c# dictionaries intersect

Question

I have a question about Linq / Lambda and the following issue:

I have two dictionaries, primary and secondary... These two dictionaries are defined as Key=string, Value=int. I need to trim down the primary dictionary if the KEYS intersect with secondary dictionary.

i.e.:

primaryDict = ["thing1", 33] ["thing2", 24] ["thing3", 21] ["thing4", 17] ["thing5", 12]

secondaryDict = ["thing1", 22] ["thing3", 20] ["thing4", 19] ["thing7", 17] ["thing9", 10]

resultDict = ["thing1", 33] ["thing3", 21] ["thing4", 17]

My attempt:

resultDict = primaryDict.Keys.Intersect(secondaryDict.Keys).ToDictionary(t => t.Key, t.Value);

This obviously does not work because the primaryDict.Keys.Intersect is returning a list of keys... how would I reestablish a new dictionary, or pair down the primary dictionary? Any help would be appreciated.

Answer 1

You could do in this way:

resultDict =  primaryDict.Keys.Intersect(secondaryDict.Keys)
                              .ToDictionary(t => t, t => primaryDict[t]);

or, alternatively:

resultDict =  primaryDict.Where(x => secondaryDict.ContainsKey(x.Key))
                         .ToDictionary(x => x.Key, x => x.Value);

the latter maybe is slightly more efficient because avoids the creation of a throw-away collection (the one generated by the Intersect method) and does not require a second access-by-key to primaryDict.

EDIT (as per comment) :

resultDict =  
primaryDict.Where(x => secondaryDict.ContainsKey(x.Key))
           .ToDictionary(x => x.Key, x => x.Value + secondaryDict[x.Key]);

Answer 2

You can still use primaryDict within your Linq statement since you are creating a new dictionary, which only gets assigned to your variable once it is created:

resultDict = primaryDict.Keys
                        .Intersect(secondaryDict.Keys)
                        .ToDictionary(t => t, primaryDict[t]);