Join two dictionaries using a common key

Question

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

var idList = new Dictionary();
idList.Add(1, 1);
idList.A         


        

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}

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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.