Use collections to organize related data

Collection Overview

The set is simply the upgraded version of the array. He can dynamically define and maintain the length of the collection (that is, the maximum number of elements in the collection)!

ArrayList

ArrayList are very similar to arrays. It's also called an array list, its capacity can be dynamically expanded as needed, and its indexes are redistributed and adjusted based on the expansion of the collection capacity. That is, the subscripts of the elements in the ArrayList collection are indeterminate and mutable.

The ArrayList class belongs to the System.Collections namespace, which contains interfaces and classes that define a collection of various objects such as lists, queues, bit arrays, hash tables, and dictionaries.

Syntax:

Using System.Collections; // Import Namespaces // defining ArrayList Objects ArrayList    ArrayList Collection name =new  ArrayList ("Length");

Common methods and properties of ArrayList

Property name	Description
Count	Gets the number of elements actually contained in the ArrayList

return value type	Method name	Description
Int	Add (Object Value)	Add an object to the end of the ArrayList
Void	RemoveAt (int index)	Removes the element at the specified index ArrayList
Void	Remove (Object Value)	Remove a specific object from the ArrayList
Void	Clear ()	Remove all elements from the ArrayList

HashTable

In ArrayList we can access the elements in the collection by index, but it becomes very troublesome to find the position (index) of each element when the index of the elements within the collection changes frequently.

C # provides a data structure called Hashtable, often referred to as a hash table and also called him a "dictionary," which is the name of the dictionary because it is very similar to the dictionary, all through a word to find out more information about the word, A hashtable is a value that organizes data by key (key)

Common properties and methods of Hashtable

Property	Description
Count	Gets the number of key-value pairs contained in the Hashtable
Keys	Gets the collection that contains the keys in the hastable
Values	Get a collection that contains hastable

return value Type	method name	description
void	ad D (Object key,object Value)
	remove (object  Key)
void	clear ()	Clears all elements in Hashtable

Generics and generic collections

Generics and generic collections

The data stored through ArrayList and Hashtable is converted to the object type, which means that it can store different types of elements in a collection, and it is possible to raise the issue of forcing type conversion errors when traversing the collection. Generic collections are type-safe and define the types of elements in the collection, and the generic and generic collections are explained in detail below.

Generic type

Generics are a new feature in c#2.0, where generics introduce a concept: type parameters that reduce the risk of runtime coercion of type conversions or boxing unboxing operations by using type parameters (T), the most common application of which is to create collection classes, to maximize code reuse, to protect type safety, and to improve performance. You can constrain the types of elements in a collection class. A more typical generic set is list<t> and DICTIONARY<K,V>, which are explained in detail below,

Generic collection

In the System.Collections.Generic namespace, you define a de-multiple generic collection class that can replace the above ArrayList

The syntax for defining a List<t> collection is as follows:

List <T> Set name =new list<t> ();

T in <T> can constrain the element types in the collection, and the T table name collection manages the element types.

The difference between list<t> and ArrayList

1. list<t> constraints on saved element types, ArrayList can save any type of element
2. List<t> saving a value type element does not carry out a boxing unboxing operation and ArrayList

Generic collection dictionary<k,v>

Generic collection dictionary can replace the above Hashtable

Defines the syntax for a generic collection dictionary<k,v>

Dictionary<k,v> Collection name =new dictionary<k,v> ();

In "<K,V>" K represents the type of key in the collection, V represents the type of value, and their meaning and list<k,v> are the same

1. dictionary<k,v> constraints on saved element types, Hashtablet can save any type of element
2. Dictionary<k,v> saving a value type element does not carry out a boxing unboxing operation and Hashtablet

Generic class

The use of generics in collections is just one of the many applications of generics that have been used in classes, methods, etc.

The syntax for defining a generic class is as follows:

 Public class     Class name <T>{    //...}

T refers to a type parameter that represents a specific data type, which can be a value type or a reference type

Use collections to organize related data