missing   Marshal FinalReleaseComObject xlBook    xlApp Quit    Marshal FinalReleaseComObject xlApp     In most code examples you ll see for cleaning up COM objects from  NET  the GC Collect   and GC WaitForPendingFinalizers   calls are made TWICE as in   GC Collect    GC WaitForPendingFinalizers    GC Collect    GC WaitForPendingFinalizers      This should not be required  however  unless you are using Visual Studio Tools for Office  VSTO   which uses finalizers that cause an entire graph of objects to be promoted in the finalization queue  Such objects would not be released until the next garbage collection  However  if you are not using VSTO  you should be able to call GC Collect   and GC WaitForPendingFinalizers   just once   I know that explicitly calling GC Collect   is a no-no  and certainly doing it twice sounds very painful   but there is no way around it  to be honest  Through normal operations you will generate hidden objects to which you hold no reference that you  therefore  cannot release through any other means other than calling GC Collect     This is a complex topic  but this really is all there is to it  Once you establish this template for your cleanup procedure you can code normally  without the need for wrappers  etc   -   I have a tutorial on this here   Automating Office Programs with VB Net   COM Interop  It s written for VB NET  but don t be put off by that  the principles are exactly the same as when using C

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There i have an idea try to kill the excel process you have opened    before open an excelapplication get all the process ids named oldProcessIds  open the excelapplication  get now all the excelapplication process ids named nowProcessIds  when need to quit kill the except ids between oldProcessIds and nowProcessIds   private static Excel Application GetExcelApp                  if   excelApp    null                        var processIds   System Diagnostics Process GetProcessesByName  EXCEL   Select a   gt  a Id  ToList                 excelApp   new Excel Application                 excelApp DisplayAlerts   false                excelApp Visible   false               excelApp ScreenUpdating   false              var newProcessIds   System Diagnostics Process GetProcessesByName  EXCEL   Select a   gt  a Id  ToList                 excelApplicationProcessId   newProcessIds Except processIds  FirstOrDefault                       return  excelApp         public static void Dispose                 try                        excelApp Workbooks Close                 excelApp Quit                System Runtime InteropServices Marshal ReleaseComObject  excelApp                excelApp   null              GC Collect                GC WaitForPendingFinalizers                if   excelApplicationProcessId    default int                                 var process   System Diagnostics Process GetProcessById  excelApplicationProcessId                   process  Kill                     excelApplicationProcessId   default int                                   catch  Exception ex                         excelApp   null

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I followed this exactly    But I still ran into issues 1 out of 1000 times  Who knows why  Time to bring out the hammer     Right after the Excel Application class is instantiated I get a hold of the Excel process that was just created   excel   new Microsoft Office Interop Excel Application    var process   Process GetProcessesByName  EXCEL   OrderByDescending p   gt  p StartTime  First      Then once I ve done all the above COM clean-up  I make sure that process isn t running  If it is still running  kill it   if   process HasExited     process Kill

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I cant believe this problem has haunted the world for 5 years     If you have created an application  you need to shut it down first before removing the link   objExcel   new Excel Application      objBook    Excel Workbook  objExcel Workbooks Add Type Missing       when closing  objBook Close true  Type Missing  Type Missing    objExcel Application Quit    objExcel Quit       When you new an excel application  it opens a excel program in the background  You need to command that excel program to quit before you release the link because that excel program is not part of your direct control  Therefore  it will stay open if the link is released   Good programming everyone

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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 inappropriate  because if the user starts another Excel instance manually  during execution of my function which works also with Excel  then that instance will also be closed by my function  I bet the user will not be happy  So  honestly  this is a lame option  no offence guys   So I spent a couple of hours before I found a good  in my humble opinion  solution  Kill excel process by hWnd of its main window  it s the first solution    Here is the simple code    DllImport  user32 dll    private static extern uint GetWindowThreadProcessId IntPtr hWnd  out uint lpdwProcessId         lt summary gt  Tries to find and kill process by hWnd to the main window of the process  lt  summary gt       lt param name  hWnd  gt Handle to the main window of the process  lt  param gt       lt returns gt True if process was found and killed  False if process was not found by hWnd or if it could not be killed  lt  returns gt  public static bool TryKillProcessByMainWindowHwnd int hWnd        uint processID      GetWindowThreadProcessId  IntPtr hWnd  out processID       if processID    0  return false      try               Process GetProcessById  int processID  Kill              catch  ArgumentException                return false            catch  Win32Exception                return false            catch  NotSupportedException                return false            catch  InvalidOperationException                return false            return true          lt summary gt  Finds and kills process by hWnd to the main window of the process  lt  summary gt       lt param name  hWnd  gt Handle to the main window of the process  lt  param gt       lt exception cref  ArgumentException  gt      Thrown when process is not found by the hWnd parameter  the process is not running        The identifier of the process might be expired       lt  exception gt       lt exception cref  Win32Exception  gt See Process Kill   exceptions documentation  lt  exception gt       lt exception cref  NotSupportedException  gt See Process Kill   exceptions documentation  lt  exception gt       lt exception cref  InvalidOperationException  gt See Process Kill   exceptions documentation  lt  exception gt  public static void KillProcessByMainWindowHwnd int hWnd        uint processID      GetWindowThreadProcessId  IntPtr hWnd  out processID       if  processID    0          throw new ArgumentException  Process has not been found by the given main window handle     hWnd        Process GetProcessById  int processID  Kill        As you can see I provided two methods  according to Try-Parse pattern  I think it is appropriate here   one method doesn t throw the exception if the Process could not be killed  for example the process doesn t exist anymore   and another method throws the exception if the Process was not killed  The only weak place in this code is security permissions  Theoretically  the user may not have permissions to kill the process  but in 99 99  of all cases  user has such permissions  I also tested it with a guest account - it works perfectly   So  your code  working with Excel  can look like this   int hWnd   xl Application Hwnd            here we try to close Excel as usual  with xl Quit       Marshal FinalReleaseComObject xl  and so on        TryKillProcessByMainWindowHwnd hWnd     Voila  Excel is terminated      Ok  let s go back to the second solution  as I promised in the beginning of the post  The second solution is to call GC Collect   and GC WaitForPendingFinalizers    Yes  they actually work  but you need to be careful here  Many people say  and I said  that calling GC Collect   doesn t help  But the reason it wouldn t help is if there are still references to COM objects  One of the most popular reasons for GC Collect   not being helpful is running the project in Debug-mode  In debug-mode objects that are not really referenced anymore will not be garbage collected until the end of the method  So  if you tried GC Collect   and GC WaitForPendingFinalizers   and it didn t help  try to do the following     1  Try to run your project in Release mode and check if Excel closed correctly    2  Wrap the method of working with Excel in a separate method  So  instead of something like this   void GenerateWorkbook          ApplicationClass xl    Workbook xlWB    try         xl           xlWB   xl Workbooks Add                     finally                 Marshal ReleaseComObject xlWB              GC Collect        GC WaitForPendingFinalizers            you write   void GenerateWorkbook          try         GenerateWorkbookInternal             finally         GC Collect        GC WaitForPendingFinalizers           private void GenerateWorkbookInternal          ApplicationClass xl    Workbook xlWB    try         xl           xlWB   xl Workbooks Add                     finally                 Marshal ReleaseComObject xlWB                  Now  Excel will close

