How to unite two List and assign result to the third List?

Question

I have 3 List<> objects.

List<T> listA = new List<t>();
List<T> listB = new List<t>();
List<T> listC = new List<t>();

I need to unite listA and ListB and assign it to ListC:

List<T> listC = new ListA +ListB;

My question is how to implement it?

http://stackoverflow.com/a/1825577/1689451 – Rudolf Mühlbauer Oct 13 '12 at 14:02 — Rudolf Mühlbauer, Oct 13 '12 at 14:02

score 7 · Accepted Answer · answered Oct 13 '12 at 14:06

7

    List<int> A = new List<int>();
    A.Add(1);
    A.Add(2);
    List<int> B = new List<int>();
    B.Add(3);
    B.Add(4);

    List<int> C = new List<int>();

    C = A.Union<int>(B).ToList<int>();

answered Oct 13 '12 at 14:06

Mohsen Afshin

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1

It wasn't specified, but `Union` would remove duplicate entries if they exist. If @Michael wants to keep duplicates, use `Concat` instead. – Chris Sinclair Oct 13 '12 at 14:27
You should have mentioned that OP needs to implement an `EqualityComparer`(like [here](http://stackoverflow.com/a/12873969/284240)) if it's a custom type. Otherwise he would compare reference types by reference instead by equality. – Tim Schmelter Oct 14 '12 at 10:17

score 5 · Answer 2 · answered Oct 13 '12 at 14:02

5

Use AddRange:

List<T> listA = new List<t>();
List<T> listB = new List<t>();
List<T> listC = new List<t>();

listC.AddRange(listA);
listC.AddRange(listB);

answered Oct 13 '12 at 14:02

Leri

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score 4 · Answer 3 · answered Oct 13 '12 at 14:03

4

Try AddRange() this:-

    listC.AddRange(listA);
    listC.AddRange(listB);

answered Oct 13 '12 at 14:03

Rahul Tripathi

168,305
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score 3 · Answer 4 · answered Oct 13 '12 at 14:02

3

Kinda depends on your requirements, but might be as easy as

listC.AddRange(listA);
listC.AddRange(listB);

answered Oct 13 '12 at 14:02

iamkrillin

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score 2 · Answer 5 · answered Oct 13 '12 at 14:02

2

listC.AddRange(listA);
listC.AddRange(listB);

answered Oct 13 '12 at 14:02

Lior

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Tim Schmelter · Answer 6 · 2012-10-13T14:49:21.683

Either use AddRange as others have already mentioned or , if you want to union both as mentioned, you need to provide a IEqualityComparer<T> and use Enumerable.Union:

List<Foo> third = first.Union(second, new FooComparer()).ToList();

Here's an examplary implementation:

public class FooComparer : IEqualityComparer<Foo>
{
    public bool Equals(Foo x, Foo y)
    {
        return x.Id == y.Id; // assuming that there's a value type like ID as key
    }

    public int GetHashCode(Foo obj)
    {
        return obj.Id.GetHashCode();
    }
}

Union returns only unique values.

How to unite two List and assign result to the third List?

6 Answers6