Array, ArrayList, List and Dictionary in ASP.Net

Array, ArrayList, List and Dictionary all represent a collection of data but each is designed to satisfy different requirements. This article attempts to explain them with very basic examples; once you go through 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;
    }
}