Convert paired lists into one-to-many lookup

Question

I have lists (of equal length) of paired strings, call them "source" and "target". One source may map to multiple targets. Can I convert this to a lookup table (mapping a source to a list of targets) using LINQ? One (long-winded) way of doing this is:

// There may be multiple targets corresponding to each source
// as in the following example lists.
// NB. Lists are guaranteed same length
List<string> sourceNames = new List<string>() { "A", "B", "C", "A", "B", "C" };
List<string> targetNames = new List<string>() { "Z", "Y", "X", "W", "V", "U" };

// Use extension methods to make list of unique source names
List<string> uniqueSourceNames = sourceNames.Distinct().ToList<string>();

// For each unique source, make a list of the corresponding targets
Dictionary<string, List<string>> nameLookup = new Dictionary<string, List<string>>();
foreach (string sourceName in uniqueSourceNames)
{
    List<string> targetsForSource = new List<string>();

    for (int index = 0; index < sourceNames.Count; index++)
    {
        if (sourceNames[index] == sourceName)
            targetsForSource.Add(targetNames[index]);
    }
    nameLookup.Add(sourceName, targetsForSource);
}

// Can now use the nameLookup dictionary to map a source name to a list of target names
// e.g. this returns "Z", "W"
List<string> targets = nameLookup["A"];

Is there a way to do this more efficiently using LINQ?

Answer 1

You can use Zip and ToLookup:

var nameLookup = sourceNames
  .Zip(targetNames, (x, y) => new { Source = x, Target = y })
  .ToLookup(x => x.Source, x => x.Target);

Answer 2

You can use GroupBy and ToDictionary:

var lookup = sourceNames
   .Select((Source, i) => new { Target = targetNames.ElementAt(i), Source})
   .GroupBy(x => x.Source)
   .ToDictionary(g => g.Key, g => g.Select(x => x.Target));

Now every distinct source-string is mapped to an IEnumerable<string> targets.

foreach (var kv in lookup)
    Console.WriteLine("{0} has destinations {1}"
        , kv.Key
        , string.Join(",", lookup[kv.Key]));

Edit: Here's the demo: http://ideone.com/b18H7X