Extensia

Get a bitmap representation of a control.

<System.Runtime.CompilerServices.Extension()> _
 Public Function ToBitmap(ByRef c As Windows.Forms.Control) As Image
  Dim bmp As New Bitmap(c.Width, c.Height)
  c.DrawToBitmap(bmp, New Rectangle(0, 0, c.Width, c.Height))
  Return bmp
 End Function

Set properties across threads.

<System.Runtime.CompilerServices.Extension()> _
 Public Sub CrossThreadSet(Of t)(ByRef [Control] As Control, _
                                 ByRef PropertyName As String, _
                                 ByRef Value As t)
  Dim PInfo As Reflection.PropertyInfo = [Control].GetType.GetProperty(PropertyName)
  If PInfo Is Nothing Then Exit Sub
  If [Control].InvokeRequired Then
   [Control].Invoke(New Delegare_CrossThreadSet(Of t)(AddressOf CrossThreadSet), _
                    New Object() {[Control], PropertyName, Value})
  Else
   PInfo.SetValue([Control], Value, Nothing)
  End If
 End Sub
 Public Delegate Sub Delegare_CrossThreadSet(Of t)(ByRef [Control] As Control, _
                                                   ByRef PropertyName As String, _
                                                   ByRef Value As t)

Usage

Me.Lbl_Name.CrossThreadSet(of String)("Text","Hello")

Me.ProgressBar1.CrossThreadSet(Of Integer)("Value",10)

ByVal creates a copy of the parameter.

In the CrossThreadSet the sub-routine can recursively call itself, if I used ByVal that be the third copy.

Using ByRef this is only one copy.

It easily overriden by enclosing the parameter with parentheses (Brackets).

That what I said. (Could have be worded slightly better)

The data in the ByVal parameter is a copy

See MSDN Doc

My belief it is ByVal by default to protect the typical vb programmer from themselves.

I would prefer to option to change the default in the ide.

I think your confused.

A Double is a Value Type

A String is a Reference Type

I think of passing;-

 ByRef as using the original which is rewritable.

 ByVal as new copy of (which happens to contain the same data.) so you do what ever you like to it as it get disposed on exit the sub.

  Unless you use it as the return value in a function. (behind the scenes the compiler secretly moves to the correct scope.)

Sven Groot & littleguru.
You're argued your cases well and you've changed the way I'll think about it the future.
 <shaking fist> damn you vb6 </shaking fist>
I add to create a diagram to convince myself.
Notes
 ][ represent the parameter/method boundary,
--> Object can flow in
<-- Object can flow out
ByVal
 ClassObj -->][   

Inside function/sub :
  ClassObj.foo -->][<--

ByRef
 ClassObj -->][<--  Can be altered to different object/Nothing

Inside function/sub:

  ClassObj.foo -->][<--
   
On doing the diagram, I wondered how I could do.
 
ByVal

 ClassObj -->][
Inside function/sub :

  ClassObj.foo -->][

ByRef
 ClassObj -->][

Inside function/sub :

  ClassObj.foo -->][

Where it object is effectly read only, irrespective of the setters.

I know these are useless extension but it illustrates why I thought they were copies.

<System.Runtime.CompilerServices.Extension()> _
 Public Sub MakeNothing(ByRef o As Object)
  o = Nothing
 End Sub
 <System.Runtime.CompilerServices.Extension()> _
Public Sub DoNothing(ByVal o As Object)
  o = Nothing
 End Sub

Using the first would change the input and second wouldn't.

I was thinking of a sub that updated the contents on a form, it sole purpose.  No processing / calcution just update the form contents.

No way of intercepting the set?

What  about?

Public Class ReadOnlyObj(Of T As Class)
 Protected m_obj As T
 Public Sub New(ByVal obj As T)
  m_obj = obj
 End Sub
 Public ReadOnly Property Obj() As T
  Get
   Return m_obj
  End Get
 End Property
End Class

 

<System.Runtime.CompilerServices.Extension()> _
  Public Function makeReadOnly(Of T As Class)(ByVal obj As T)
  Return New ReadOnlyObj(Of T)(obj)
 End Function

That to me that not a readonly collection, more of fixed collection.

Read Only to me uggest you can't alter anything, for example a music cd is a read only collection of music.

On the quest on of the readonly object. This is only a theory .

Could it be implemented with a new class which essenstially the same as object but with the difference that FieldSetter doesn't set?