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The Two Dot rule unfortunately is not very well documented in msdn. The only mention of it happens to be in the All-In-One Code Framework.
What is Two Dot Rule?
The Two Dots tunnels your call into the Com object model to access it’s properties.
Let’s take an example
Dim xlApp As New Excel.Application Dim xlWorkBook As Excel.Workbook xlWorkBook = xlApp.Workbooks.Add
Do you notice the two dots? One after the xlApp and the other after Workbooks?
Using Two Dots when calling xlApp.Workbooks.Add creates an RCW (Runtime Callable Wrapper) for the Workbooks object. If you invoke these accessors, the RCW for Workbooks is created on the GC heap. However what’s worth noting is that the reference is created under the hood on the stack and are then discarded. Because of this there is no way to call MarshalFinalReleaseComObject on this RCW. Therefore, if all references have not been released on the RCW, the COM object does not quit and this results in an instance of your Excel Application (in this case) being left in Task Manager.
Is ignoring Two Dot Rule Bad?
Honestly, if I may say so, it all depends on how you flush the toilet after use!
As mentioned above, there is no way to call MarshalFinalReleaseComObject on this RCW. You will have to either force a garbage collection as soon as the calling function is off the stack or you would need to explicitly assign each accessor object to a variable and free it.
Let’s take an example
Dim xlApp As New Excel.Application Dim xlWorkBooks As Excel.Workbooks = xlApp.Workbooks Dim xlWorkBook As Excel.Workbook = xlWorkBooks.Add() ' '~~> rest of the code ' xlApp.Quit() If Not xlWorkBook Is Nothing Then Marshal.FinalReleaseComObject (xlWorkBook) xlWorkBook = Nothing End If If Not xlWorkBooks Is Nothing Then Marshal.FinalReleaseComObject (xlWorkBooks) xlWorkBooks = Nothing End If If Not xlApp Is Nothing Then Marshal.FinalReleaseComObject (xlApp) xlApp = Nothing End If
Imports Excel = Microsoft.Office.Interop.Excel Public Class Form1 '~~> Define your Excel Objects Dim xlApp As New Excel.Application Dim xlWorkBook As Excel.Workbook Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click '~~> Add a New Workbook (IGNORING THE TWO DOT RULE) xlWorkBook = xlApp.Workbooks.Add '~~> Display Excel xlApp.Visible = True '~~> Do some stuff Here '~~> Save the file xlWorkBook.SaveAs(Filename:="C:\Tutorial\SampleNew.xlsx", FileFormat:=51) '~~> Close the File xlWorkBook.Close() '~~> Quit the Excel Application xlApp.Quit() '~~> Clean Up releaseObject (xlApp) releaseObject (xlWorkBook) End Sub '~~> Release the objects Private Sub releaseObject(ByVal obj As Object) Try System.Runtime.InteropServices.Marshal.ReleaseComObject (obj) obj = Nothing Catch ex As Exception obj = Nothing Finally GC.Collect() End Try End Sub End Class