Parenthesising arguments: What is MyByRefSub((arg)) doing?

[cjard]

Suppose we have the following:

    Public Sub MyByRefSub(ByRef sb As StringBuilder)
        sb = New StringBuilder("All New SB By Ref")
    End Sub

And we call it with:

  Dim abc as New StringBuilder("Old StringBuilder")
  MyByRefSub(abc)

When the method completes, we now have abc as the "All New SB By Ref" string builder.
I know why this is

But my question is, what, behind the scenes exactly is this doing:

  MyByRefSub((abc))

It behaves exactly like ByVal but I cant think of a way of investigating whether it is just ignoring ByRef and passing ByVal, or whether it's doing some funky cloning.

Interestingly, I had also the same Sub, but with an Object arg:

    Public Sub MyByRefSub(ByRef o As Object)
        o = New Object()
    End Sub

When I pass in a child type:

  MyByRefSub((abc))
  [U]MyByRefSub(abc)[/U]

only the second one gets a wiggly line and a: "Option Strict On disallows narrowing from type 'Object' to type 'System.Text.StringBuilder' in copying the value of 'ByRef' parameter 'o' back to the matching argument. " error

Is this a clue to what is going on?

 

[jmcilhinney]

Open your local MSDN library, select the Index tab, set the filter to Visual Basic, type arguments into the search box, go down the index list to the arguments [Visual Basic] -> in parentheses topic.

 

[cjard]

I dont have a local MSDN and I havent succeeded in finding any info on the net that might be what you are referring to.. Would you mind pasting your local MSDN entry to the forum?

 

[JohnH]

How to: Force an Argument to Be Passed by Value

I find the local MSDN of great value, all VS editions have it, the Express a limited version. Wish MSDN online was fast as that, searching out the class documentation and related articles requires a great deal of cross-lookups

 

[cjard]

Ah, so there is no magic or voodoo here.. its just the ByRef is conceptually rewritten to ByVal. Thanks guys!