Safely disposing Excel interop objects in C#?

Question

i am working on a winforms c# visual studio 2008 application. the app talks to excel files and i am using Microsoft.Office.Interop.Excel; to do this.

i would like to know how can i make sure that the objects are released even when there is an error?

here's my code:

private void button1_Click(object sender, EventArgs e)
{
    string myBigFile="";
    OpenFileDialog openFileDialog1 = new OpenFileDialog();
    DialogResult result = openFileDialog1.ShowDialog(); // Show the dialog.
    if (result == DialogResult.OK) // Test result.
        myBigFile=openFileDialog1.FileName;

    Excel.Application xlApp;
    Excel.Workbook xlWorkBook;
    Excel.Worksheet xlWorkSheet;
    Excel.Range range;

    string str;
    int rCnt = 0;
    int cCnt = 0;

    xlApp = new Excel.ApplicationClass();
    xlWorkBook = xlApp.Workbooks.Open(myBigFile, 0, true, 5, "", "", true, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "\t", true, false, 0, true, 1, 0);
    xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);

    range = xlWorkSheet.UsedRange;

    /*
    for (rCnt = 1; rCnt <= range.Rows.Count; rCnt++)
    {
        for (cCnt = 1; cCnt <= range.Columns.Count; cCnt++)
        {
            str = (string)(range.Cells[rCnt, cCnt] as Excel.Range).Value2;
            MessageBox.Show(str);
        }
    }
     */
    xlWorkSheet..EntireRow.Delete(Excel.XLDirection.xlUp)

    xlWorkBook.SaveAs(xlWorkBook.Path + @"\XMLCopy.xls",         Excel.XlFileFormat.xlXMLSpreadsheet, Type.Missing, Type.Missing,
   false, false, Excel.XlSaveAsAccessMode.xlNoChange,
   Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

    xlWorkBook.Close(true, null, null);
    xlApp.Quit();

    releaseObject(xlWorkSheet);
    releaseObject(xlWorkBook);
    releaseObject(xlApp);
}

private void releaseObject(object obj)
{
    try
    {
        System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
        obj = null;
    }
    catch (Exception ex)
    {
        obj = null;
        MessageBox.Show("Unable to release the Object " + ex.ToString());
    }
    finally
    {
        GC.Collect();
    }
}

how can i make sure that even if i get an error after the workbook opened, that i make sure to dispose of the objects:

Excel.Application xlApp;
Excel.Workbook xlWorkBook;
Excel.Worksheet xlWorkSheet;
Excel.Range range;

In other words no matter what i need the following lines to run

xlWorkBook.Close(true, null, null);
xlApp.Quit();

releaseObject(xlWorkSheet);
releaseObject(xlWorkBook);
releaseObject(xlApp);

please note that i have tried this as well, resulting in the same issue

xlWorkBook.Close(false, System.Reflection.Missing.Value, System.Reflection.Missing.Value);


                xlApp.Quit();

                Marshal.ReleaseComObject(xlWorkSheet);
                Marshal.ReleaseComObject(xlWorkBook);
                Marshal.ReleaseComObject(xlApp);

                xlWorkSheet = null;
                xlWorkBook = null;
                xlApp = null;

                GC.GetTotalMemory(false);
                GC.Collect();
                GC.WaitForPendingFinalizers();
                GC.Collect();
                GC.GetTotalMemory(true);  

and i did this as well:

GC.Collect()                   ;
                GC.WaitForPendingFinalizers();
                GC.Collect()                  ; 
                GC.WaitForPendingFinalizers();

                Marshal.FinalReleaseComObject(xlWorkSheet);

                xlWorkBook.Close(Type.Missing, Type.Missing, Type.Missing);
                Marshal.FinalReleaseComObject(xlWorkBook); 

                xlApp.Quit();
                Marshal.FinalReleaseComObject(xlApp); 

at this point i do not think it's possible to close excel from visual studio 2008. it must be a bug or something, but i've tried the top 20 websites on this and getting the same result: excel is opening two instances for some reason and when i do the garbage collection etc.. (or not) it closes just ONE instance.

when i try to open the file, it says there's an error or it's corrupt.

when i go to task manager and kill the excel process, the file will open without problems.]

is there a way to close excel with visual studio 2008? if so, can you please provide me with guidance or a solution to this

Answer 1

First I will present a modified releaseObject, and then I will provide a pattern to use it.

using Marshal = System.Runtime.InteropServices.Marshal;
private void releaseObject(ref object obj) // note ref!
{
    // Do not catch an exception from this.
    // You may want to remove these guards depending on
    // what you think the semantics should be.
    if (obj != null && Marshal.IsComObject(obj)) {
        Marshal.ReleaseComObject(obj);
    }
    // Since passed "by ref" this assingment will be useful
    // (It was not useful in the original, and neither was the
    //  GC.Collect.)
    obj = null;
}

Now, a pattern to use:

private void button1_Click(object sender, EventArgs e)
{
    // Declare. Assign a value to avoid a compiler error.
    Excel.Application xlApp = null;
    Excel.Workbook xlWorkBook = null;
    Excel.Worksheet xlWorkSheet = null;

    try {
        // Initialize
        xlApp = new Excel.ApplicationClass();
        xlWorkBook = xlApp.Workbooks.Open(myBigFile, 0, true, 5, "", "", true, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "\t", true, false, 0, true, 1, 0);
        // If the cast fails this like could "leak" a COM RCW
        // Since this "should never happen" I wouldn't worry about it.
        xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);
        ...
    } finally {
        // Release all COM RCWs.
        // The "releaseObject" will just "do nothing" if null is passed,
        // so no need to check to find out which need to be released.
        // The "finally" is run in all cases, even if there was an exception
        // in the "try". 
        // Note: passing "by ref" so afterwords "xlWorkSheet" will
        // evaluate to null. See "releaseObject".
        releaseObject(ref xlWorkSheet);
        releaseObject(ref xlWorkBook);
        // The Quit is done in the finally because we always
        // want to quit. It is no different than releasing RCWs.
        if (xlApp != null) {
            xlApp.Quit();
        }
        releaseObject(ref xlApp);    
    }
}

This simple approach can be extended/nested over most situations. I use a custom wrapper class that implements IDisposable to make this task easier.