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Use    DllImport  user32 dll    private static extern uint GetWindowThreadProcessId IntPtr hWnd  out uint lpdwProcessId     Declare it  add code in the finally block   finally       GC Collect        GC WaitForPendingFinalizers        if  excelApp    null                excelApp Quit            int hWnd   excelApp Application Hwnd          uint processID          GetWindowThreadProcessId  IntPtr hWnd  out processID           Process   procs   Process GetProcessesByName  EXCEL            foreach  Process p in procs                        if  p Id    processID                  p Kill                      Marshal FinalReleaseComObject excelApp

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You need to be aware that Excel is very sensitive to the culture you are running under as well   You may find that you need to set the culture to EN-US before calling Excel functions  This does not apply to all functions - but some of them       CultureInfo en US   new System Globalization CultureInfo  en-US         System Threading Thread CurrentThread CurrentCulture   en US      string filePathLocal    applicationObject ActiveWorkbook Path      System Threading Thread CurrentThread CurrentCulture   orgCulture    This applies even if you are using VSTO   For details  http   support microsoft com default aspx scid kb en-us Q320369

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The accepted answer did not work for me  The following code in the destructor did the job   if  xlApp    null        xlApp Workbooks Close        xlApp Quit       System Diagnostics Process   processArray   System Diagnostics Process GetProcessesByName  EXCEL    foreach  System Diagnostics Process process in processArray        if  process MainWindowTitle Length    0    process Kill

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Anything that is in the Excel namespace needs to be released  Period  You can t be doing   Worksheet ws   excel WorkBooks 1  WorkSheets 1     You have to be doing  Workbooks books   excel WorkBooks  Workbook book   books 1   Sheets sheets   book WorkSheets  Worksheet ws   sheets 1     followed by the releasing of the objects

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My solution   DllImport  user32 dll    static extern int GetWindowThreadProcessId int hWnd  out int lpdwProcessId    private void GenerateExcel         var excel   new Microsoft Office Interop Excel Application        int id         Find the Excel Process Id  ath the end  you kill him     GetWindowThreadProcessId excel Hwnd  out id       Process excelProcess   Process GetProcessById id    try          Your code   finally       excel Quit            Kill him       excelProcess Kill

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I am currently working on Office automation and have stumbled across a solution for this that works every time for me  It is simple and does not involve killing any processes   It seems that by merely looping through the current active processes  and in any way  accessing  an open Excel process  any stray hanging instance of Excel will be removed  The below code simply checks for processes where the name is  Excel   then writes the MainWindowTitle property of the process to a string  This  interaction  with the process seems to make Windows catch up and abort the frozen instance of Excel    I run the below method just before the add-in which I am developing quits  as it fires it unloading event  It removes any hanging instances of Excel every time  In all honesty I am not entirely sure why this works  but it works well for me and could be placed at the end of any Excel application without having to worry about double dots  Marshal ReleaseComObject  nor killing processes  I would be very interested in any suggestions as to why this is effective   public static void SweepExcelProcesses                            if  Process GetProcessesByName  EXCEL   Length    0                                Process   processes   Process GetProcesses                    foreach  Process process in processes                                        if  process ProcessName ToString       excel                                                                            string title   process MainWindowTitle

