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C# List Add Method, Append Element to List

Use the Add method on the List. Append value types and reference types to Lists.
Add. Consider an empty list. It has no elements. We must add elements to it. With Add() we place (or append) an element at the end of a List.List
Add, details. Add() handles both value types and reference types. This method has internal logic that allows us to quickly add elements to the end of the collection.
First example. We declare a List with an int type parameter. We use Add() 4 times. This example shows how to create a List of unspecified size, and append 4 numbers to it.Int, uintPrimes

Note: The angle brackets are part of the declaration type, not numeric operators. They are treated differently in the language.

AddRange: For adding many elements at once, you can use the AddRange method on List for less code.

AddRange, InsertRange
C# program that uses Add method using System.Collections.Generic; class Program { static void Main() { // Add first 4 numbers to the List. List<int> primes = new List<int>(); primes.Add(2); primes.Add(3); primes.Add(5); primes.Add(7); } }
Example 2. Next we declare a custom class, and then add instances of it to a new List. The type in between the angle brackets is the name of the new class.

And: Because the Test type is a reference type, the List stores references to the Test objects on the managed heap.

C# program that adds objects to List using System.Collections.Generic; class Program { static void Main() { // Add 3 objects to a List. List<Test> list = new List<Test>(); list.Add(new Test(1, 2)); list.Add(new Test(3, 4)); list.Add(new Test(5, 6)); } class Test { int _a; int _b; public Test(int a, int b) { _a = a; _b = b; } }; }
Performance, Add. This benchmark tests the Add() method and the AddRange method. A 3-element int array is appended to a newly-created list.

Version 1: This code creates a new, empty List, and then appends 3 elements from an array directly with AddRange.

Version 2: This version of the code uses a foreach-loop and calls Add() on each element of the 3-element int array.

Result: It is faster to call Add() over each element of the array. Using AddRange seems to make the program slower.

Benchmark
C# program that benchmarks List Add, AddRange using System; using System.Collections.Generic; using System.Diagnostics; class Program { const int _max = 1000000; static void Main() { int[] array = { 10, 20, 30 }; // Version 1: use AddRange. var s1 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { var list = new List<int>(); list.AddRange(array); if (list.Count != 3) { return; } } s1.Stop(); // Version 2: use Add. var s2 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { var list = new List<int>(); foreach (int value in array) { list.Add(value); } 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")); } } Output 131.78 ns AddRange 25.01 ns Add
Internals. When you call Add each time, the EnsureCapacity method may be invoked. If you are adding an element that will not fit in the array size, the array is resized with Array.Copy.Array.Copy

Info: Resizing an array is slow, but if you are using a reference type in your List, the references themselves are small and fast to copy.

Tip: Faster copying is an advantage to reference types. References are also faster to copy to arguments during method calls.

Capacity. Because the List is dynamically resized, it must manage its capacity in a way that is algorithmically efficient. It first allocates the internal array with a length of 4.

Then: It doubles the array size each time it runs out of room during an Add call.

Capacity
A summary. Add receives a single parameter. We noted the usage of Add() with integers and object references. We mentioned how the capacity of the List is managed internally.
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