Visual Basic .NET Language Tutorial => Collection Initializer

Example

Arrays

 Dim names = {"Foo", "Bar"} ' Inferred as String()
 Dim numbers = {1, 5, 42} ' Inferred as Integer()

Containers (List(Of T), Dictionary(Of TKey, TValue), etc.)

 Dim names As New List(Of String) From {
     "Foo",
     "Bar"
     '...
 }

 Dim indexedDays As New Dictionary(Of Integer, String) From {
     {0, "Sun"},
     {1, "Mon"}
     '...
 }

Is equivalent to

 Dim indexedDays As New Dictionary(Of Integer, String)
 indexedDays.Add(0, "Sun")
 indexedDays.Add(1, "Mon")
 '...

Items can be the result of a constructor, a method call, a property access. It can also be mixed with Object initializer.

 Dim someList As New List(Of SomeClass) From {
     New SomeClass(argument),
     New SomeClass With { .Member = value },
     otherClass.PropertyReturningSomeClass,
     FunctionReturningSomeClass(arguments)
     '...
 }

It is not possible to use Object initializer syntax AND collection initializer syntax for the same object at the same time. For example, these won't work

 Dim numbers As New List(Of Integer) With {.Capacity = 10} _
                                     From { 1, 5, 42 }

 Dim numbers As New List(Of Integer) From {
     .Capacity = 10,
     1, 5, 42
 }

 Dim numbers As New List(Of Integer) With {
     .Capacity = 10,
     1, 5, 42
 }

Custom Type

We can also allow collection initializer syntax by providing for a custom type.
It must implement IEnumerable and have an accessible and compatible by overload rules Add method (instance, Shared or even extension method)

Contrived example :

 Class Person
     Implements IEnumerable(Of Person) ' Inherits from IEnumerable

     Private ReadOnly relationships As List(Of Person)

     Public Sub New(name As String)
         relationships = New List(Of Person)
     End Sub

     Public Sub Add(relationName As String)
         relationships.Add(New Person(relationName))
     End Sub

     Public Iterator Function GetEnumerator() As IEnumerator(Of Person) _
         Implements IEnumerable(Of Person).GetEnumerator

         For Each relation In relationships
             Yield relation
         Next
     End Function

     Private Function IEnumerable_GetEnumerator() As IEnumerator _ 
         Implements IEnumerable.GetEnumerator

         Return GetEnumerator()
     End Function
 End Class

 ' Usage
 Dim somePerson As New Person("name") From {
     "FriendName",
     "CoWorkerName"
     '...
 }

If we wanted to add Person object to a List(Of Person) by just putting the name in the collection initializer (but we can't modify the List(Of Person) class) we can use an Extension method

 ' Inside a Module
 <Runtime.CompilerServices.Extension>
 Sub Add(target As List(Of Person), name As String)
     target.Add(New Person(name))
 End Sub

 ' Usage
 Dim people As New List(Of Person) From {
     "Name1", ' no need to create Person object here
     "Name2"
 }

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