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My answer is late and its only purpose is to support the solution proposed by Govert  Porkbutts  and Dave Cousineau with a complete example  Automating Excel or other COM objects from the COM-agnostic  NET world is a    tough nut     as we say in German  and you can easily go nuts  I rely on the following steps    For each interaction with Excel  get one and only one local instance ExcelApp of the Application interface and create a scope within which ExcelApp lives  This is necessary  because the CLR won   t free the resources of Excel before any reference to Excel goes out of scope  A new Excel process is started in the background  Implement functions that do the tasks by using ExcelApp to generate via Collection properties new objects like Workbook s   Worksheet s   and Cell s   In these functions don   t care for the voodoo one-dot-good  two-dot-bad rule  don   t try to get a reference for each implicitly created object and don   t Marshall ReleaseComObject anything  That is the job of the Garbage Collection  Within the scope of ExcelApp  call these functions and pass the reference ExcelApp  While your Excel instance is loaded  don   t allow any user actions that would bypass the Quit function that unloads this instance again  When you are done with Excel  call the separate Quit function within the scope made for Excel handling  This should be the last statement within this scope    Before you run my app  open the task manager and watch in the Processes tab the entries in background processes  When you start the program  a new Excel process entry appears in the list and stays there until the thread wakes up again after 5 seconds  After that the Quit function will be called  stopping the Excel process that gracefully vanishes from the list of background processes   using System  using System Threading  using Excel   Microsoft Office Interop Excel   namespace GCTestOnOffice     class Program           Don t  private  static Excel Application ExcelApp   new Excel Application         private static void DoSomething Excel Application ExcelApp                Excel Workbook Wb   ExcelApp Workbooks Open   D  Aktuell SampleWorkbook xlsx            Excel Worksheet NewWs   Wb Worksheets Add             for  int i   1  i  lt  10  i                          NewWs Cells i  1    i                    Wb Save               public static void Quit Excel Application ExcelApp                if  ExcelApp    null                        ExcelApp Quit      Don t forget                  ExcelApp   null                    GC Collect            GC WaitForPendingFinalizers            GC Collect            GC WaitForPendingFinalizers               static void Main string   args                        Excel Application ExcelApp   new Excel Application            Thread Sleep 5000           DoSomething ExcelApp           Quit ExcelApp             ExcelApp goes out of scope  the CLR can and will    release Excel                Console WriteLine  Input a digit            int k   Console Read                   If I changed the Main function to  static void Main string   args      Excel Application ExcelApp   new Excel Application      DoSomething ExcelApp      Console WriteLine  Input a digit        int k   Console Read       Quit ExcelApp       the user could instead of inputting a number  hit the Close button of the console and my Excel instance lived happily ever after  So  in cases where your Excel instance remains stubbornly loaded  your cleanup feature might not be wrong  but is bypassed by unforeseen user actions   If the Program class would have a member for the Excel instance  the CLR would not unload the Excel instance before the app terminates  That s why I prefer local references that go out of scope when they are no longer needed

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To add to reasons why Excel does not close  even when you create direct refrences to each object upon read  creation  is the  For  loop   For Each objWorkBook As WorkBook in objWorkBooks  local ref  created from ExcelApp WorkBooks to avoid the double-dot    objWorkBook Close  or whatever    FinalReleaseComObject objWorkBook     objWorkBook   Nothing Next    The above does not work  and this is the workaround   For intCounter As Integer   1 To mobjExcel WorkBooks Count    Dim objTempWorkBook As Workbook   mobjExcel WorkBooks Item intCounter     objTempWorkBook Saved   True    objTempWorkBook Close False  Type Missing  Type Missing     FinalReleaseComObject objTempWorkBook     objTempWorkBook   Nothing Next

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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      lt -- 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 Clean up Excel Interop Objects with IDisposable

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Just to add another solution to the many listed here  using C   ATL automation  I imagine you could use something similar from VB C      Excel   ApplicationPtr pXL           SendMessage     HWND   m pXL- gt GetHwnd      WM DESTROY  0  0       This works like a charm for me

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Tested with Microsoft Excel 2016  A really tested solution   To C  Reference please see  https   stackoverflow com a 1307180 10442623  To VB net Reference please see  https   stackoverflow com a 54044646 10442623  1 include the class job  2 implement the class to handle the apropiate dispose of excel proces

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The two dots rule did not work for me  In my case I created a method to clean my resources as follows   private static void Clean         workBook Close        Marshall ReleaseComObject workBook       excel Quit        CG Collect        CG WaitForPendingFinalizers

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I found a useful generic template that can help implement the correct disposal pattern for COM objects  that need Marshal ReleaseComObject called when they go out of scope   Usage   using  AutoReleaseComObject lt Application gt  excelApplicationWrapper   new AutoReleaseComObject lt Application gt  new Application           try               using  AutoReleaseComObject lt Workbook gt  workbookWrapper   new AutoReleaseComObject lt Workbook gt  excelApplicationWrapper ComObject Workbooks Open namedRangeBase FullName  false  false  missing  missing  missing  true  missing  missing  true  missing  missing  missing  missing  missing                            do something with your workbook                         finally                excelApplicationWrapper ComObject Quit               Template   public class AutoReleaseComObject lt T gt    IDisposable       private T m comObject      private bool m armed   true      private bool m disposed   false       public AutoReleaseComObject T comObject                Debug Assert comObject    null           m comObject   comObject          if DEBUG      AutoReleaseComObject                    We should have been disposed using Dispose            Debug WriteLine  Finalize being called  should have been disposed             if  this ComObject    null                        Debug WriteLine string Format  ComObject was not null  0   name  1     this ComObject  this ComObjectName                         Debug Assert false          endif      public T ComObject               get                       Debug Assert  m disposed               return m comObject                       private string ComObjectName               get                       if this ComObject is Microsoft Office Interop Excel Workbook                                return   Microsoft Office Interop Excel Workbook this ComObject  Name                             return null                       public void Disarm                 Debug Assert  m disposed           m armed   false              region IDisposable Members      public void Dispose                 Dispose true    if DEBUG         GC SuppressFinalize this    endif             endregion      protected virtual void Dispose bool disposing                if   m disposed                        if  m armed                                int refcnt   0                  do                                       refcnt   System Runtime InteropServices Marshal ReleaseComObject m comObject                     while  refcnt  gt  0                    m comObject   default T                              m disposed   true                      Reference   http   www deez info sengelha 2005 02 11 useful-idisposable-class-3-autoreleasecomobject

