How can I initialize a C# List in the same line I declare it. (IEnumerable string Collection Example)

Question

I am writing my testcode and I do not want wo write:

List<string> nameslist = new List<string>();
nameslist.Add("one");
nameslist.Add("two");
nameslist.Add("three");

I would love to write

List<string> nameslist = new List<string>({"one", "two", "three"});

However {"one", "two", "three"} is not an "IEnumerable string Collection". How can I initialise this in one line using the IEnumerable string Collection"?

score 221 · Accepted Answer · answered Dec 14 '10 at 10:33

221

var list = new List<string> { "One", "Two", "Three" };

Essentially the syntax is:

new List<Type> { Instance1, Instance2, Instance3 };

Which is translated by the compiler as

List<string> list = new List<string>();
list.Add("One");
list.Add("Two");
list.Add("Three");

answered Dec 14 '10 at 10:33

Matthew Abbott

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1

I like this no-parentheses method, what c# version did that start with? – SilverbackNet Dec 14 '10 at 10:36
10

It's not *quite* translated into that, at least not in general. The assignment to the variable happens *after* all the `Add` calls have been made - it's as if it uses a temporary variable, with `list = tmp;` at the end. This can be important if you're *reassigning* a variable's value. – Jon Skeet Dec 14 '10 at 10:38
Automatic Properties and Object Initialisers were introduced with .NET 3 I believe, it's the same Framework version, just an updated compiler to support LINQ. – Matthew Abbott Dec 14 '10 at 10:38
@Jon, Cheers, I never knew that part. :D – Matthew Abbott Dec 14 '10 at 10:39
@Jon Skeet : "...This can be important if you're reassigning a variable's value." can you explain a bit more on this remark? – Tony May 24 '15 at 02:48
1

@Tony Consider: `list = new List { list[1], list[2], list[0] };` - you don't want `list` replaced with an empty `List` before the elements are added. – NetMage Apr 01 '20 at 21:31

score 22 · Answer 2 · answered Dec 14 '10 at 10:32

22

Change the code to

List<string> nameslist = new List<string> {"one", "two", "three"};

or

List<string> nameslist = new List<string>(new[] {"one", "two", "three"});

answered Dec 14 '10 at 10:32

Adriaan Stander

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what is the purpose of using the second line " List nameslist = new List(new[] {"one", "two", "three"}); " when can we use it ? Also what is meaning of "new[] {...} " in the second syntax ?why new keyword is used along with the parenthesis [] ??? – Tony May 24 '15 at 02:52

score 7 · Answer 3 · edited Jan 01 '14 at 10:47

7

Just lose the parenthesis:

var nameslist = new List<string> { "one", "two", "three" };

edited Jan 01 '14 at 10:47

Joe the Person

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answered Dec 14 '10 at 10:36

Richard Fawcett

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Oops, look like five people beat me to it. – Richard Fawcett Dec 14 '10 at 10:36

score 7 · Answer 4 · answered Feb 22 '21 at 10:11

Posting this answer for folks wanting to initialize list with POCOs and also coz this is the first thing that pops up in search but all answers only for list of type string.

You can do this two ways one is directly setting the property by setter assignment or much cleaner by creating a constructor that takes in params and sets the properties.

class MObject {        
        public int Code { get; set; }
        public string Org { get; set; }
    }

List<MObject> theList = new List<MObject> { new MObject{ PASCode = 111, Org="Oracle" }, new MObject{ PASCode = 444, Org="MS"} };

OR by parameterized constructor

class MObject {
        public MObject(int code, string org)
        {
            Code = code;
            Org = org;
        }

        public int Code { get; set; }
        public string Org { get; set; }
    }

List<MObject> theList = new List<MObject> {new MObject( 111, "Oracle" ), new MObject(222,"SAP")};

score 4 · Answer 5 · edited Aug 10 '20 at 22:06

4

This is one way.

List<int> list = new List<int>{ 1, 2, 3, 4, 5 };

This is another way.

List<int> list2 = new List<int>();

list2.Add(1);

list2.Add(2);

Same goes with strings.

Eg:

List<string> list3 = new List<string> { "Hello", "World" };

edited Aug 10 '20 at 22:06

Dima Kozhevin

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answered Aug 10 '20 at 21:42

Minhajuddin Khaja

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5

score 3 · Answer 6 · answered Dec 14 '10 at 10:32

3

List<string> nameslist = new List<string> {"one", "two", "three"} ?

answered Dec 14 '10 at 10:32

Romain Meresse

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score 3 · Answer 7 · answered Dec 14 '10 at 10:33

3

Remove the parentheses:

List<string> nameslist = new List<string> {"one", "two", "three"};

answered Dec 14 '10 at 10:33

Tim Robinson

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score 3 · Answer 8 · answered Dec 14 '10 at 10:34

3

It depends which version of C# you're using, from version 3.0 onwards you can use...

List<string> nameslist = new List<string> { "one", "two", "three" };

answered Dec 14 '10 at 10:34

Sam Salisbury

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score 3 · Answer 9 · answered Sep 03 '19 at 15:10

3

I think this will work for int, long and string values.

List<int> list = new List<int>(new int[]{ 2, 3, 7 });


var animals = new List<string>() { "bird", "dog" };

answered Sep 03 '19 at 15:10

Muhammad Awais

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How can I initialize a C# List in the same line I declare it. (IEnumerable string Collection Example)

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