LINQ Convert Dictionary to Lookup

How about:

var lookup = dictionary.SelectMany(pair => pair.Value,
                                   (pair, Value) => new { pair.Key, Value })
                       .ToLookup(pair => pair.Key, pair => pair.Value);

It does feel like a little bit of a waste doing this when the dictionary already has all the information grouped appropriately, but I can't see a simple way round that. Of course you could implement ILookup<TKey, TValue> yourself with a wrapper around the dictionary...

Same as Jon's method, but avoiding the creation of an anonymous type:

var lookup = dictionary
            .SelectMany(p => p.Value, Tuple.Create)
            .ToLookup(p => p.Item1.Key, p => p.Item2);

Already a few answers here, but putting this here for reference. This flips a dictionary with a list of values, to having those values as the keys of look up list.

var myLookup = myDict.SelectMany(p => p.Value, 
        (pair, id) => Tuple.Create(id, pair.Key))
    .ToLookup(p => p.Item1, p => p.Item2);

Annotated

var myLookup = myDict.SelectMany(
        // specify that the select many is to be based off the Value which is a list of items
        p => p.Value, 
        // Using the individual items from that list, create a tuple of that item and the dictionary key it was under
        (pair, id) => Tuple.Create(id, pair.Key))
        // use the item as the lookup key, and put the original dictionary key (that
        // had that value underneath them) in the list of lookup values.
    .ToLookup(p => p.Item1, p => p.Item2);