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I think that some of that is just the way that the framework handles Office applications  but I could be wrong  On some days  some applications clean up the processes immediately  and other days it seems to wait until the application closes  In general  I quit paying attention to the details and just make sure that there aren t any extra processes floating around at the end of the day   Also  and maybe I m over simplifying things  but I think you can just     objExcel   new Excel Application    objBook    Excel Workbook  objExcel Workbooks Add Type Missing    DoSomeStuff objBook   SaveTheBook objBook   objBook Close false  Type Missing  Type Missing   objExcel Quit      Like I said earlier  I don t tend to pay attention to the details of when the Excel process appears or disappears  but that usually works for me   I also don t like to keep Excel processes around for anything other than the minimal amount of time  but I m probably just being paranoid on that

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Shoot Excel proc and chew bubble gum                  public class MyExcelInteropClass       Excel Application xlApp      Excel Workbook xlBook       public void dothingswithExcel                  try      Do stuff manipulating cells sheets and workbooks                  catch            finally  KillExcelProcess xlApp               static void KillExcelProcess Excel Application xlApp                if  xlApp    null                        int excelProcessId   0              GetWindowThreadProcessId xlApp Hwnd  out excelProcessId               Process p   Process GetProcessById excelProcessId               p Kill                xlApp   null                        DllImport  user32 dll        static extern int GetWindowThreadProcessId int hWnd  out int lpdwProcessId

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This sure seems like it has been over-complicated  From my experience  there are just three key things to get Excel to close properly   1  make sure there are no remaining references to the excel application you created  you should only have one anyway  set it to null   2  call GC Collect    3  Excel has to be closed  either by the user manually closing the program  or by you calling Quit on the Excel object   Note that Quit will function just as if the user tried to close the program  and will present a confirmation dialog if there are unsaved changes  even if Excel is not visible  The user could press cancel  and then Excel will not have been closed    1 needs to happen before 2  but 3 can happen anytime   One way to implement this is to wrap the interop Excel object with your own class  create the interop instance in the constructor  and implement IDisposable with Dispose looking something like  That will clean up excel from your program s side of things  Once Excel is closed  manually by the user or by you calling Quit  the process will go away  If the program has already been closed  then the process will disappear on the GC Collect   call    I m not sure how important it is  but you may want a GC WaitForPendingFinalizers   call after the GC Collect   call but it is not strictly necessary to get rid of the Excel process    This has worked for me without issue for years  Keep in mind though that while this works  you actually have to close gracefully for it to work  You will still get accumulating excel exe processes if you interrupt your program before Excel is cleaned up  usually by hitting  stop  while your program is being debugged

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UPDATE  Added C  code  and link to Windows Jobs  I spent sometime trying to figure out this problem  and at the time XtremeVBTalk was the most active and responsive   Here is a link to my original post  Closing an Excel Interop process cleanly  even if your application crashes  Below is a summary of the post  and the code copied to this post     Closing the Interop process with Application Quit   and Process Kill   works for the most part  but fails if the applications crashes catastrophically  I e  if the app crashes  the Excel process will still be running loose  The solution is to let the OS handle the cleanup of your processes through Windows Job Objects using Win32 calls   When your main application dies  the associated processes  i e  Excel  will get terminated as well      I found this to be a clean solution because the OS is doing real work of cleaning up   All you have to do is register the Excel process   Windows Job Code  Wraps the Win32 API Calls to register Interop processes   public enum JobObjectInfoType       AssociateCompletionPortInformation   7      BasicLimitInformation   2      BasicUIRestrictions   4      EndOfJobTimeInformation   6      ExtendedLimitInformation   9      SecurityLimitInformation   5      GroupInformation   11     StructLayout LayoutKind Sequential   public struct SECURITY ATTRIBUTES       public int nLength      public IntPtr lpSecurityDescriptor      public int bInheritHandle      StructLayout LayoutKind Sequential   struct JOBOBJECT BASIC LIMIT INFORMATION       public Int64 PerProcessUserTimeLimit      public Int64 PerJobUserTimeLimit      public Int16 LimitFlags      public UInt32 MinimumWorkingSetSize      public UInt32 MaximumWorkingSetSize      public Int16 ActiveProcessLimit      public Int64 Affinity      public Int16 PriorityClass      public Int16 SchedulingClass      StructLayout LayoutKind Sequential   struct IO COUNTERS       public UInt64 ReadOperationCount      public UInt64 WriteOperationCount      public UInt64 OtherOperationCount      public UInt64 ReadTransferCount      public UInt64 WriteTransferCount      public UInt64 OtherTransferCount      StructLayout LayoutKind Sequential   struct JOBOBJECT EXTENDED LIMIT INFORMATION       public JOBOBJECT BASIC LIMIT INFORMATION BasicLimitInformation      public IO COUNTERS IoInfo      public UInt32 ProcessMemoryLimit      public UInt32 JobMemoryLimit      public UInt32 PeakProcessMemoryUsed      public UInt32 PeakJobMemoryUsed     public class Job   IDisposable        DllImport  kernel32 dll   CharSet   CharSet Unicode       static extern IntPtr CreateJobObject object a  string lpName         DllImport  kernel32 dll        static extern bool SetInformationJobObject IntPtr hJob  JobObjectInfoType infoType  IntPtr lpJobObjectInfo  uint cbJobObjectInfoLength         DllImport  kernel32 dll   SetLastError   true       static extern bool AssignProcessToJobObject IntPtr job  IntPtr process        private IntPtr m handle      private bool m disposed   false       public Job                 m handle   CreateJobObject null  null            JOBOBJECT BASIC LIMIT INFORMATION info   new JOBOBJECT BASIC LIMIT INFORMATION            info LimitFlags   0x2000           JOBOBJECT EXTENDED LIMIT INFORMATION extendedInfo   new JOBOBJECT EXTENDED LIMIT INFORMATION            extendedInfo BasicLimitInformation   info           int length   Marshal SizeOf typeof JOBOBJECT EXTENDED LIMIT INFORMATION            IntPtr extendedInfoPtr   Marshal AllocHGlobal length           Marshal StructureToPtr extendedInfo  extendedInfoPtr  false            if   SetInformationJobObject m handle  JobObjectInfoType ExtendedLimitInformation  extendedInfoPtr   uint length               throw new Exception string Format  Unable to set information   Error   0    Marshal GetLastWin32Error                  region IDisposable Members      public void Dispose                 Dispose true           GC SuppressFinalize this               endregion      private void Dispose bool disposing                if  m disposed              return           if  disposing              Close            m disposed   true             public void Close                 Win32 CloseHandle m handle           m handle   IntPtr Zero             public bool AddProcess IntPtr handle                return AssignProcessToJobObject m handle  handle              Note about Constructor code   In the constructor  the info LimitFlags   0x2000  is called  0x2000 is the JOB OBJECT LIMIT KILL ON JOB CLOSE enum value  and this value is defined by MSDN as       Causes all processes associated with the job to terminate when the   last handle to the job is closed    Extra Win32 API Call to get the Process ID  PID        DllImport  user32 dll   SetLastError   true       public static extern uint GetWindowThreadProcessId IntPtr hWnd  out uint lpdwProcessId     Using the code      Excel Application app   new Excel ApplicationClass        Job job   new Job        uint pid   0      Win32 GetWindowThreadProcessId new IntPtr app Hwnd   out pid       job AddProcess Process GetProcessById  int pid  Handle

