C# : Generic Collection - List

 An ArrayList resizes automatically as it grows. The List<T> collection is the same as an ArrayList except that List<T> is a generic collection whereas ArrayList is a non-generic collection.

List<T> can be initialized in the following two ways.

Example: List<T> Initialization

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

//Or

IList<int> intList = new List<int>();

In the above example, the first statement uses List type variable, whereas the second statement uses IList type variable to initialize List. List<T> is a concreate implementation of IList<T> interface. In the object-oriented programming, it is advisable to program to interface rather than concreate class. So use IList<T> type variable to create an object of List<T>.

List<T> includes more helper methods than IList<T> interface. The table shown below lists important properties and methods of List<T>, which are initialized using a List<T>:

Property	Usage
Items	Gets or sets the element at the specified index
Count	Returns the total number of elements exists in the List<T>

Method	Usage
Add	Adds an element at the end of a List<T>.
AddRange	Adds elements of the specified collection at the end of a List<T>.
BinarySearch	Search the element and returns an index of the element.
Clear	Removes all the elements from a List<T>.
Contains	Checks whether the speciied element exists or not in a List<T>.
Find	Finds the first element based on the specified predicate function.
Foreach	Iterates through a List<T>.
Insert	Inserts an element at the specified index in a List<T>.
InsertRange	Inserts elements of another collection at the specified index.
Remove	Removes the first occurence of the specified element.
RemoveAt	Removes the element at the specified index.
RemoveRange	Removes all the elements that match with the supplied predicate function.
Sort	Sorts all the elements.
TrimExcess	Sets the capacity to the actual number of elements.
TrueForAll	Determines whether every element in the List<T> matches the conditions defined by the specified predicate.

Add Elements into List:

Use the Add() method to add an element into a List collection. The following example adds int value into a List<T> of int type.

Add() signature: void Add(T item)

Example: Adding elements into List

IList<int> intList = new List<int>();
intList.Add(10);
intList.Add(20);
intList.Add(30);
intList.Add(40);

IList<string> strList = new List<string>();
strList.Add("one");
strList.Add("two");
strList.Add("three");
strList.Add("four");
strList.Add("four");
strList.Add(null);
strList.Add(null);

IList<Student> studentList = new List<Student>();
studentList.Add(new Student());
studentList.Add(new Student());
studentList.Add(new Student());

Try it

You can also add elements at the time of initialization using object initializer syntax as below:

Example: Add elements using object initializer syntax

IList<int> intList = new List<int>(){ 10, 20, 30, 40 };

//Or

IList<Student> studentList = new List<Student>() { 
                new Student(){ StudentID=1, StudentName="Bill"},
                new Student(){ StudentID=2, StudentName="Steve"},
                new Student(){ StudentID=3, StudentName="Ram"},
                new Student(){ StudentID=1, StudentName="Moin"}
            };

Try it

AddRange:

The AddRange() method adds all the elements from another collection.

AddRange() signature: void AddRange(IEnumerable<T> collection)

Example: AddRange

IList<int> intList1 = new List<int>();
intList1.Add(10);
intList1.Add(20);
intList1.Add(30);
intList1.Add(40);

List<int> intList2 = new List<int>();

intList2.AddRange(intList1);

Try it

  

Note :The AddRange() method will only be applicable if you initialized with a List<T> variable. IList<T> doesn't include the AddRange() method.

Access List collection:

Use a foreach or for loop to iterate a List<T> collection.

Example: Accessing List

List<int> intList = new List<int>() { 10, 20, 30 };

intList.ForEach(el => Console.WriteLine(el));

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If you have initialized the List<T> with an IList<T> interface then use seperate foreach statement with implicitly typed variable:

Example: Accessing List

IList<int> intList = new List<int>() { 10, 20, 30, 40 };

foreach (var el in intList)
        Console.WriteLine(el);

Try it

Output:

10 
20 
30 
40

Access individual items by using an indexer (i.e., passing an index in square brackets):

Example: Accessing List

IList<int> intList = new List<int>() { 10, 20, 30, 40 };

int elem = intList[1]; // returns 20

Use the Count property to get the total number of elements in the List.

Example: Access List elements

IList<int> intList = new List<int>() { 10, 20, 30, 40 };

Console.Write("Total elements: {0}", intList.Count);

Output:

Total elements: 4

Use for loop to access list as shown below:

Example: Accessing List using for loop example:

IList<int> intList = new List<int>() { 10, 20, 30, 40 };

for (int i = 0; i < intList.Count; i++)
        Console.WriteLine(intList[i]);

Try it

Output:

10 
20 
30 
40

List<T> implements IList<T>, so List<T> implicitly type cast to IList<T>.

Example: Access List

static void Print(IList<string> list)
{
    Console.WriteLine("Count: {0}", list.Count);
    foreach (string value in list)
    {
        Console.WriteLine(value);
    }
}

static void Main(string[] args)
{

    string[] strArray = new string[2];
    strArray[0] = "Hello";
    strArray[1] = "World";
    Print(strArray);

    List<string> strList = new List<string>();
    strList.Add("Hello");
    strList.Add("World");
    Print(strList);
}

Try it

Output:

Hello 
World 
Hello 
World

Insert into List:

The Insert() method inserts an element into a List<T> collection at the specified index.

Insert() signature:void Insert(int index, T item);

Example: Insert elements into List

IList<int> intList = new List<int>(){ 10, 20, 30, 40 };

intList.Insert(1, 11);// inserts 11 at 1st index: after 10.

foreach (var el in intList)
        Console.Write(el);

Try it

Output:

10 
11 
20 
30 
40

Remove Elements from List:

The Remove() and RemoveAt() methods remove items from a List<T> collection.

Remove() signature: bool Remove(T item)

RemoveAt() signature: void RemoveAt(int index)

Example: Remove elements from List

IList<int> intList = new List<int>(){ 10, 20, 30, 40 };

intList.Remove(10); // removes the 10 from a list

intList.RemoveAt(2); //removes the 3rd element (index starts from 0)

foreach (var el in intList)
    Console.Write(el);

Try it

Output:

20 
30

TrueForAll:

TrueForAll() is a method of the List<T> class. It returns true if the specified condition turns out to be true, otherwise false. Here, the condition can be specified as a predicate type deligate or lambda expression.

TrueForAll() signature: bool TrueForAll(Predicate<T> match)

Example: TrueForAll()

List<int> intList = new List<int>(){ 10, 20, 30, 40 };

bool res = intList.TrueForAll(el => el%2 == 0);// returns true

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The following example uses isPositiveInt() as a Predicate<int> type delegate as a parameter to TrueForAll.

Example: TrueForAll()

static bool isPositiveInt(int i)
{
    return i > 0;
}

static void Main(string[] args)
{
    List<int> intList = new List<int>(){10, 20, 30, 40};

    bool res = intList.TrueForAll(isPositiveInt);

}

Try it

Points to Remember :

1. List<T> stores elements of the specified type and it grows automatically.
2. List<T> can store multiple null and duplicate elements.
3. List<T> can be assigned to IList<T> or List<T> type of variable. It provides more helper method When assigned to List<T> variable
4. List<T> can be access using indexer, for loop or foreach statement.
5. LINQ can be use to query List<T> collection.
6. List<T> is ideal for storing and retrieving large number of elements.