How do you concatenate Lists in C#?

Question

If I have:

List<string> myList1;
List<string> myList2;

myList1 = getMeAList();
// Checked myList1, it contains 4 strings

myList2 = getMeAnotherList();
// Checked myList2, it contains 6 strings

myList1.Concat(myList2);
// Checked mylist1, it contains 4 strings... why?

I ran code similar to this in Visual Studio 2008 and set break points after each execution. After myList1 = getMeAList();, myList1 contains four strings, and I pressed the plus button to make sure they weren't all nulls.

After myList2 = getMeAnotherList();, myList2 contains six strings, and I checked to make sure they weren't null... After myList1.Concat(myList2); myList1 contained only four strings. Why is that?

score 384 · Accepted Answer · edited May 23 '17 at 12:10

384

Concat returns a new sequence without modifying the original list. Try myList1.AddRange(myList2).

edited May 23 '17 at 12:10

Community

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1

answered Jun 25 '09 at 04:45

John Kugelman

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score 127 · Answer 2 · answered Jun 25 '09 at 04:44

127

Try this:

myList1 = myList1.Concat(myList2).ToList();

Concat returns an IEnumerable<T> that is the two lists put together, it doesn't modify either existing list. Also, since it returns an IEnumerable, if you want to assign it to a variable that is List<T>, you'll have to call ToList() on the IEnumerable<T> that is returned.

answered Jun 25 '09 at 04:44

Jonathan Rupp

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Now that I re-read the question, .AddRange() does sound like what the OP really wants. – Jonathan Rupp Jun 25 '09 at 04:47
@Kartiikeya if it's saying the arguments are invalid, you don't have a using statement for System.Linq, or one of them is not an `IEnumerable` – Jonathan Rupp Jun 11 '15 at 17:28

score 16 · Answer 3 · edited Nov 22 '15 at 03:08

16

targetList = list1.Concat(list2).ToList();

It's working fine I think so. As previously said, Concat returns a new sequence and while converting the result to List, it does the job perfectly.

edited Nov 22 '15 at 03:08

Peter Mortensen

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answered Jun 23 '14 at 09:17

Balasubramani M

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score 8 · Answer 4 · answered Nov 22 '14 at 01:13

It also worth noting that Concat works in constant time and in constant memory. For example, the following code

        long boundary = 60000000;
        for (long i = 0; i < boundary; i++)
        {
            list1.Add(i);
            list2.Add(i);
        }
        var listConcat = list1.Concat(list2);
        var list = listConcat.ToList();
        list1.AddRange(list2);

gives the following timing/memory metrics:

After lists filled mem used: 1048730 KB
concat two enumerables: 00:00:00.0023309 mem used: 1048730 KB
convert concat to list: 00:00:03.7430633 mem used: 2097307 KB
list1.AddRange(list2) : 00:00:00.8439870 mem used: 2621595 KB

score 3 · Answer 5 · answered Sep 28 '15 at 18:16

I know this is old but I came upon this post quickly thinking Concat would be my answer. Union worked great for me. Note, it returns only unique values but knowing that I was getting unique values anyway this solution worked for me.

namespace TestProject
{
    public partial class Form1 :Form
    {
        public Form1()
        {
            InitializeComponent();

            List<string> FirstList = new List<string>();
            FirstList.Add("1234");
            FirstList.Add("4567");

            // In my code, I know I would not have this here but I put it in as a demonstration that it will not be in the secondList twice
            FirstList.Add("Three");  

            List<string> secondList = GetList(FirstList);            
            foreach (string item in secondList)
                Console.WriteLine(item);
        }

        private List<String> GetList(List<string> SortBy)
        {
            List<string> list = new List<string>();
            list.Add("One");
            list.Add("Two");
            list.Add("Three");

            list = list.Union(SortBy).ToList();

            return list;
        }
    }
}

The output is:

One
Two
Three
1234
4567

score 3 · Answer 6 · edited Apr 17 '20 at 22:14

Take a look at my implementation. It's safe from null lists.

 IList<string> all= new List<string>();

 if (letterForm.SecretaryPhone!=null)// first list may be null
     all=all.Concat(letterForm.SecretaryPhone).ToList();

 if (letterForm.EmployeePhone != null)// second list may be null
     all= all.Concat(letterForm.EmployeePhone).ToList(); 

 if (letterForm.DepartmentManagerName != null) // this is not list (its just string variable) so wrap it inside list then concat it 
     all = all.Concat(new []{letterForm.DepartmentManagerPhone}).ToList();

How do you concatenate Lists in C#?

6 Answers6

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