Add comprehensive COM object cleanup documentation and examples for Excel Interop

docs/csharp/advanced-topics/interop/snippets/OfficeInterop/program.cs

@@ -8,6 +8,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 //<Snippet1>
 using Excel = Microsoft.Office.Interop.Excel;
 using Word = Microsoft.Office.Interop.Word;
@@ -48,39 +49,128 @@ static void Main(string[] args)
         //<Snippet4>
         static void DisplayInExcel(IEnumerable<Account> accounts)
         {
-            var excelApp = new Excel.Application();
-            // Make the object visible.
-            excelApp.Visible = true;
+            Excel.Application excelApp = null;
+            Excel.Workbook workbook = null;
+            Excel.Worksheet workSheet = null;
+
+            try
+            {
+                excelApp = new Excel.Application();
+                // Make the object visible.
+                excelApp.Visible = true;
 
-            // Create a new, empty workbook and add it to the collection returned
-            // by property Workbooks. The new workbook becomes the active workbook.
-            // Add has an optional parameter for specifying a particular template.
-            // Because no argument is sent in this example, Add creates a new workbook.
-            excelApp.Workbooks.Add();
+                // Create a new, empty workbook and add it to the collection returned
+                // by property Workbooks. The new workbook becomes the active workbook.
+                // Add has an optional parameter for specifying a particular template.
+                // Because no argument is sent in this example, Add creates a new workbook.
+                workbook = excelApp.Workbooks.Add();
 
-            // This example uses a single workSheet. The explicit type casting is
-            // removed in a later procedure.
-            Excel._Worksheet workSheet = (Excel.Worksheet)excelApp.ActiveSheet;
+                // This example uses a single workSheet. The explicit type casting is
+                // removed in a later procedure.
+                workSheet = (Excel.Worksheet)excelApp.ActiveSheet;
+
+                // Establish column headings in cells A1 and B1.
+                workSheet.Cells[1, "A"] = "ID Number";
+                workSheet.Cells[1, "B"] = "Current Balance";
+
+                var row = 1;
+                foreach (var acct in accounts)
+                {
+                    row++;
+                    workSheet.Cells[row, "A"] = acct.ID;
+                    workSheet.Cells[row, "B"] = acct.Balance;
+                }
+
+                workSheet.Columns[1].AutoFit();
+                workSheet.Columns[2].AutoFit();
+
+                // Put the spreadsheet contents on the clipboard.
+                workSheet.Range["A1:B3"].Copy();
+
+                // Save the workbook before closing
+                string fileName = System.IO.Path.Combine(
+                    Environment.GetFolderPath(Environment.SpecialFolder.Desktop), 
+                    "BankAccounts.xlsx");
+                workbook.SaveAs(fileName);
+            }
+            finally
+            {
+                // Clean up COM objects in reverse order of creation
+                if (workSheet != null)
+                {
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(workSheet);
+                    workSheet = null;
+                }
+                if (workbook != null)
+                {
+                    workbook.Close(true); // Save changes
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(workbook);
+                    workbook = null;
+                }
+                if (excelApp != null)
+                {
+                    excelApp.DisplayAlerts = true;
+                    excelApp.Quit();
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelApp);
+                    excelApp = null;
+                }
+
+                // Force garbage collection
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+            }
         }
         //</Snippet4>
 
