Question

Have the following:

class Hal
{
    public int zip { get; set; }
    public string Phone { get; set; }
    ...
}

List<Hal> data;
Dictionary<int, List<int>> zipList;

zipList
– key is “zip”
– value is a collection of related “zip”

Want to select all “Hal” objects with “zip” from ziplist “key” AND all related “zip” from ziplist “value”.

How on earth do I do this in c# linq?

Answer 1

So you mean every Hal in data where the zip is either in the key or the value of zipList? I’d probably use:

var zips = new HashSet<int>(zipList.Keys
                                   .Concat(zipList.Values.SelectMany(x => x));
var hals = data.Where(x => zips.Contains(x.zip));

To explain:

• zipList.Values.SelectMany(x => x) will just create a flattened view of all the values
• That’s concatenated with the keys within the dictionary
• I’m creating a HashSet<int> from that for simplicity and efficiency of checking in the Where clause in a moment; alternatively a join would do this for us, but it feels simpler not to join when we’re really only interested in one side
• The Where clause just filters the list of Hal objects to those with a required zip