How to: Create a Collection Used by a Collection Initializer (Visual Basic)

When you use a collection initializer to create a collection, the Visual Basic compiler searches for an Add method of the collection type for which the parameters for the Add method match the types of the values in the collection initializer. This Add method is used to populate the collection with the values from the collection initializer.

The following example shows an OrderCollection collection that contains a public Add method that a collection initializer can use to add objects of type Order. The Add method enables you to use the shortened collection initializer syntax.

Public Class Customer
    Public Property Id As Integer 
    Public Property Name As String 
    Public Property Orders As OrderCollection

    Public Sub New(ByVal id As Integer, ByVal name As String, ByVal orders As OrderCollection)
        Me.Id = id
        Me.Name = name
        Me.Orders = orders
    End Sub 
End Class 

Public Class Order
    Public Property Id As Integer 
    Public Property CustomerId As Integer 
    Public Property OrderDate As DateTime

    Public Sub New(ByVal id As Integer,
                   ByVal customerId As Integer,
                   ByVal orderDate As DateTime)
        Me.Id = id
        Me.CustomerId = customerId
        Me.OrderDate = orderDate
    End Sub 
End Class
Public Class OrderCollection
    Implements IEnumerable(Of Order)

    Dim items As New List(Of Order)

    Public Property Item(ByVal index As Integer) As Order
        Get 
            Return CType(Me(index), Order)
        End Get 
        Set(ByVal value As Order)
            items(index) = value
        End Set 
    End Property 

    Public Sub Add(ByVal id As Integer, ByVal customerID As Integer, ByVal orderDate As DateTime)
        items.Add(New Order(id, customerID, orderDate))
    End Sub 

    Public Function GetEnumerator() As IEnumerator(Of Order) Implements IEnumerable(Of Order).GetEnumerator
        Return items.GetEnumerator()
    End Function 

    Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
        Return Me.GetEnumerator()
    End Function 
End Class
Imports System.Runtime.CompilerServices

Module Module1

    <Extension()>
    Sub Add(ByVal genericList As List(Of Customer),
            ByVal id As Integer,
            ByVal name As String,
            ByVal orders As OrderCollection)

        genericList.Add(New Customer(id, name, orders))
    End Sub 
End Module
Dim customerList = New List(Of Customer) From
  {
    {1, "John Rodman", New OrderCollection From {{9, 1, #6/12/2008#},
                                                 {8, 1, #6/11/2008#},
                                                 {5, 1, #5/1/2008#}}},
    {2, "Ariane Berthier", New OrderCollection From {{2, 2, #1/18/2008#},
                                                     {4, 2, #3/8/2008#},
                                                     {6, 2, #3/18/2008#},
                                                     {7, 2, #5/14/2008#},
                                                     {5, 2, #4/4/2008#}}},
     {3, "Brian Perry", New OrderCollection From {{1, 3, #1/15/2008#},
                                                  {3, 3, #3/8/2008#}}}
  }
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