         //<Snippet9>
         static void CreateIconInWordDoc()
         {
-            var wordApp = new Word.Application();
-            wordApp.Visible = true;
-
-            // The Add method has four reference parameters, all of which are
-            // optional. Visual C# allows you to omit arguments for them if
-            // the default values are what you want.
-            wordApp.Documents.Add();
-
-            // PasteSpecial has seven reference parameters, all of which are
-            // optional. This example uses named arguments to specify values
-            // for two of the parameters. Although these are reference
-            // parameters, you do not need to use the ref keyword, or to create
-            // variables to send in as arguments. You can send the values directly.
-            wordApp.Selection.PasteSpecial( Link: true, DisplayAsIcon: true);
+            Word.Application wordApp = null;
+            Word.Document document = null;
+
+            try
+            {
+                wordApp = new Word.Application();
+                wordApp.Visible = true;
+
+                // The Add method has four reference parameters, all of which are
+                // optional. Visual C# allows you to omit arguments for them if
+                // the default values are what you want.
+                document = wordApp.Documents.Add();
+
+                // PasteSpecial has seven reference parameters, all of which are
+                // optional. This example uses named arguments to specify values
+                // for two of the parameters. Although these are reference
+                // parameters, you do not need to use the ref keyword, or to create
+                // variables to send in as arguments. You can send the values directly.
+                wordApp.Selection.PasteSpecial( Link: true, DisplayAsIcon: true);
+
+                // Save the document
+                string fileName = System.IO.Path.Combine(
+                    Environment.GetFolderPath(Environment.SpecialFolder.Desktop), 
+                    "BankAccountsLink.docx");
+                document.SaveAs(fileName);
+            }
+            finally
+            {
+                // Clean up COM objects in reverse order of creation
+                if (document != null)
+                {
+                    document.Close(true); // Save changes
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(document);
+                    document = null;
+                }
+                if (wordApp != null)
+                {
+                    wordApp.Quit(true); // Save changes to all documents
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(wordApp);
+                    wordApp = null;
+                }
+
+                // Force garbage collection
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+            }
         }
         //</Snippet9>
 

docs/csharp/advanced-topics/interop/snippets/OfficeWalkthrough/ThisAddIn.cs

@@ -1,6 +1,7 @@
 using System;
 //<Usings>
 using System.Collections.Generic;
+using System.Runtime.InteropServices;
 using Excel = Microsoft.Office.Interop.Excel;
 using Word = Microsoft.Office.Interop.Word;
 //</Usings>
@@ -43,10 +44,41 @@ private void ThisAddIn_Startup(object sender, System.EventArgs e)
             //</CallDisplay>
 
             //<PasteIntoWord>
-            var wordApp = new Word.Application();
-            wordApp.Visible = true;
-            wordApp.Documents.Add();
-            wordApp.Selection.PasteSpecial(Link: true, DisplayAsIcon: true);
+            Word.Application wordApp = null;
+            Word.Document document = null;
+
+            try
+            {
+                wordApp = new Word.Application();
+                wordApp.Visible = true;
+                document = wordApp.Documents.Add();
+                wordApp.Selection.PasteSpecial(Link: true, DisplayAsIcon: true);
+
+                // Save the document
+                string fileName = System.IO.Path.Combine(
+                    Environment.GetFolderPath(Environment.SpecialFolder.Desktop), 
+                    "BankAccountsLink.docx");
+                document.SaveAs(fileName);
+            }
+            finally
+            {
+                // Clean up COM objects
+                if (document != null)
+                {
+                    document.Close(true);
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(document);
+                    document = null;
+                }
+                if (wordApp != null)
+                {
+                    wordApp.Quit(true);
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(wordApp);
+                    wordApp = null;
+                }
+
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+            }
             //</PasteIntoWord>
         }
 
@@ -72,6 +104,83 @@ void DisplayInExcel(IEnumerable<Account> accounts,
         }
         //</Display>
 
+        //<ProperCleanup>
+        static void CleanupComObject(object comObject)
+        {
+            if (comObject != null)
+            {
+                System.Runtime.InteropServices.Marshal.FinalReleaseComObject(comObject);
+                comObject = null;
+            }
+        }
+        //</ProperCleanup>
+
+        //<DisplayWithCleanup>
+        void DisplayInExcelWithCleanup(IEnumerable<Account> accounts,
+                   Action<Account, Excel.Range> DisplayFunc)
+        {
+            Excel.Application excelApp = null;
+            Excel.Workbook workbook = null;
+            Excel.Worksheet worksheet = null;
+
+            try
+            {
+                excelApp = new Excel.Application();
+                excelApp.Visible = true;
+
+                // Add a new Excel workbook.
+                workbook = excelApp.Workbooks.Add();
+                worksheet = workbook.ActiveSheet;
+
+                worksheet.Cells[1, 1].Value = "ID";
+                worksheet.Cells[1, 2].Value = "Balance";
+
+                int row = 2;
+                foreach (var ac in accounts)
+                {
+                    var cell = worksheet.Cells[row, 1];
+                    DisplayFunc(ac, cell);
+                    row++;
+                }
+
+                // Save the workbook
+                string fileName = System.IO.Path.Combine(
+                    Environment.GetFolderPath(Environment.SpecialFolder.Desktop), 
+                    "BankAccounts.xlsx");
+                workbook.SaveAs(fileName);
+
+                // Copy the results to the Clipboard.
+                worksheet.Range["A1:B3"].Copy();
+            }
+            finally
+            {
+                // Always clean up COM objects in reverse order of creation
+                if (worksheet != null)
+                {
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(worksheet);
+                    worksheet = null;
+                }
+                if (workbook != null)
+                {
+                    workbook.Close(true); // Save changes
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(workbook);
+                    workbook = null;
+                }
+                if (excelApp != null)
+                {
+                    excelApp.DisplayAlerts = true;
+                    excelApp.Quit();
+                    System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelApp);
+                    excelApp = null;
+                }
+
+                // Force garbage collection to help release any remaining references
+                GC.Collect();
+                GC.WaitForPendingFinalizers();
+            }
+        }
+        //</DisplayWithCleanup>
+
 
