When using a LINQ Where clause on a Dictionary, how can I return a dictionary of the same type?

Question

I'm currently using the following code to achieve this, but is seems there should be something better... suggestions? It just seems to me there should be a way to skip the foreach...

Dictionary<string,string> getValidIds(Dictionary<string,string> SalesPersons,List<string> ids)
{
    Dictionary<string,string> o = new Dictionary<string,string>();
    var ie = SalesPersons.Where<KeyValuePair<string, string>>(t => ids.Contains(t.Key));
    foreach (var i in ie)
    {
        o.Add(i.Key, i.Value);
    }
    return o;
}

Answer 1

Pretty sure you could just call ToDictionary on the result of the Where call:

Dictionary<string, string> GetValidIds(Dictionary<string, string> salesPersons,
    IList<string> ids)
{
    return salesPersons
        .Where(p => ids.Contains(p.Key))
        .ToDictionary(p => p.Key, p => p.Value);
}

Answer 2

Interestingly, if you enumerate over the dictionary (length N) first and check the list (length M) for inclusion, then you get O(NM) performance.

You could build a HashSet<> of the ids, but that seems redundant since we already have a (pre-hashed) dictionary available.

I would instead iterate over the ids first; since dictionary lookup (by key) is O(1), this gives O(M) performance - this might, however, mean that you don't use LINQ (since TryGetValue won't love LINQ (and introducing a tuple is too much like hard work)...

    Dictionary<string, string> getValidIds(
            IDictionary<string, string> salesPersons,
            IEnumerable<string> ids) {
        var result = new Dictionary<string, string>();
        string value;
        foreach (string key in ids) {
            if (salesPersons.TryGetValue(key, out value)) {
                result.Add(key, value);
            }
        }
        return result;
    }

It doesn't overly concern me that this is more lines than the LINQ version; it removes an O(N) of complexity...

---

Edit; the following might work (I haven't tested it), but I think it is an abuse of LINQ, and certainly won't scale to PLINQ etc... use with extreme caution!! I also believe the foreach approach simply has fewer overheads, so will be quicker... anyway:

    Dictionary<string, string> getValidIds(
        IDictionary<string, string> salesPersons,
        IEnumerable<string> ids)
    {
        string value = null;
        return  (from key in ids
                where salesPersons.TryGetValue(key, out value) // HACK: v. dodgy
                select new { key, value })
                .ToDictionary(x=>x.key, x=>x.value);
    }

Answer 3

Seems to be a lot of fussing about looking things up in the List. If the list only contains a few elements, then no big deal. If the List contains thousands of elements, you're going to want O(1) lookups into it. HashSet can provide this.

Dictionary<string, string> getValidIds(
  Dictionary<string, string> SalesPersons,
  List<string> ids
)
{
  HashSet<string> idsFast = new HashSet<string>(ids);
  Dictionary<string, string> result = SalesPersons
    .Where(kvp => idsFast.Contains(kvp.Key))
    .ToDictionary(kvp => kvp.Key, kvp => kvp.Value)
  return result;
}