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After trying   Release COM objects in reverse order Add GC Collect   and GC WaitForPendingFinalizers   twice at the end No more than two dots Close workbook and quit application Run in release mode   the final solution that works for me is to move one set of  GC Collect    GC WaitForPendingFinalizers      that we added to the end of the function to a wrapper  as follows   private void FunctionWrapper string sourcePath  string targetPath        try               FunctionThatCallsExcel sourcePath  targetPath             finally               GC Collect            GC WaitForPendingFinalizers

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I had this same problem getting PowerPoint to close after newing up the Application object in my VSTO AddIn   I tried all the answers here with limited success   This is the solution I found for my case - DONT use  new Application   the AddInBase base class of ThisAddIn already has a handle to  Application   If you use that handle where you need it  make it static if you have to  then you don t need to worry about cleaning it up and PowerPoint won t hang on close

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When all the stuff above didn t work  try giving Excel some time to close its sheets   app workbooks Close    Thread Sleep 500      adjust  for me it works at around 300  app Quit         FinalReleaseComObject app

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Make sure that you release all objects related to Excel   I spent a few hours by trying several ways  All are great ideas but I finally found my mistake  If you don t release all objects  none of the ways above can help you like in my case  Make sure you release all objects including range one   Excel Range rng    Excel Range worksheet Cells 1  1   worksheet Paste rng  false   releaseObject rng     The options are together here

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As some have probably already written  it s not just important how you close the Excel  object   it s also important how you open it and also by the type of the project   In a WPF application  basically the same code is working without or with very few problems   I have a project in which the same Excel file is being processed several times for different parameter value - e g  parsing it based on values inside a generic list    I put all Excel-related functions into the base class  and parser into a subclass  different parsers use common Excel functions   I didn t want that Excel is opened and closed again for each item in a generic list  so I ve opened it only once in the base class and close it in the subclass  I had problems when moving the code into a desktop application  I ve tried many of the above mentioned solutions  GC Collect   was already implemented before  twice as suggested   Then I ve decided that I will move the code for opening Excel to a subclass  Instead of opening only once  now I create a new object  base class  and open Excel for every item and close it at the end  There is some performance penalty  but based on several tests Excel processes are closing without problems  in debug mode   so also temporary files are removed  I will continue with testing and write some more if I will get some updates   The bottom line is  You must also check the initialize code  especially if you have many classes  etc

