The VB6 Collection is quite a versatile object: it allows you to add and remove elements by their index like in an array, but without requiring to initialize it to a fixed number of elements. It also allows you to associate a string key to an element, so that you can quickly retrieve (or delete) that element without having to scan the collection one element at a time. (An important detail: string keys are compared in case-insensitive mode.)

Both the Upgrade Wizard and VB Migration Partner convert VB6 Collection objects into the Microsoft.VisualBasic.Collection objects, a choice that ensure the highest degree of compatibility and functional equivalence. A few articles around the Internet suggest that you should try to use a "pure" .NET collection, if possible. 

Unfortunately, this is easier said that done, because no other .NET collection exactly mimicks the behavior of the VB6 Collection. If you never use string keys you can translate a VB6 Collection using the ArrayList or the List(Of T) objects, except these objects have their lower index equal to 0 instead of 1, a detail that would force you to carefully scrutinize and fix your code as necessary. On the other hand, a VB6 collection that is only used with string key might be converted to VB.NET using the HashTable or the Dictionary(Of T), but be careful because these .NET collections deal with string key in case-sensitive mode and therefore aren't perfectly equivalent to the original VB6 Collection object. (TIP: you can use the CollectionUtils.CreateCaseInsensitiveHashtable method to create a case-insensitive dictionary.)

The main reason for replacing the VB.NET Collection with a native .NET collection is performance. The following table recaps the timing (in milliseconds) to perform a given operation 10,000 times. As you see, while the VB.NET collection is marginally faster than the VB6 collection in most cases, it is also much slower than the ArrayList collection at retrieving and deleting elements. (It is also slower than the Hashtable object, but the difference in absolute terms is virtually negligible.)

Test  VB6   VB.NET   ArrayList   Hashtable   VB6Collection  
Add(item) 0 1 0 n/a 2
Add(item,key) 47 7 n/a 4 6
Item(ndx) 203 215 0 n/a 0
Item(key) 16 7 n/a 0 9
For Each 15 7 0 0 1
Remove(ndx) 609 539 46 n/a 247
Remove(ndx) (*)    140 131 46 n/a 70
Remove(key) 16 12 n/a 3 3820
  (*) if items aren't associated with keys


The table suggests that, if you are sure that you never use keys, then you should replace a collection with an ArrayList object. Likewise, if you are sure that the collection is always going to contain elements of a specific type - for example, strings - you can improve performance and code robustness by replacing the collection with a List(Of T) object. However, if your code uses all the features of the VB6 Collection - namely, numeric indexes and string keys, you apparently have no choice other than using the standard VB.NET collection, right?

Well, not if you use VB Migration Partner. Starting with forthcoming version 1.10, you can replace a VB.NET collection with the new VB6Collection object, provided in our support library. As shown in previous table, this custom object is remarkably faster than the VB.NET collection in all tests (except when passing a key to the Remove method), and is as fast as the ArrayList object at retrieving elements by their numeric index.

When version 1.10 is released, you can replace one or more instances of the VB.NET Collection with our custom VB6Collection type by means of a ReplaceStatement pragma:

    '## ReplaceStatement Dim col As New VB6Collection
    Dim col As New Collection

Even better: we provide also a generic, strong-typed version of this collection, named VB6Collection(Of T). If you are sure that the collection is going to contain only element of a certain kind - integers, for example - you can write better code by using the generic version of this collection:

    '## ReplaceStatement Dim col As New VB6Collection(Of Integer)
    Dim col As New Collection

The VB6Collection class is 100% compatible with the original VB6 collection and the VB.NET collection, therefore you don't need to edit the VB.NET source code in any other way.