Making a Collection Multithread safe

[fred]

I need some help in trying to understand how to make myCollection (inherited
from CollectionBase) multithread safe.

Taking my implementation of the Add Sub and a readonly property Item.

Public Sub Add(ByVal aDoc As myDocument)
List.Add(aDoc)
End Sub

Can I make this multithread safe by and is this the best way to do it.

Public Sub Add(ByVal aDoc As myDocument)
Synclock myCollection.SyncRoot
List.Add(aDoc)
End Synclock
End Sub

I notice that the ArrayList Class has a Synchronized method. How would I
implement such a function for myCollection.
As for the Item property:

Default Public ReadOnly Property Item(ByVal index As Integer) As myDocument
Get
Return CType(List.Item(index), myDocument)
End Get
End Property

Am I correct in thinking that since it is readonly I do not need to make
this thread safe in order to protect myCollection?
If I do make it thread safe as per the Add Sub, then will it prevent
multiple access to the Item (myDocument) through the same instance of
myCollection? I realise that there may be many other ways of accessing the
Item.

Does using Synclock as per the Add Sub above have much affect on
performance?
Thanks
Fred