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Of the three general strategies considered in other answers  killing the excel process is clearly a hack  whereas invoking the garbage collector is a brutal shotgun approach meant to compensate for incorrect deallocation of COM-objects  After lots of experimentation and rewriting the management of COM objects in my version-agnostic and late-bound wrapper  I have come to the conclusion that accurate and timely invocations of Marshal ReleaseComObject   is the most efficient and elegant strategy  And no  you do not ever need FinalReleaseComObject    because in a well-writtin program each COM acquired on once and therefore requires a single decrement of the reference counter  One shall make sure to release every single COM object  preferably as soon as it is no longer needed  But it is perfectly possible to release everything right after quitting the Excel application  at the only expense of higher memory usage  Excel will close as expected as long as one does not loose or forget to release a COM object  The simplest and most obvious aid in the process is wrapping every interop object into a  NET class implementing IDisposable  where the Dispose   method invokes ReleaseComObject   on its interop object  Doing it in the destructor  as proposed in here  makes no sense because destructors are non-deterministic  Show below is our wrapper s method that obtains a cell from WorkSheet bypassing the intermediate Cells member  Notice the way it disposes of the intermediate object after use  public ExcelRange XCell  int row  int col      ExcelRange anchor  res      using  anchor   Range   quot A1 quot             res   anchor Offset  row - 1  col - 1           return res     The next step may be a simple memory manager that will keep track of every COM object obtained and make sure to release it after Excel quits if the user prefers to trade some RAM usage for simpler code  Futher reading  How to properly release Excel COM objects  Releasing COM objects  Garbage Collector vs  Marshal RelseaseComObject

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So far it seems all answers involve some of these    Kill the process Use GC Collect   Keep track of every COM object and release it properly    Which makes me appreciate how difficult this issue is     I have been working on a library to simplify access to Excel  and I am trying to make sure that people using it won t leave a mess  fingers crossed    Instead of writing directly on the interfaces Interop provides  I am making extension methods to make live easier  Like ApplicationHelpers CreateExcel   or workbook CreateWorksheet  mySheetNameThatWillBeValidated    Naturally  anything that is created may lead to an issue later on cleaning up  so I am actually favoring killing the process as last resort  Yet  cleaning up properly  third option   is probably the least destructive and most controlled   So  in that context I was wondering whether it wouldn t be best to make something like this   public abstract class ReleaseContainer lt T gt        private readonly Action lt T gt  actionOnT       protected ReleaseContainer T releasible  Action lt T gt  actionOnT                this actionOnT   actionOnT          this Releasible   releasible              ReleaseContainer                 Release               public T Releasible   get  private set         private void Release                 actionOnT Releasible           Releasible   default T             I used  Releasible  to avoid confusion with Disposable  Extending this to IDisposable should be easy though   An implementation like this   public class ApplicationContainer   ReleaseContainer lt Application gt        public ApplicationContainer             base new Application    ActionOnExcel                   private static void ActionOnExcel Application application                application Show       extension method  want to make sure the app is visible          application Quit            Marshal FinalReleaseComObject application             And one could do something similar for all sorts of COM objects   In the factory method       public static Application CreateExcelApplication bool hidden   false                var excel   new ApplicationContainer   Releasible          excel Visible    hidden           return excel          I would expect that every container will be destructed properly by the GC  and therefore automatically make the call to Quit and Marshal FinalReleaseComObject   Comments  Or is this an answer to the question of the third kind

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quot Never use two dots with COM objects quot  is a great rule of thumb to avoid leakage of COM references  but Excel PIA can lead to leakage in more ways than apparent at first sight  One of these ways is subscribing to any event exposed by any of the Excel object model s COM objects  For example  subscribing to the Application class s WorkbookOpen event  Some theory on COM events COM classes expose a group of events through call-back interfaces  In order to subscribe to events  the client code can simply register an object implementing the call-back interface and the COM class will invoke its methods in response to specific events  Since the call-back interface is a COM interface  it is the duty of the implementing object to decrement the reference count of any COM object it receives  as a parameter  for any of the event handlers  How Excel PIA expose COM Events Excel PIA exposes COM events of Excel Application class as conventional  NET events  Whenever the client code subscribes to a  NET event  emphasis on  a    PIA creates an instance of a class implementing the call-back interface and registers it with Excel  Hence  a number of call-back objects get registered with Excel in response to different subscription requests from the  NET code  One call-back object per event subscription  A call-back interface for event handling means that  PIA has to subscribe to all interface events for every  NET event subscription request  It cannot pick and choose  On receiving an event call-back  the call-back object checks if the associated  NET event handler is interested in the current event or not and then either invokes the handler or silently ignores the call-back  Effect on COM instance reference counts All these call-back objects do not decrement the reference count of any of the COM objects they receive  as parameters  for any of the call-back methods  even for the ones that are silently ignored   They rely solely on the CLR garbage collector to free up the COM objects  Since GC run is non-deterministic  this can lead to the holding off of Excel process for a longer duration than desired and create an impression of a  memory leak   Solution The only solution as of now is to avoid the PIA   s event provider for the COM class and write your own event provider which deterministically releases COM objects  For the Application class  this can be done by implementing the AppEvents interface and then registering the implementation with Excel by using IConnectionPointContainer interface  The Application class  and for that matter all COM objects exposing events using callback mechanism  implements the IConnectionPointContainer interface

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You should be very careful using Word Excel interop applications  After trying all the solutions we still had a lot of  WinWord  process left open on server  with more than 2000 users    After working on the problem for hours  I realized that if I open more than a couple of documents using Word ApplicationClass Document Open   on different threads simultaneously  IIS worker process  w3wp exe  would crash leaving all WinWord processes open   So I guess there is no absolute solution to this problem  but switching to other methods such as Office Open XML development

