C# - List Initialize - Dot Net Perls

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.

List

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.

Part 1 These first 2 declarations specify 3 strings to be added to a new List.

Part 2 Here the code copies an external array to the internal buffer of the List at runtime. This avoids unnecessary resizing.

Part 3 This code is an unclear way of initializing a List variable in the normal case. But it works.

List Add

Part 4 We can avoid a List initializer, and just create an empty list and Add() elements to it.

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.

Version 1 We use a list collection initializer. In the initialization of list 1, 2 Test instances (with properties A and B) are specified.

Property

Version 2 The object initializer syntax is used, and each object is initialized with its constructor.

Detail The 2 lists have the same contents—the same number of objects, and each object has the same properties.

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); } }

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Benchmark, List initializer. Here we test whether List initializers are as fast as a calling the Add method. We create 3-element lists with 2 syntax forms.

Version 1 This code uses the List initializer syntax to create a 3-element List of strings.

Version 2 This version of the code uses 3 Add() calls to populate an empty list. The Count is tested to ensure correctness.

Result In 2021 (with .NET 5 on Linux) we find that calling Add() is faster than using a collection initializer. Add() has been sped up.

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.