Array, ArrayList, List and Dictionary in ASP.Net

Array, ArrayList, List and Dictionary all represent a collection of data but each are designed to satisfy differnt requirements. This article attempts to explain them with very basic examples; once you go this article I hope the next time you will feel a little more comfortable deciding what to choose and why.

Array

dataType[] myArray = new dataType[size];

• Fixed number of array size
• Strictly type-safe
• Collection of elements of the same data type only
• Elements are accessed by zero-based index
• Array class is in the System Namespace (in other words, it is automatically available)
• Values are assigned by assignment operator as in the following:
  
  int[] intArray = new int[5];
  intArray[0] = 2;
  intArray[1] = 4;

  intArray[2] = 6;

  intArray[3] = 8;

  intArray[4] = 10;
  intArray[5] = 12; //Run time error
  intArray[6] = "Hello";//Compile time error
  foreach (int i in intArray)
  {

      Console.WriteLine("From array " + i);

  }
      Console.WriteLine("From array " + intArray[6]); //Run time error           

ArrayList

ArrayList myArrayList = new ArrayList();

• Size varies as required
• Not type-safe
• Collection of elements of the same or various data types
• Reverse, Contains, Sort and a few other usefull methods are available that we don't get with Arrays
• Useful when you are not sure about the size of the collection or you may need to have more than one data type in the collection
• Elements are accessed by zero-based index
• The data type of an ArrayList is an object type
• Dictionary classes are in the System.Collections Namespace
• Values are assigned by calling the Add method as in the following:
   

  ArrayList myArrayList = new ArrayList();

  myArrayList.Add(2);

  myArrayList.Add("OOPS");

  foreach (object obj in myArrayList)

  {

     Console.WriteLine("From arrayList " + obj.ToString());

  }

  Console.WriteLine("-------");

List

List<T> myList = new List<T>(); // T for data type

• Size varies as required
• Strictly type-safe
• Collection of elements of the same data type only
• Reverse, Contains, Sort and a few other useful methods are available that we don't get with Arrays
• Useful if you want to have the features of both Array and ArrayList
• Elements are accessed by zero-based index
• Dictionary classes are in the System.Collections.Generic Namespace
• Values are assigned by calling the Add method as in the following:
   

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

  intList.Add(2);

  intList.Add(8);

  intList.Add(4);

  intList.Add(6);

  foreach (int i in intList)

  {

     Console.WriteLine("From List " + i);

  }

Dictionary

Dictionary<keyDataType, valueDataType> myCollection = new Dictionary<keyDataType, valueDataType>();

• Size can vary
• Can't have mixed data types, type-safe
• The Collection is defined as a key-value pair
• All of the keys must be of the same data type
• All of the values must be of the same data type
• Useful to deal with custom data type
• Elements are accessed based on key
• Dictionary classes are in the System.Collections.Generic Namespace
• Key-Values are assigned by calling the Add method as in the following:
  
  

   Dictionary<string,Employee> Employees =new Dictionary<string,Employee>();

  Employee emp1 = new Employee("Emp01","Employee1");

  Employee emp2 = new Employee("Emp02","Employee2");

  Employee emp3 = new Employee("Emp03","Employee3");

  Employee emp4 = new Employee("Emp04","Employee4");

  Employee emp5 = new Employee("Emp05","Employee5");

  Employees.Add(emp1.empID, emp1);

  Employees.Add(emp2.empID, emp2);

  Employees.Add(emp3.empID, emp3);

  Employees.Add(emp4.empID, emp4);

  Employees.Add(emp5.empID, emp5);

  Console.WriteLine(Employees["Emp05"].getNameAndID());

  foreach (KeyValuePair<string,Employee> emp in Employees)

  {

     Console.WriteLine(emp.Key +"  >>>>>  " + emp.Value.empName);

     Console.WriteLine(emp.Value.ToString());

  }

   

      public class Employee

      {

          public string empID;

          public readonly string empName;

          public Employee(string empID, string name)

          {

              this.empID = empID;

              this.empName = name;

          }

   

      }