真·VB.NET彻底释放Interop.Excel对象

使用 Microsoft.Office.Interop.Excel 虽然有速度慢的缺点；但是作为自带引用，兼容性最好，而且是COM对象模型也很熟悉(Excel里直接录个宏，很方便把VBA代码转成VB.NET)。所以处理几百上千条的小数据时还是很方便的。

而 Microsoft.Office.Interop.Excel 用得不多的最大问题其实就是拿简单例子可以正确释放Excel，做了大量操作后却发现在任务管理器中依然有多余Excel进程存在。

问题原因当然是COM对象映射到Interop交互对象之后，.NET下的交互对象释放次序不符合COM对象预期，导致不能正确释放。比如

Dim xlApp New Excel.Application() With {.Visible = False}
Dim xlWorkbooks As Excel.Workbooks = xlApp.Workbooks
Dim xlWorkbook As Excel.Workbook = xlWorkbooks.Open("...")Dim value As Object = xlWorkbook.Sheets(1).Cells(1,1).ValuexlWorkbook.Close()
System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkbook)
xlWorkbook = Nothing
System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkbooks)
xlWorkbooks = Nothing
xlApp.Quit()
System.Runtime.InteropServices.Marshal.ReleaseComObject(xlApp)
xlApp = Nothing
System.GC.Collect()

中间取value这行代码看起来很正常，没有保留任何交互对象。其实在整个对象访问路径上隐式使用了以下交互对象，要靠GC来释放(通常是延后的————即调用Close()时交互对象未释放、工作簿关闭不了，之后的Quit()Excel不会退出)，

xlWorkbook.Sheets 'Excel.Sheets
xlWorkbook.Sheets(1) 'Excel.Worksheet
xlWorkbook.Sheets(1).Cells 'Excel.Range
xlWorkbook.Sheets(1).Cells(1,1) 'Excel.Range

要做到正确释放，要把这些交互对象全部在Close()前释放。为了方便使用，把 Excel.Application 和 Excel.Workbook 封装在类中，用 IDisposable 接口确保释放。用类似下面的属性封装访问

    Public Property CellValue(sheetIndex As Object, rowNo As Integer, colNo As Integer) As ObjectGetDim xlSheets As Excel.Sheets = m_xlWorkbook.SheetsDim xlSheet As Excel.Worksheet = xlSheets.Item(sheetIndex)Dim xlCells As Excel.Range = xlSheet.CellsDim xlCell As Excel.Range = xlCells.Item(rowNo, colNo)Dim value As Object = xlCell.ValueSystem.Runtime.InteropServices.Marshal.ReleaseComObject(xlCell)System.Runtime.InteropServices.Marshal.ReleaseComObject(xlCells)System.Runtime.InteropServices.Marshal.ReleaseComObject(xlSheet)System.Runtime.InteropServices.Marshal.ReleaseComObject(xlSheets)System.GC.Collect(0)Return valueEnd GetSet(value As Object)' 同理所有交互对象保留变量、释放End SetEnd Property