Collections and Lists in .NET Framework

Introduction

In this article I will explain on Collections especially classes that is in System.Collections Namespace in .NET Framework.  Some of the Collection objects that I will explain include Arrays, ArrayList, SortedList, Hashtables, Queues , Stacks, and Lists.

As you all probably has heard and use all of these in OO languages or non OO languages. Collections has been used by all programmers regardless of programming languages in the world and whether you realize it or not, you are using it every day when you are doing programming. In the first section, I will explain about Arrays

Arrays

The simplest collection of objects in programming languages is an Array. You can insert an objects into an array or even a collection of basic primitive types such as integer, double, strings.
In this example,lets start inserting Collection of Person objects into an Array

Code Sample (A Simple Person Class)

public class Person {
    string FirstName;
    string LastName;
    
    public Person (string first,string Last) {
        FirstName = first;
        LastName = last;
    }
    public string FirstName {
       get{
          return FirstName + " " + LastName;
       }
    }
}

public partial class _Default :System.Web.UI.Page {
    protected void Page_Load(object sender,EventArgs e) {
        Person michael = new Person("Michael","Jackson");
        Person bill = new Person("Bill","Gate");
        Person michelle = new Person("Michelle","Sullivan");
 
        Person[] people = new Person[3];
        people[0] = michael;
        people[1] = bill;
        people[2] = michelle;
        
       //Using For Each to print out the Objects from Array
        foreach (Person p in people) {
            Response.Write(p.FullName + " <BR>");
        }
       
       //Using ForLoop to print out the objetcs from Array
       for(int i=0;i < people.Length;i++) { 
            Response.Write(people[i].FullName + " <BR>");
       }
    }
}

Resizing Arrays

VB language can resize an array and keep the existing values with the ReDim statement. Here you can double the size of the people array by using ReDim.

e.g ReDim Preserve people (5)

In C#, it doesn't support a convenient array resizing statement such as ReDim. Instead you copy people to a larger new array.

Code sample below

Person[] people2 = new Person[8];
Array.Copy(people,people2,people.Length);

Finding Objects in Arrays

Searching objects from the array is quite easy and straight forward in .NET framework.

int indexOfMichael = Array.IndexOf(people,michael);
Response.Write ("Michael is at " + indexOfMichael + ");

Sorting Objects in Arrays

For objects to have the sorting features, it needs to implement the IComparable interface and the IComparable interface basically consists of one method that is called CompareTo(). The CompareTo Method will basically return 3 values to determine which object is greater than the other objects being compared.

It will return 0, if both objects are equal.
It will return less than 0, if current object is less than the other object.
It will return greater than 0 , if current object is greater than the other object.

In our Person class, We can try to do a sorting based on the Age.  Since Age is an Integer objects and Integer objects already implement the CompareTo methods, we can make use of this to compare two instance of the Person objects based on the Age. Below is the sample code.

public class Person:IComparable {
    string FirstName;
    string LastName;
    int iAge;
    public Person (string first,string Last,int age) {
        FirstName = first;
        LastName = last;
        Age = age;
    }
    public string FirstName {
       get{
          return FirstName + " " + LastName;
       }
    }
    public int Age {
       get {
           return iAge;
       }
    }
    int iComparable.CompareTo(object obj) {
       Person p2 = obj as Person;
       if(p2 == null) throw new ArgumentException("Object is not a Person");
       int iAgeCompared = this.Age.CompareTo(p2.Age);
       return iAgeCompared;
    }
}

 Person michael = new Person("Michael","Jackson",12);
 Person bill = new Person("Bill","Gate",11);
 Person michelle = new Person("Michelle","Sullivan",14);
 
 Person[] people = new Person[3];
 people[0] = michael;
 people[1] = bill;
 people[2] = michelle;
        
 //Before Sorted
 foreach (Person p in people) {
     Response.Write(p.FullName + " <BR>");
 }
 
 //After Sorted
 Array.Sort(people);
 foreach (Person p in people) {
     Response.Write(p.FullName + " <BR>");
 }

ArrayList

If you have all the trouble and error when working and dealing with the size of the Array. Then ArrayList might be perfect for you. You don't have to worry about the Array size, as ArrayList has the features for automatic sizing. You have all the helper methods called Add,Insert,Remove,Sort. The only downside of the ArrayList is you have to type cast the objects upon retrieval from the Array.

public partial class _Default:System.Web.UI.Page
{
    protected void Page_Load(object sender,EventArgs e) 
    {
        Person michael = new Person("Michael","Jackson",12);
        Person bill = new Person("Bill","Gate",11);
        Person michelle = new Person("Michelle","Sullivan",14);
        
        ArrayList people = new ArrayList(); 
        people.Add(michael); 
        people.Add(bill);
        people.Add(michelle);

        //Using ForEach to print people
        foreach(Person p in people) {
            Response.Write(p.FullName + " <BR>");
        }
        //Sorting
        people.Sort();
        foreach(Person p in people) {
            Response.Write(p.FullName + " <BR>");
        }

        //Search By index
        int indexOfMichael = people.IndexOf(michael);
        Response.Write("Index Of Michael is " + indexOfMichael);
    }

}

Hashtables

If you like to store a collection of objects but with the key and value pair, then the Hashtable might be the best choice for you. Hashtable allow you to retrieve the objects based on the key you specified. It can be a string or integer. Unlike an array, you can only retrieve the value only by the index.

protected void Page_Load(object sender,EventArgs e) {
    Person michael = new Person("Michael","Jackson",12);
    Person bill = new Person("Bill","Gate",11);
    Person michelle = new Person("Michelle","Sullivan",14);
    
    Hashtable ht = new Hashtable();
    ht.Add("mc",michael);
    ht.Add("bl",bill);
    ht.Add("me",michelle);
    
    Person hashtablePerson = (Person)ht["sh"];
    Response.Write(hashtablePerson.FullName);
}

Queues and Stacks

Queue oQueue = new Queue();
oQueue.Enqueue(michael);
oQueue.Enqueue(bill);
oQueue.Enqueue(michelle);

Person x = (Person)oQueue.Dequeue();
Response.Write(x.FullName + "<BR>);

x = (Person)oQueue.Dequeue();
Response.Write(x.FullName + "<BR>);

x = (Person)oQueue.Dequeue();
Response.Write(x.FullName + "<BR>);

Stack oStack = new Stack();
oStack.Push(michael);
oStack.Push(bill);
oStack.Push(michelle);

Person x = (Person)oStack.Pop();
Response.Write(x.FullName + "<BR>);

Person x = (Person)oStack.Pop();
Response.Write(x.FullName + "<BR>);

Person x = (Person)oStack.Peek();
Response.Write(x.FullName + "&lt;BR>);

Person x = (Person)oStack.Pop();
Response.Write(x.FullName + "<BR>);