Linq join two dictionaries using a common key

Question

Hello,

I am trying to join two Dictionary collections together based on a common lookup value.

        var idList = new Dictionary<int, int>();
        idList.Add(1, 1);
        idList.Add(3, 3);
        idList.Add(5, 5);

        var lookupList = new Dictionary<int, int>();
        lookupList.Add(1, 1000);
        lookupList.Add(2, 1001);
        lookupList.Add(3, 1002);
        lookupList.Add(4, 1003);
        lookupList.Add(5, 1004);
        lookupList.Add(6, 1005);
        lookupList.Add(7, 1006);

// Something like this:
            var q = from id in idList.Keys
                    join entry in lookupList on entry.Key equals id
                    select entry.Value;

The Linq statement above is only an example and does not compile. For each entry in the idList, pull the value from the lookupList based on matching Keys.

The result should be a list of Values from lookupList (1000, 1002, 1004).

What’s the easiest way to do this using Linq?

Thank you,

Rick

Answer 1

from id in idList.Keys
where lookupList.ContainsKey(id)
let value1 = idList[id]
let value2 = lookupList[id]
select new {id, value1, value2}

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Or, more classically

from kvp1 in idList
join kvp2 in lookupList on kvp1.Key equals kvp2.Key
select new {key = kvp1.Key, value1 = kvp1.Value, value2.kvp2.Value}

---

The mistake in your query is one of scoping:

from a in theAs
join b in theBs on (leftside) equals (rightside)

a is in scope in the leftside area. b is in scope in the rightside area.

Answer 2

I apologize if I misinterpretted your question, but do you just want to retrieve the Values from list B only where list A has a KeyValuePair with the same Key?

from lookup in lookupList
where idList.Keys.Contains(lookup.Key)
select lookup.Value;

Answer 3

        var q = from id in idList
                join entry in lookupList
                  on id.Key equals entry.Key
                select entry.Value;

Your desired linq statement will look like that, ID and Entry needed to be switched around on the condition.

Answer 4

What do you think of this ?

var values = idList.Keys.Select(i => lookupList[i]);