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Collections. A collection is a way of grouping a set of related items. Properties and Methods a Collection. How is a Collection stored?. A Collection object stores each item in a Variant. The list of things you can add to a Collection object is the

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slide1

Collections

A collection is a way of grouping a set of related items.

Properties and Methods a Collection

slide2

How is a Collection stored?

  • A Collection object stores each item in a Variant.
  • The list of things you can add to a Collection object is the
  • same as the list of things that can be stored in a Variant.
  • This include standard data types, objects, and arrays.
  • Variants always take up 16 bytes, no matter what\'s stored in them.
  • Using a Collection object is not as efficient as using arrays.
  • However, you never have to ReDim a Collection object
  • Collection objects have extremely fast look-ups by key, which arrays do not.
  • Note:   A Variant always takes up 16 bytes even if the data are actually stored elsewhere.
  • TradeOff: Collection objects allow you to write very clean, maintainable code
  • — at the cost of storing items in Variants.
slide3

How is a Collection stored?

Use the Item method to retrieve specific items from a collection.

The syntax is:

[Set] variable = object.Item(index)

Set woCurrent = colWorkOrders.Item(3)

or

Set woCurrent = colWorkOrders.Item("W017493")

Item is the default method

Set woCurrent = colWorkOrders(3)

or

Set woCurrent = colWorkOrders ("W017493")

colWorkOrders("W017493").Priority = 3

slide4

colCourses

objCourse

objCourse

objCourse

objCourse

objCourse

Creating your own collection class

The most robust way to implement a collection is by making it a class module .

Create a class that is a wrapper around a collection.

colCourses

collection

The student/courses project demonstrates this.

clsStudent

colCourses

clsCourse

colCourses is a container for

objects of type clsCourse

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Making Item the Default Property

  • To make Item the default property
  • On the Tools menu, click Procedure Attributes to open the Procedure Attributes dialog box. In Name box, select the Item method.
  • Click Advanced to show the advanced features. In the Procedure ID box, select (Default) to make the Item method the default. Click OK.
  • Note   A class can have only one default member (property or method).
slide6

Enabling For Each

  • Write the NewEnum property
  • Public Property Get NewEnum() As Iunknown
  • Set NewEnum = mcol.[_NewEnum]
  • End Property
  • To hide the NewEnum method and give it the necessary procedure ID
  • On the Tools menu, click Procedure Attributes to open the Procedure Attributes dialog box. In Name box, select the NewEnum method.
  • Click Advanced to show the advanced features. Check Hide this member to make NewEnum hidden in the type library.
  • In the Procedure ID box, type –4 (minus four) to give NewEnum the procedure ID required by For Each … Next. Click OK.
  • Important   In order for your collection classes to work with For Each … Next, you must provide a hidden NewEnum method with the correct procedure ID.
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