Imports System
Imports System.Collections.Generic
Imports System.Globalization

' ひらがな・カタカナの違いを無視するIEqualityComparer(Of String)
Class IgnoreKanaTypeComparer
  Inherits EqualityComparer(Of String)

  Public Overrides Function Equals(ByVal x As String, ByVal y As String) As Boolean
    ' CompareOptions.IgnoreKanaTypeを指定して2つの文字列が等しいか否かを判定する
    Return String.Compare(x, y, CultureInfo.InvariantCulture, CompareOptions.IgnoreKanaType) = 0
  End Function

  Public Overrides Function GetHashCode(ByVal s As String) As Integer
    ' 実装は省略
    Return 0
  End Function
End Class

Class Sample
  Shared Sub Main()
    ' 独自に実装したIEqualityComparer(Of String)を指定してDictionaryを作成する
    Dim dict As New Dictionary(Of String, String)(New IgnoreKanaTypeComparer())

    dict("ほげ") = "hoge"
    dict("ぴよ") = "piyo"
    dict("ふが") = "fuga"

    dict("ピヨ") = "PIYO" ' ひらがな・カタカナの違いは無視されるため、キー"ぴよ"の値が上書きされる

    For Each p As KeyValuePair(Of String, String) In dict
      Console.WriteLine("{0} => {1}", p.Key, p.Value)
    Next

    Console.WriteLine(dict.ContainsKey("ピヨ"))
  End Sub
End Class

ほげ => hoge
ぴよ => PIYO
ふが => fuga
True