Question

My question has been flagged as a possible duplicate of this question: How to combine two dictionaries without looping?

I believe my question is different because I am asking how to combine two dictionaries in a particular way: I want all items from Dictionary1 plus all items from Dictionary2 that are not in (ie the key does not exist) in Dictionary1.

I have two dictionaries like this:

var d1 = new Dictionary<string,object>();
var d2 = new Dictionary<string,object>();

d1["a"] = 1;
d1["b"] = 2;
d1["c"] = 3;

d2["a"] = 11;
d2["e"] = 12;
d2["c"] = 13;

I would like to combine them into a new Dictionary (technically, it does not have to be a dictionary, it could just be a sequence of KeyValuePairs) such that the output contains all of the KeyValuePairs from d1 and only the KeyValuePairs from d2 whose Key does not appear in d1.

Conceptually:

var d3 = d1.Concat(d2.Except(d1))

But that is giving me all of the elements from d1 and d2.

Seems like it should be obvious, but I must be missing something.

Answer 1

When you use Except by default it uses the default equality comparer, which for the KeyValuePair type compares both the keys and the values. You could this approach instead:

var d3 = d1.Concat(d2.Where(kvp => !d1.ContainsKey(kvp.Key)));