MultiMap. Sometimes we want to map one String to a single String, but more often, we may want to map one String to many Strings. This is a MultiMap, and it can be implemented in VB.NET code.

With a Dictionary field, and some subroutines and properties, we can create a simple MultiMap class. Then we can reuse this class for mapping one key to multiple values.

Example. We introduce the MultiMap class, which contains a Dictionary field. We then have the Add Function, a Keys property, and an indexer (a This property).

Class MultiMap
    Dim _dictionary as Dictionary(Of String, List(Of String)) = New Dictionary(Of String, List(Of String))

    Public Sub Add(key As String, value As String)
        ' Add a String key.
        ' For the list of values, add a value onto the end of the List if one exists.
        Dim list as List(Of String) = Nothing
        If _dictionary.TryGetValue(key, list)
            list.Add(value)
        Else
            list = New List(Of String)
            list.Add(value)
            _dictionary(key) = list
        End If
    End Sub

    Public ReadOnly Property Keys As IEnumerable(Of String)
        Get
            ' Get all keys.
            Return _dictionary.Keys
        End Get
    End Property

    Public ReadOnly Default Property This(key As String) As List(Of String)
        Get
            ' Get list at a key.
            Dim list As List(Of String) = Nothing
            ' Create empty list if none exists.
            If Not _dictionary.TryGetValue(key, list)
                list = New List(Of String)
                _dictionary(key) = list
            End If
            Return list
        End Get
    End Property
End Class

Module Module1
    Sub Main()
        ' Part 1: Create MultiMap by calling Add Function.
        Dim multiMap = New MultiMap()
        multiMap.Add("animal", "cat")
        multiMap.Add("animal", "dog")
        multiMap.Add("color", "blue")
        multiMap.Add("color", "orange")
        multiMap.Add("mineral", "calcium")

        ' Part 2: Loop over keys in MultiMap with property.
        For Each key in multiMap.Keys
            ' Part 3: Access all values for a specific key, and print them.
            For Each value in multiMap(key)
                Console.WriteLine($"MULTIMAP: {key} = {value}")
            Next
        Next
    End Sub
End Module
MULTIMAP: animal = cat
MULTIMAP: animal = dog
MULTIMAP: color = blue
MULTIMAP: color = orange
MULTIMAP: mineral = calcium

Property syntax. In VB.NET, we must use the ReadOnly keyword to describe a Property that only has a Get function. And for an indexer (This) we must use the Default keyword.

Summary. Though the MultiMap implementation here is simple, it could be useful in VB.NET programs that need to map unique Strings to multiple String values. Generic types could be added.