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As others have pointed out  you need to create an explicit reference for every Excel object you use  and call Marshal ReleaseComObject on that reference  as described in this KB article   You also need to use try finally to ensure ReleaseComObject is always called  even when an exception is thrown   I e  instead of   Worksheet sheet   excelApp Worksheets 1      do something with sheet   you need to do something like   Worksheets sheets   null  Worksheet sheet   null try        sheets   excelApp Worksheets      sheet   sheets 1             finally       if  sheets    null  Marshal ReleaseComObject sheets       if  sheet    null  Marshal ReleaseComObject sheet       You also need to call Application Quit before releasing the Application object if you want Excel to close   As you can see  this quickly becomes extremely unwieldy as soon as you try to do anything even moderately complex   I have successfully developed  NET applications with a simple wrapper class that wraps a few simple manipulations of the Excel object model  open a workbook  write to a Range  save close the workbook etc    The wrapper class implements IDisposable  carefully implements Marshal ReleaseComObject on every object it uses  and does not pubicly expose any Excel objects to the rest of the app   But this approach doesn t scale well for more complex requirements     This is a big deficiency of  NET COM Interop   For more complex scenarios  I would seriously consider writing an ActiveX DLL in VB6 or other unmanaged language to which you can delegate all interaction with out-proc COM objects such as Office   You can then reference this ActiveX DLL from your  NET application  and things will be much easier as you will only need to release this one reference

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This sure seems like it has been over-complicated  From my experience  there are just three key things to get Excel to close properly   1  make sure there are no remaining references to the excel application you created  you should only have one anyway  set it to null   2  call GC Collect    3  Excel has to be closed  either by the user manually closing the program  or by you calling Quit on the Excel object   Note that Quit will function just as if the user tried to close the program  and will present a confirmation dialog if there are unsaved changes  even if Excel is not visible  The user could press cancel  and then Excel will not have been closed    1 needs to happen before 2  but 3 can happen anytime   One way to implement this is to wrap the interop Excel object with your own class  create the interop instance in the constructor  and implement IDisposable with Dispose looking something like  if   mDisposed       mExcel   null     GC Collect       mDisposed   true      That will clean up excel from your program s side of things  Once Excel is closed  manually by the user or by you calling Quit  the process will go away  If the program has already been closed  then the process will disappear on the GC Collect   call    I m not sure how important it is  but you may want a GC WaitForPendingFinalizers   call after the GC Collect   call but it is not strictly necessary to get rid of the Excel process    This has worked for me without issue for years  Keep in mind though that while this works  you actually have to close gracefully for it to work  You will still get accumulating excel exe processes if you interrupt your program before Excel is cleaned up  usually by hitting  stop  while your program is being debugged

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This is the only way that really works for me          foreach  Process proc in System Diagnostics Process GetProcessesByName  EXCEL                          proc Kill

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This worked for a project I was working on   excelApp Quit    Marshal ReleaseComObject  excelWB   Marshal ReleaseComObject  excelApp   excelApp   null    We learned that it was important to set every reference to an Excel COM object to null when you were done with it  This included Cells  Sheets  and everything

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The accepted answer here is correct  but also take note that not only  two dot  references need to be avoided  but also objects that are retrieved via the index   You also do not need to wait until you are finished with the program to clean up these objects  it s best to create functions that will clean them up as soon as you re finished with them  when possible   Here is a function I created that assigns some properties of a Style object called xlStyleHeader   public Excel Style xlStyleHeader   null   private void CreateHeaderStyle         Excel Styles xlStyles   null      Excel Font xlFont   null      Excel Interior xlInterior   null      Excel Borders xlBorders   null      Excel Border xlBorderBottom   null       try               xlStyles   xlWorkbook Styles          xlStyleHeader   xlStyles Add  Header   Type Missing               Text Format         xlStyleHeader NumberFormat                    Bold         xlFont   xlStyleHeader Font          xlFont Bold   true              Light Gray Cell Color         xlInterior   xlStyleHeader Interior          xlInterior Color   12632256              Medium Bottom border         xlBorders   xlStyleHeader Borders          xlBorderBottom   xlBorders Excel XlBordersIndex xlEdgeBottom           xlBorderBottom Weight   Excel XlBorderWeight xlMedium            catch  Exception ex                throw ex            finally               Release xlBorderBottom           Release xlBorders           Release xlInterior           Release xlFont           Release xlStyles            private void Release object obj           Errors are ignored per Microsoft s suggestion for this type of function         http   support microsoft com default aspx kb 317109     try               System Runtime InteropServices Marshal ReleaseComObject obj             catch          Notice that I had to set xlBorders Excel XlBordersIndex xlEdgeBottom  to a variable in order to clean that up  Not because of the two dots  which refer to an enumeration which does not need to be released  but because the object I m referring to is actually a Border object that does need to be released    This sort of thing is not really necessary in standard applications  which do a great job of cleaning up after themselves  but in ASP NET applications  if you miss even one of these  no matter how often you call the garbage collector  Excel will still be running on your server     It requires a lot of attention to detail and many test executions while monitoring the Task Manager when writing this code  but doing so saves you the hassle of desperately searching through pages of code to find the one instance you missed   This is especially important when working in loops  where you need to release EACH INSTANCE of an object  even though it uses the same variable name each time it loops

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Excel is not designed to be programmed via C   or C   The COM API is specifically designed to work with Visual Basic  VB NET  and VBA    Also all the code samples on this page are not optimal for the simple reason that each call must cross a managed unmanaged boundary and further ignore the fact that the Excel COM API is free to fail any call with a cryptic HRESULT indicating the RPC server is busy    The best way to automate Excel in my opinion is to collect your data into as big an array as possible   feasible and send this across to a VBA function or sub  via Application Run  which then performs any required processing  Furthermore - when calling Application Run - be sure to watch for exceptions indicating excel is busy and retry calling Application Run

