使用Linq在两个列表之间进行规范化的最佳方法是什么？

Question

Given two IEnumerable instances in C# (call them a and b), what's the best way to remove all values in a that are not in b and add all values of b that are not in a?

I know I could just set a = b, normally, but this is ultimately for persisting to the DB via Entity Framework CodeFirst in an MVC application so there's some wonkiness of state to watch out for. In fact, we're talking about updating a record based on stuff posted from the client.

The closest that seems to work involves about for foreach loops, one to iterate the 'a' list and populate a collection of 'items to be removed', another to iterate the 'b' list to identify the 'items to be added', and then one each on the 'items to be removed' and 'items to be added' collections to add and remove items, respectively (since you can't modify the 'a' collection while you're iterating on it.

That feels clunky, though; is there a better way?

UPDATE For clarity, I'll make an example. Let's say I have an entity I fetch from the DB which represents a blog Post (since that example never gets tired...) and said Post has a collection of Tags. From the client, I get a list of Tags that should be now the 'canonical' list of tags, but none of them are entities, it's just an in-memory collection. What I want to do is ensure that Post.Tags matches the tags being posted by the client, without creating duplicate tags in the database.

Answer 1

You can use Intersect , Concat and Except :

a = a.Intersect(b).Concat(b.Except(a));

Intersect returns items that exist in both collections, so a.Intersect(b) will give you all items that are in a and b .

Except returns elements that are in first collection, but not in the other, so b.Except(a) returns elements that are in b but not in a .

Concat concatenates these two collections.

But I don't really get your questions, so I'm not sure it's what you're looking for.

Answer 2

It sounds like your enumerable a is actually a list, so I did it this way:

var a = new List<int>() { 1, 2, 3, };
var b = new List<int>() { 1, 3, 4, 5, };

foreach (var x in a.Except(b).ToArray())
{
    a.Remove(x);
}

foreach (var x in b.Except(a).ToArray())
{
    a.Add(x);
}

At the end a has the same elements as b .

However, you need to be careful if you have duplicates in b .