Question

I got two collections of objects.

For example:

List<Foo> firstFoos = new List<Foo>();
List<Foo> secondFoos = new List<Foo>();

firstFoos.Add(new Foo() { Id = 1, ValueA = 10, ValueB = 15 });
firstFoos.Add(new Foo() { Id = 2, ValueA = 20, ValueB = 25 });
firstFoos.Add(new Foo() { Id = 3, ValueA = 30, ValueB = 35 });
firstFoos.Add(new Foo() { Id = 4, ValueA = 40, ValueB = 45 });

secondFoos.Add(new Foo() { Id = 1, ValueA = 100, ValueB = 150 });
secondFoos.Add(new Foo() { Id = 2, ValueA = 200, ValueB = 250 });

Using LINQ, how can I merge the two collection overriding firstFoos by secondFoos which have the same ID?

Expected result is:

|   Id    | ValueA | ValueB |
|---------|--------|--------|
|    1    |   100  |   150  |
|    2    |   200  |   250  |
|    3    |   30   |   35   |
|    4    |   40   |   45   |

Please note that this example case has only two value columns (ValueA and ValueB), but an actual case could have many more.

Answer 1

I’d convert it to an Id -> Foo dictionary, and then just update with a regular foreach:

var fooDict = firstFoos.ToDictionary(foo => foo.Id, foo => foo);
foreach (var foo in secondFoos) 
    fooDict[foo.Id] = foo;
var newFoos = fooDict.Values.OrderBy(foo => foo.Id).ToList();