Collections in .NET allow storing and accessing multiple elements as a single unit. There are normal and generic collections, with the latter providing type safety without duplicating code. Common collection classes include stacks, queues, lists, and hash tables, with generic versions like Stack<T>, Queue<T>, and Dictionary<T> that allow specifying the element type. Collections implement interfaces like IEnumerable to support iteration over elements.

1. A collection — sometimes called a container — is simply an object
that groups multiple elements into a single unit.
Collection classes are specialized classes for data storage and
retrieval. These classes provide support for stacks, queues, lists, and
hash tables.
Collection classes serve various purposes, such as allocating
memory dynamically to elements and accessing a list of items on the
basis of an index etc. These classes create collections of objects of
the Object class, which is the base class for all data types in C#.

2. Dot Net Collections are divided into 2types:
1. NormalCollections
2. Generic Collections
 Normal Collections are representing with a namespace called
System.Collections;
 Generic Collections are representing with a namespace called
System.Collection.Generic;

3. Generics
Generics are the most powerful feature of C# Generics
allow you to define type-safe data structures, without
committing to actual data types.
This results in a significant performance boost and higher
quality code, because you get to reuse data processing
algorithms without duplicating type-specific code.

4. Collections
• Normal Collections
1. Stack
2. Queue
3. ArrayList
4. HashTable
5. Sorted
6. Bit Array
• Generic Collections
1. Stack<T>
2. Queue<T>
3. List<T>
4. Dictionary<T>

5. Stack:
Stack<Data type> name = new Stack<Data type>();
Queue:
Queue <Data type> name = new Queue <Data type>();
ArrayList or List:
ArrayList <Data type> name = new ArrayList <Data type>();
HashTable or Dictionary:
HashTable <Data type, Data type> name= new
HashTable<Data type, Data type>();
BitArray:
BitSet<Data type> bs = new BitSet();
Data type Get[data type, Index];
These are the some of important collection classes in the
collections Namespace

6. Generics - Prototype
class Stack<T>
{
T[ ] data;
void Push(T top) { …}
T Pop() { … }
}
Stack<string> ss = new Stack<string>;
ss.Push(“Hello”);
Stack<int> si = new Stack<int>;
ss.Push(4);

7. Iterating over a Collection
foreach(object o in collection) {
// do something with o
}
foreach(int i in array) {
// do something with i
}
 Collection objects have a GetEnumerator() method
which returns an Enumerator object.
 An Enumerator object has a MoveNext method and a
Current property.

8. IEnumerable
Collections in .NET use the Iterator Design Pattern, hence
they do not implement IEnumerator.
Instead they implement IEnumerable.
Creates a new instance of an IEnumerator and returns it.
public interface IEnumerable {
IEnumerator GetEnumerator();
}

9. Enumerable Collections
What collections are enumerable?
That is, support the IEnumerable or IEnumerable<T> interfaces?
System.Array – all arrays support IEnumerable<T>
All .NET collections except dictionaries.
System.String or string.
Any user-defined class supporting the interface.

11. Collection-Drawbacks
No type checking enforcement at compile time
Doesn’t prevent adding unwanted types
Can lead to difficult to troubleshoot issues
Runtime errors!
All items are stored as objects
Must be cast going in and coming out
Performance overhead of boxing and unboxing
specific types