         private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
         {

docs/csharp/advanced-topics/interop/walkthrough-office-programming.md

@@ -96,6 +96,52 @@ This code demonstrates several of the features in C#: the ability to omit the `r
 
 Press F5 to run the application. Excel starts and displays a table that contains the information from the two accounts in `bankAccounts`. Then a Word document appears that contains a link to the Excel table.
 
+## Important: COM object cleanup and resource management
+
+The examples shown above demonstrate basic Office Interop functionality, but they don't include proper cleanup of COM objects. This is a critical issue in production applications because failing to properly release COM objects can result in orphaned Office processes that remain in memory even after your application closes.
+
+### Why COM object cleanup is necessary
+
+COM objects in Office Interop require explicit cleanup because:
+
+- The .NET garbage collector doesn't automatically release COM objects
+- Each Excel or Word object you create holds resources that must be manually released
+- Without proper cleanup, Office applications remain running in the background
+- This applies to all COM objects: Application, Workbooks, Worksheets, Ranges, and more
+
+### Proper cleanup pattern
+
+Use the following pattern to ensure all COM objects are properly released:
+
+:::code language="csharp" source="./snippets/OfficeWalkthrough/ThisAddIn.cs" id="ProperCleanup":::
+
+Add the following enhanced version of the `DisplayInExcel` method that includes proper COM object cleanup:
+
+:::code language="csharp" source="./snippets/OfficeWalkthrough/ThisAddIn.cs" id="DisplayWithCleanup":::
+
+This pattern ensures that:
+
+- COM objects are released even if an exception occurs
+- Excel processes don't remain orphaned in Task Manager
+- Memory is properly freed
+- The application behaves reliably in production environments
+
+For production applications, always implement this cleanup pattern for every COM object you create, including Application, Workbooks, Worksheets, Ranges, and other Office objects.
+
+### Common questions about COM object cleanup
+
+**Why can't garbage collection handle this automatically?**
+COM objects use reference counting for memory management, which is different from .NET's garbage collection. The .NET runtime creates a Runtime Callable Wrapper (RCW) around each COM object, but the RCW doesn't automatically release the underlying COM object when it's garbage collected.
+
+**Why must I set objects to null after calling Marshal.FinalReleaseComObject?**
+Setting references to null ensures that your code can't accidentally use a released COM object, which would throw an exception. It also helps the garbage collector by removing any remaining managed references.
+
+**Why call GC.Collect() and GC.WaitForPendingFinalizers()?**
+These calls force immediate garbage collection, which helps ensure that any remaining RCWs are cleaned up promptly. While not always strictly necessary, they provide additional safety in COM interop scenarios.
+
+**What happens if I don't follow this pattern?**
+Without proper cleanup, Office applications remain running in the background even after your application exits. You can verify this by checking Task Manager - you'll see excel.exe or winword.exe processes that weren't properly terminated. These orphaned processes consume memory and can cause issues with future Office automation.
+
 ## Clean up the completed project
 
 In Visual Studio, select **Clean Solution** on the **Build** menu. Otherwise, the add-in runs every time that you open Excel on your computer.