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Common developers  none of your solutions worked for me   so I decide to implement a new trick   First let specify  What is our goal       Not to see excel object after our job in task manager    Ok  Let no to challenge and start destroying it  but consider not to destroy other instance os Excel which are running in parallel   So   get the list of current processors and fetch PID of EXCEL processes   then once your job is done  we have a new guest in processes list with a unique PID  find and destroy just that one    lt  keep in mind any new excel process during your excel job will be detected as new and destroyed     lt  A better solution is to capture PID of new created excel object and just destroy that   Process   prs   Process GetProcesses    List lt int gt  excelPID   new List lt int gt     foreach  Process p in prs     if  p ProcessName     EXCEL          excelPID Add p Id            your job   prs   Process GetProcesses    foreach  Process p in prs     if  p ProcessName     EXCEL   amp  amp   excelPID Contains p Id          p Kill      This resolves my issue  hope yours too

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I ve traditionally followed the advice found in VVS s answer   However  in an effort to keep this answer up-to-date with the latest options  I think all my future projects will use the  NetOffice  library   NetOffice is a complete replacement for the Office PIAs and is completely version-agnostic  It s a collection of Managed COM wrappers that can handle the cleanup that often causes such headaches when working with Microsoft Office in  NET   Some key features are    Mostly version-independent  and version-dependant features are documented  No dependencies No PIA No registration No VSTO   I am in no way affiliated with the project  I just genuinely appreciate the stark reduction in headaches

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First - you never have to call Marshal ReleaseComObject      or Marshal FinalReleaseComObject      when doing Excel interop  It is a confusing anti-pattern  but any information about this  including from Microsoft  that indicates you have to manually release COM references from  NET is incorrect  The fact is that the  NET runtime and garbage collector correctly keep track of and clean up COM references  For your code  this means you can remove the whole  while       loop at the top   Second  if you want to ensure that the COM references to an out-of-process COM object are cleaned up when your process ends  so that the Excel process will close   you need to ensure that the garbage collector runs  You do this correctly with calls to GC Collect   and GC WaitForPendingFinalizers    Calling this twice is safe  and ensures that cycles are definitely cleaned up too  though I m not sure it s needed  and would appreciate an example that shows this    Third  when running under the debugger  local references will be artificially kept alive until the end of the method  so that local variable inspection works   So  GC Collect   calls are not effective for cleaning object like rng Cells from the same method  You should split the code doing the COM interop from the GC cleanup into separate methods   This was a key discovery for me  from one part of the answer posted here by  nightcoder    The general pattern would thus be   Sub WrapperThatCleansUp          NOTE  Don t call Excel objects in here                 Debugger would keep alive until end  preventing GC cleanup        Call a separate function that talks to Excel     DoTheWork          Now let the GC clean up  twice  to clean up cycles too      GC Collect           GC WaitForPendingFinalizers       GC Collect           GC WaitForPendingFinalizers    End Sub  Sub DoTheWork       Dim app As New Microsoft Office Interop Excel Application     Dim book As Microsoft Office Interop Excel Workbook   app Workbooks Add       Dim worksheet As Microsoft Office Interop Excel Worksheet   book Worksheets  Sheet1       app Visible   True     For i As Integer   1 To 10         worksheet Cells Range  A   amp  i  Value    Hello      Next     book Save       book Close       app Quit          NOTE  No calls the Marshal ReleaseComObject   are ever needed End Sub   There is a lot of false information and confusion about this issue  including many posts on MSDN and on Stack Overflow  and especially this question     What finally convinced me to have a closer look and figure out the right advice was blog post Marshal ReleaseComObject Considered Dangerous together with finding the issue with references kept alive under the debugger that was confusing my earlier testing

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I found a useful generic template that can help implement the correct disposal pattern for COM objects  that need Marshal ReleaseComObject called when they go out of scope   Usage   using  AutoReleaseComObject lt Application gt  excelApplicationWrapper   new AutoReleaseComObject lt Application gt  new Application           try               using  AutoReleaseComObject lt Workbook gt  workbookWrapper   new AutoReleaseComObject lt Workbook gt  excelApplicationWrapper ComObject Workbooks Open namedRangeBase FullName  false  false  missing  missing  missing  true  missing  missing  true  missing  missing  missing  missing  missing                            do something with your workbook                         finally                excelApplicationWrapper ComObject Quit               Template   public class AutoReleaseComObject lt T gt    IDisposable       private T m comObject      private bool m armed   true      private bool m disposed   false       public AutoReleaseComObject T comObject                Debug Assert comObject    null           m comObject   comObject          if DEBUG      AutoReleaseComObject                    We should have been disposed using Dispose            Debug WriteLine  Finalize being called  should have been disposed             if  this ComObject    null                        Debug WriteLine string Format  ComObject was not null  0   name  1     this ComObject  this ComObjectName                         Debug Assert false          endif      public T ComObject               get                       Debug Assert  m disposed               return m comObject                       private string ComObjectName               get                       if this ComObject is Microsoft Office Interop Excel Workbook                                return   Microsoft Office Interop Excel Workbook this ComObject  Name                             return null                       public void Disarm                 Debug Assert  m disposed           m armed   false              region IDisposable Members      public void Dispose                 Dispose true    if DEBUG         GC SuppressFinalize this    endif             endregion      protected virtual void Dispose bool disposing                if   m disposed                        if  m armed                                int refcnt   0                  do                                       refcnt   System Runtime InteropServices Marshal ReleaseComObject m comObject                     while  refcnt  gt  0                    m comObject   default T                              m disposed   true                      Reference   http   www deez info sengelha 2005 02 11 useful-idisposable-class-3-autoreleasecomobject

[c#] How do I properly clean up Excel interop objects?

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