The meaning of "setting to Nothing"

Origin:http://www.dotnet2themax.com/blogs/fbalena/CategoryView,category,Migrating%20from%20VB6.aspx

 

In the VB6 world - and in the COM world, in general - you "destroy" an object by simply putting it to Nothing, unless of course there are other variables that are pointing to that specific instance. The VB6 runtime invokes the Class_Terminate method, if it exists, then it deallocates all the resources belonging to the object, memory included. This is a recursive process, and destroys any object that is owned by the object being set to Nothing.

We know well that setting a VB.NET object variable to Nothing doesn't fire any event, because the Terminate event isn't supported in .NET. If the object implements the Finalize method, this method is invoked by the garbage collection, but this invocation occurs some time later. The time interval between the Set to Nothing statement and the invocation of Finalize depends on many factors, but it could be as long as a few minutes or even hours, if the application doesn't allocate many objects and doesn't stress the garbate collector.

This behavioral difference can be a serious issue when migrating a VB6 application to VB.NET. For example, if the code in Class_Terminate closes a database connection, or a file, or a serial port, or deletes confidential information from disk, or unloads a form, then the delay between the "logical" destruction of the object (i.e. the Set to Nothing) and its "physical" destruction (when the Finalize method runs) can compromise the correct working of the application after its migration to the .NET world.

Unfortunately, it isn't possible to have VB.NET "automagically" behave like VB6 in this respect. However, it is possible to take a step that can greatly reduce the problem, without much impact on the code structure. Once again, this is possible thanks to generics. To see how, create a Module and add the following code to it, so that the SetNothing method is visible to the entire application:

 

Public Sub SetNothing( Of T)( ByRef obj As T)
   ' Dispose of the object if possible
   If obj IsNot Nothing AndAlso TypeOf obj Is IDisposable Then
      DirectCast (obj, IDisposable).Dispose()
   End If
   ' Decrease the reference counter, if it's a COM object
   If Marshal.IsComObject(obj) Then
      Marshal.ReleaseComObject(obj)
   End If
   obj = Nothing
End Sub

Next, you can apply the search-and-replace feature in Visual Studio to the code produced by the migration wizard, to replace all occurrences of the var = Nothing pattern with the SetNothing.(var) pattern. Obviously, this technique doesn't really solve all the abovementioned problems, because it works only with the variables that are explicitly set to Nothing via code, and doesn't work with variables that are implicitly cleared when they exit the current scope (for example at the end of the method).

Notice that the search-and-replace operation include a variable part - that is, the name of the instance that is set to Nothing - therefore making this replacementement automatically seems impossible or unpractical. But, fortunately, Visual Studio supports regular expressions in search-and-replace operations (as I explain here), therefore you can search for the <{:i} = Nothing string and specify the SetNothing(/1) string in the Replace with field. Et voilà

NOTE for those who are unfamiliar with regexes: the Find What string specifies that you want to search for a variable name (:i) that is located at the beginning of a word (<); curly braces in {:i} cause the variable name to be tagged, so that you can refer to the variable name in the Replace With regular expression, by means of the /1 placeholder. That's it.