Union two collections but only for objects with a matching id

c#, c#-4.0, collections, linq

Solution

Try the following

var result = collection1
  .Where(x => collection2.Any(c => c.Type == x.Type))
  .Union(collection2);

Essentially first filter `collection1` down to the items which have `Type` values in `collection2` and then run the union operation

Problem

First off apologies if this is a blatant duplicate, I searched for all phrases that might yield a result but struggled to find an answer/construct my search terms coherently. I have a Collection 1 and a Collection 2 of Object A. ``` // Object A Structure Date Type Value ``` Collection 1 covers dates 1st to 2nd for 2 types and Collection 2 covers dates 3rd to 4th for 1 type: ``` // Collection 1 [0] = 1st, TypeA, 3 [1] = 2nd, TypeA, 3 [2] = 1st, TypeB, 57 [3] = 2nd, TypeB, 57 // Collection 2 [0] = 3rd, TypeB, 57 [1] = 4th, TypeB, 58 ``` My original requirement was to combine these two collections. Not a problem: ``` var result = collection1.Union(collection2); ``` However, I now would like to combine both collections but excluding objects from Collection 1 where the Type does not exist in Collection 2: ``` // Result Collection [0] = 1st, TypeB, 57 [1] = 2nd, TypeB, 57 [2] = 3rd, TypeB, 57 [3] = 4th, TypeB, 58 ``` I'm sure there's a simple (hopefully linq) one line solution to this but I just can't get my brain into gear to figure out what it is. The best (two lines) I've got so far is: ``` // Get distinct Types from Collection 2 var typesToInclude = collection2.GroupBy(c => c.Type).Select(group => group.First().Type); var result = collection2.Union(collection1.Where(c => typesToInclude.Contains(c.Type))); ``` But that feels really clunky and not optimal :(

Original source