Extension to the normal Dictionary. This class can store more than one value for every key. It keeps a HashSet for every Key value. Calling Add with the same Key and multiple values will store each value under the same Key in the Dictionary. Obtaining the values for a Key will return the HashSet with the Values of the Key. It can also merge with other instances of MultiValueDictionary, as long as the TKey and TValue types are equal.

[SerializableAttribute]
public class MultiValueDictionary<TKey, TValue> : Dictionary<TKey, HashSet<TValue>>

<SerializableAttribute>
Public Class MultiValueDictionary(Of TKey, TValue)
	Inherits Dictionary(Of TKey, HashSet(Of TValue))