ToLookup, LINQ and Index

I have the following situation (VB.NET, .NET 4):

Generic list of custom objects letters(Of Letter)

A, B, C, B, D

need a collection containing items and its indexes:

A (0)
B (1, 3)
C (2)
D (4)

Some tries:

Dim letters As New List(Of Letter)
Dim query As ??? = letters.Select(Function(letter, index) _
                            New With {index, letter})

Dim lookup As Lookup(Of Letter, Intege开发者_StackOverflow中文版r)
lookup = letters (query).ToLookup(???

First use the overload of Select that supports an index to create a tuple(or KeyValuePair) of letter and index. Then use the overload of ToLookUp that takes selectors for both key and value.

You might need to either supply an IEqualityComparer<Letter> to ToLookUp, or override both Letter.Equals and Letter.GetHashCode(). Those two need to be overridden together so they are consistent, or you'll get wrong results.

Dim lookup = Enumerable.Range(0, letters.Count).ToLookup(Function(i) letters(i))