C# Function Chaining

Question

Why do i receive error in the following declaration ?

List<int> intrs = new List<int>().AddRange(new int[]{1,2,3,45});

Error :Can not convert type void to List ?

Answer 1

Because AddRange function does not return a value. You might need to perform this in two steps:

List<int> intrs = new List<int>();
intrs.AddRange(new int[]{1,2,3,45});

Answer 2

AddRange does not return the list it has added items to (unlike StringBuilder). You need to do something like this:

List<int> intrs = new List<int>();
intrs.AddRange(new int[]{1,2,3,45});

Answer 3

AddRange() is declared as:

public void AddRange(object[]);

It does not return the list.

Answer 4

Because AddRange modifies the specified list instead of returning a new list with the added items. To indicate this, it returns void.

Try this:

List<int> intrs = new List<int>();
intrs.AddRange(new int[]{1,2,3,45});

If you want to create a new list without modifying the original list, you can use LINQ:

List<int> intrs = new List<int>();
List<int> newIntrs = intrs.Union(new int[]{1,2,3,45}).ToList();
// intrs is unchanged

Answer 5

You could also use a collection initializer (assuming C# 3.0+).

List<int> intrs = new List<int> { 1, 2, 3, 45 };

Edit by 280Z28: This works for anything with an Add method. The constructor parenthesis are optional - if you want to pass thing to a constructor such as the capacity, you can do so with List<int>(capacity) instead of just List<int> written above.

Here's an MSDN reference for details on the Object and Collection Initializers.

Dictionary<string, string> map = new Dictionary<string, string>()
    {
        { "a", "first" },
        { "b", "second" }
    };

Answer 6

By the way in C# 3.x (not sure about 2.0) you can do either of

List<int> intrs = new List<int>{1,2,3,45};
List<int> intrs = new []{1,2,3,45}.ToList(); // with Linq extensions

Besides other answers, you can add your own extension method that will add range and return list (not that it's a good practice).

Answer 7

BTW, if you had already declared intrs, you could have done it with parentheses:

(intrs = new List<int>()).AddRange(new int[] { 1, 2, 3, 45 });

However, I like the initialization syntax better.

Answer 8

Although others have already mentioned that AddRange does not return a value, based on the samples given for alternatives it should also be remembered that the constructor of List will take an IEnumerable of T as well in addition to the code previously mentioned that is .NET 3.5+

For example:

List<int> intrs = new List<int>(new int[]{2,3,5,7});