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.NET Framework - Collections
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  Using collection initializers

Some collection types can be initialized at the declaration time. For example, the following statement creates and initializes the numbers with some integers:

List<int> numbers = new List<int>(){10, 9, 8, 7, 7, 6, 5, 10, 4, 3, 2, 1};

Internally, the C# compiler actually converts this initialization to a series of calls to the Add method. Consequently, you can use this syntax only for collections that actually support the Add method.

The Stack<T> and Queue<T> classes do not support it.

For complex collections such as the Dictionary<TKey, TValue> class, that take key/value pairs, you can specify each key/value pair as an anonymous type in the initializer list.

Dictionary<int, string> employee = new Dictionary<int, string>()
     {{44, "John"}, {45, "Bob"}, {47, "James"}, {48, "Franklin"}};

The first item in each pair is the key, and the second is the value.
  Creating an initialized List with Custom Types 
public class Model
{
    public string Name { get; set; }
    public bool? Selected { get; set; }
}

Here we have a Class with no constructor with two properties: Name and a nullable boolean property Selected. If we wanted to initialize a List<Model>, there are a few different ways to execute this.

var SelectedEmployees = new List<Model>
 {
      new Model() {Name = "Item1", Selected = true},
      new Model() {Name = "Item2", Selected = false},
      new Model() {Name = "Item3", Selected = false},
      new Model() {Name = "Item4"}
 };

Here, we are creating several new instances of our Model class, and initializing them with data. What if we added a constructor?

public class Model
{

    public Model(string name, bool? selected = false)
    {
        Name = name;
        selected = Selected;
    }
    public string Name { get; set; }
    public bool? Selected { get; set; }
}

This allows us to initialize our List a little differently.

var SelectedEmployees = new List<Model>
{
    new Model("Mark", true),
    new Model("Alexis"),
    new Model("")
};

What about a Class where one of the properties is a class itself?

public class Model
{
    public string Name { get; set; }
    public bool? Selected { get; set; }
}

public class ExtendedModel : Model
{
    public ExtendedModel()
    {
        BaseModel = new Model();
    }

    public Model BaseModel { get; set; }
    public DateTime BirthDate { get; set; }
}

Notice we reverted the constructor on the Model class to simplify the example a little bit.

var SelectedWithBirthDate = new List<ExtendedModel>
{
    new ExtendedModel()
    {
        BaseModel = new Model { Name = "Mark", Selected = true},
        BirthDate = new DateTime(2015, 11, 23)
    },
                    new ExtendedModel()
    {
        BaseModel = new Model { Name = "Random"},
        BirthDate = new DateTime(2015, 11, 23)
    }
};

Note that we can interchange our List<ExtendedModel> with Collection<ExtendedModel>, ExtendedModel[], object[], or even simply [].
  Queue

There is a collection in .Net used to manage values in a Queue (opens new window) that uses the FIFO (first-in first-out) (opens new window) concept. The basics of queues is the method Enqueue(T item) (opens new window) which is used to add elements in the queue and Dequeue() (opens new window) which is used to get the first element and remove it from the queue. The generic version can be used like the following code for a queue of strings.

First, add the namespace:

using System.Collections.Generic;

and use it:

Queue<string> queue = new Queue<string>();
queue.Enqueue("John");
queue.Enqueue("Paul");
queue.Enqueue("George");
queue.Enqueue("Ringo");

string dequeueValue;
dequeueValue = queue.Dequeue(); // return John
dequeueValue = queue.Dequeue(); // return Paul
dequeueValue = queue.Dequeue(); // return George
dequeueValue = queue.Dequeue(); // return Ringo

There is a non generic version of the type, which works with objects.

The namespace is:

using System.Collections;

Adn a code sample fo non generic queue:

Queue queue = new Queue();
queue.Enqueue("Hello World"); // string
queue.Enqueue(5); // int
queue.Enqueue(1d); // double
queue.Enqueue(true); // bool
queue.Enqueue(new Product()); // Product object

object dequeueValue;
dequeueValue = queue.Dequeue(); // return Hello World (string)
dequeueValue = queue.Dequeue(); // return 5 (int)
dequeueValue = queue.Dequeue(); // return 1d (double)
dequeueValue = queue.Dequeue(); // return true (bool)
dequeueValue = queue.Dequeue(); // return Product (Product type)

There is also a method called Peek() (opens new window) which returns the object at the beginning of the queue without removing it the elements.

Queue<int> queue = new Queue<int>();
queue.Enqueue(10);
queue.Enqueue(20);
queue.Enqueue(30);
queue.Enqueue(40);
queue.Enqueue(50);

foreach (int element in queue)
{
    Console.WriteLine(i);
}

The output (without removing):

10
20
30
40
50
  Stack

There is a collection in .Net used to manage values in a Stack (opens new window) that uses the LIFO (last-in first-out) (opens new window) concept. The basics of stacks is the method Push(T item) (opens new window) which is used to add elements in the stack and Pop() (opens new window) which is used to get the last element added and remove it from the stack. The generic version can be used like the following code for a queue of strings.

First, add the namespace:

using System.Collections.Generic;

and use it:

Stack<string> stack = new Stack<string>();
stack.Push("John");
stack.Push("Paul");
stack.Push("George");
stack.Push("Ringo");

string value;
value = stack.Pop(); // return Ringo
value = stack.Pop(); // return George
value = stack.Pop(); // return Paul
value = stack.Pop(); // return John

There is a non generic version of the type, which works with objects.

The namespace is:

using System.Collections;

And a code sample of non generic stack:

Stack stack = new Stack();
stack.Push("Hello World"); // string
stack.Push(5); // int
stack.Push(1d); // double
stack.Push(true); // bool
stack.Push(new Product()); // Product object

object value;
value = stack.Pop(); // return Product (Product type)
value = stack.Pop(); // return true (bool)
value = stack.Pop(); // return 1d (double)
value = stack.Pop(); // return 5 (int)
value = stack.Pop(); // return Hello World (string)

There is also a method called Peek() (opens new window) which returns the last element added but without removing it from the Stack.

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);

var lastValueAdded = stack.Peek(); // 20

It is possible to iterate on the elements on the stack and it will respect the order of the stack (LIFO).

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
stack.Push(30);
stack.Push(40);
stack.Push(50);

foreach (int element in stack)
{
   Console.WriteLine(element);
}

The output (without removing):

50
40
30
20
10

Remarks

There are several kinds of collection:

 

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