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, Va

Answer 1

Untested:

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

Answer 2

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 3

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]);

Answer 4

If you want to select out an object with the "value" of both dictionaries, because that what you probably want, the you can try something like this. Assuming that the keys for both dictionaries are the same i.e. that map each together, like a GUID between two systems

dictA.Keys.Intersect(dictB.Keys).Select(x => new MyMappingClass
{
    dictAValue= dictA[x], dictBValue= dictB[x]
})