List, initialize

A C# List can be initialized in many ways—we can call Add in a for-loop. But for more clarity, we can use the special initializer syntax.

With initializer syntax, objects inside a List can set up more easily. In modern times, .NET has advanced optimizations, so syntax here is not that important.

First example

The C# language has many ways to populate a List with values. Many of them are compiled into the same code. We use curly brackets and add elements in expressions.

using System;
using System.Collections.Generic;

// Part 1: use collection initializer.
// ... Use it with var keyword.
List<string> list1 = ["carrot", "fox", "explorer"];
var list2 = new List<string>() { "carrot", "fox", "explorer" };

// Part 2: use new array as parameter.
string[] array = { "carrot", "fox", "explorer" };
List<string> list3 = new List<string>(array);

// Part 3: use capacity in constructor and assign.
List<string> list4 = new List<string>(3);
list4.Add(null); // Add empty references (BAD).
list4.Add(null);
list4.Add(null);
list4[0] = "carrot"; // Assign those references.
list4[1] = "fox";
list4[2] = "explorer";

// Part 4: use Add method for each element.
List<string> list5 = new List<string>();
list5.Add("carrot");
list5.Add("fox");
list5.Add("explorer");

// Make sure they all have the same number of elements.
Console.WriteLine($"{list1.Count} {list2.Count} {list3.Count} {list4.Count} {list5.Count}");
3 3 3 3 3

Object List

You can allocate and assign the properties of objects inline with the List initialization. Object initializers and collection initializers share similar syntax.

using System;
using System.Collections.Generic;

class Test // Used in Lists.
{
    public int A { get; set; }
    public string B { get; set; }
}

class Program
{
    static void Main()
    {
        // Version 1: initialize list with collection initializer.
        List<Test> list1 = new List<Test>()
        {
            new Test(){ A = 1, B = "B1" },
            new Test(){ A = 2, B = "B2" }
        };

        // Version 2: initialize list with new objects.
        List<Test> list2 = new List<Test>();
        list2.Add(new Test() { A = 1, B = "B1" });
        list2.Add(new Test() { A = 2, B = "B2" });

        // Write number of elements in the lists.
        Console.WriteLine(list1.Count);
        Console.WriteLine(list2.Count);
    }
}
2
2

Benchmark, List initializer

Here we test whether List initializers are as fast as a calling the Add method. We create three-element lists with two syntax forms.

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 10000000;
// Version 1: use collection initializer.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    List<string> list = new List<string>()
    {
        "bird",
        "frog",
        "dog"
    };
    if (list.Count != 3)
    {
        return;
    }
}
s1.Stop();
// Version 2: use Add method.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    List<string> list = new List<string>();
    list.Add("bird");
    list.Add("frog");
    list.Add("dog");
    if (list.Count != 3)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
41.32 ns    Collection initializer
31.58 ns    Add, Add, Add

Notes, compiler

The C# compiler encounters the curly braces and turns the contents into individual Add method calls on the List. This is true for any List initializer.

Initializing many lists, we explored some specifics of List initializations. We used expression-based syntax to reduce the size of the code file.