amotif wrote:
Anyone know how to format this to "5=>a,e" etc. in a linq-esque fashion?


Okay, I think I've settled on this:

    var result = from pair in data
                 group pair by pair.Value into g
                 select g.FormatReverseMapping();

    foreach (string s in result)
        Console.WriteLine(s);


with an extension method:

    internal static string FormatReverseMapping(this IGrouping> grouping)
    {
        StringBuilder buffer = new StringBuilder();

        buffer.Append(grouping.Key);
        buffer.Append("=>");
        foreach (var x in grouping)
            buffer.Append(x.Key);

        return buffer.ToString();
    }

Smiley

(psst-- take a look at Systems.Collections.Generic.KeyValuePair.)