Question

我继承了一个现有的应用程序来维护并遇到障碍：它一旦完成就不会从内存中释放它。我在这里提取了相关代码。这是一种使用COM Interop创建Excel工作簿的方法：

using System.Runtime.InteropServices;
using Excel = Microsoft.Office.Interop.Excel;
...

private void CreateWorkbook(string workingDirectory, string fileName, object[,] cellValues)
{
    Excel.Application excel = null;
    Excel.Workbooks workbooks = null;
    Excel.Workbook workbook = null;
    Excel.Sheets worksheets = null;
    Excel.Worksheet worksheet = null;
    Excel.Range startRange = null;
    Excel.Range endRange = null;
    Excel.Range selectedRange = null;

    try
    {
        excel = new Excel.Application() { DisplayAlerts = false, Visible = false, ScreenUpdating = false, EnableAutoComplete = false };

        // Statement which stops the excel instance exiting:
        if(excel == null)
        {
            MessageBox.Show("Excel could not be started, please check your machine has office 2013 installed.");
            return;
        }

        workbooks = excel.Workbooks;
        workbook = workbooks.Add(Type.Missing);
        worksheets = workbook.Sheets;
        worksheet = (Excel.Worksheet)worksheets[1];            

        startCellRange = (Excel.Range)worksheet.Cells[1,1];
        endCellRange = (Excel.Range)worksheet.Cells[1 + cellValues.GetLength(0), 1 + cellValues.GetLength(1)]; 
        selectedCells = worksheet.Range[startCellRange, endCellRange];
        selectedCells.Value2 = cellValues;

        workbook.SaveAs(workingDirectory + fileName, Excel.XlFileFormat.xlOpenXMLWorkbook, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
        workbook.Close(true, Type.Missing, Type.Missing);
        excel.Quit();
    }
    catch(Exception ex)
    {
        try
        {
            // Catch methods should ideally never throw exceptions, so try close the workbook and quit the excel instance in a try/catch block:
            workbook.Close(false, Type.Missing, Type.Missing);
            excel.Quit();
        }
        catch { }
        MessageBox.Show("An error was encountered while trying to create the excel workbook.\n" + ex.Message + (ex.InnerException == null ? "" : "\n" + ex.InnerException.Message));
    }
    finally
    {
        // Now clean up the com interop stuff:
        // ===================================
        if (Marshal.IsComObject(rangeFont)) Marshal.ReleaseComObject(rangeFont);
        if (Marshal.IsComObject(rangeBorders)) Marshal.ReleaseComObject(rangeBorders);
        if (Marshal.IsComObject(selectedRange)) Marshal.ReleaseComObject(selectedRange);
        if (Marshal.IsComObject(endRange)) Marshal.ReleaseComObject(endRange);
        if (Marshal.IsComObject(startRange)) Marshal.ReleaseComObject(startRange);
        if (Marshal.IsComObject(worksheet)) Marshal.ReleaseComObject(worksheet);
        if (Marshal.IsComObject(worksheets)) Marshal.ReleaseComObject(worksheets);
        if (Marshal.IsComObject(workbook)) Marshal.ReleaseComObject(workbook);
        if (Marshal.IsComObject(workbooks)) Marshal.ReleaseComObject(workbooks);
        if (Marshal.IsComObject(excel)) Marshal.ReleaseComObject(excel);
        if (Marshal.IsComObject(excel)) Marshal.FinalReleaseComObject(excel);

        System.Threading.Thread.Sleep(100);
        GC.Collect();
        GC.WaitForPendingFinalizers();
        System.Threading.Thread.Sleep(500);
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }
}

如果我在if之后注释掉excel = new Excel.Application()...语句，该方法可以正常工作并处理excel对象。但是当包含if (excel == null)测试时，excel对象不会从内存中释放出来。我一直想弄清楚为什么，我很难过。我知道运行时可以自动在com对象周围放置一个包装器（即使用超过“2点”访问com对象时）。但只是检查一个com对象是否等于null似乎不是运行时这样做的逻辑位置，如果确实是这样的话。

那么如何正确检查com对象是否为空？还有其他我想要检查的地方，例如在尝试关闭工作簿并退出Excel时在catch块内部。但是，我现在担心检查catch块中的excel object == null是否会生成一个不可见的包装器，所以我已经使用了嵌套的try..catch块。但这也感觉不对。

问题是，发生了什么，以及如果我的方法不正确，检查Excel com对象是否正确实例化的最佳做法是什么。

Answer 1

尝试将垃圾收集器调用移动到包装器方法。这样，当您调用它时，您可以确保所有自动创建的引用都超出范围。

Marshal.ReleaseComObject

另外，根据我的经验，您不需要调用$movie = Movie::whereIn('id', function($query) use ($id, $date) { $query->select(DB::raw('session.movie_id')) ->from('session') ->whereRaw('session.id = ? and session.date = ? array($id, $date)); }) ->get();，如果您释放了所有引用，则垃圾收集器会释放所有内容。也不需要睡觉。无论如何，GC呼叫都在阻塞。

如果我将其检查为null，为什么Excel com对象不会从内存中释放？

1 个答案: