c# - Joining two dictionaries in linq -


suppose have 2 distinct dictionaries dictionary<string,string> da , dictionary<string,int> db

db holds counts of sub-level items 

var db = new dictionary<string,int?>{{"sub1",10},{"sub2",30},{"sub3",70}...};

where da holds mapping of sub-level top level:

var da = new dictionary<string,string>{{"sub1","top1"},{"sub2","top1"},{"sub3","top2"}};

if want aggregate numbers @ top level, 

{{"top1",40},{"top2",70}...}

how go in linq? help!

here how can it:

var result =     da     .groupby(x => x.value)     .todictionary(         g => g.key,         g => g.select(kvp => kvp.key).sum(sub => db[sub]));

basically, group da value (which "top1" , "top2").

then such grouping, create dictionary. key such dictionary items key of grouping (e.g. "top1").

to calculate value each item in new dictionary, first select subs each group "sub1" , "sub2" , use db dictionary counts each 1 of them , calculate sum.

if db dictionary dictionary<string,int?>, need handle null values using getvalueordefault method this:

var result =     da     .groupby(x => x.value)     .todictionary(         g => g.key,         g => g.select(kvp => kvp.key).sum(sub => db[sub].getvalueordefault()));

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